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Plagiarism Statement
I understand that I must use research conventions to cite and clearly mark other people’s ideas and words within my paper. I
understand that plagiarism is an act of intellectual dishonesty. I understand it is academically unethical and unacceptable to
do any of the following acts of which I will be immediately expelled without refund:

To submit an essay written in whole or in part by another student as if it were my own.
To download an essay from the internet, then quote or paraphrase from it, in whole or in part, without acknowledging
the original source.
To restate a clever phrase verbatim from another writer without acknowledging the source.
To paraphrase part of another writer’s work without acknowledging the source.
To reproduce the substance of another writer’s argument without acknowledging the source.
To take work originally done for one instructor’s assignment and re-submit it to another teacher.
To cheat on tests or quizzes through the use of crib sheets, hidden notes, viewing another student’s paper, revealing
the answers on my own paper to another student through verbal or textual communication, sign language, or other
means of storing and communicating information–including electronic devices, recording devices, cellular telephones,
headsets, and portable computers.
To copy another student’s work and submit the work as if it were the product of my own labor.
“The Habitable Planet”
Use the link to access the entire course and all weekly sections
Each of the 13 Unit videos introduces key scientists and their research. They provide a strong overview of the
topic under discussion, and may show the actual natural systems being discussed, or illustrate the nature of a
phenomenon. Through these video interviews, viewers will get a sense of how and why these scientists do
their research, have a look at some of the equipment and techniques they use, and learn about recognized
recent shifts in each field.
The Habitable Planet: A Systems Approach to Environmental Science
• 1Many Planets, One Earth
• 2Atmosphere
• 3Oceans
• 4Ecosystems
• 5Human Population Dynamics
• 6Risk, Exposure, and Health
• 7Agriculture
• 8Water Resources
• 9Biodiversity Decline
• 10Energy Challenges
• 11Atmospheric Pollution
• 12Earth’s Changing Climate
• 13Looking Forward: Our Global Experiment
• 14Carbon Lab
• 15Demographics Lab
• 16Disease Lab
• 17Ecology Lab
• 18Energy Lab
Be very detailed and explain each LAB clearly, responding to each question in full.
ASSIGNMENT: Watch the video for each unit using the link in the top box
“Video Index”. Provide a written summary for each individual video.
Read and study each Unit
1.) Many Planets, One Earth: Summarize the video
2.) Atmosphere: Summarize the video
3.) Oceans: Summarize the video
Do the complete lab and submit written components below.
Be very detailed and explain each LAB clearly, responding to each question in full.
Carbon Lab (Units 1-3, 13)
Throughout this course, the carbon cycle is featured as one of the most important planetary systems.
This lab uses a robust model of the carbon cycle to give you an intuitive sense for how the system
works. It also allows you to experiment with how human inputs to the cycle might change global
outcomes to the year 2100 and beyond. One especially relevant human impact is the increase in
atmospheric CO2 levels. Between 1850 and today, atmospheric concentrations have risen from 287
ppm (parts per million) to over 380 ppm – a level higher than any known on Earth in more than 30
million years (see Unit 12 to find out how scientists measure ancient atmospheric carbon levels). You
will experiment with the human factors that contribute to this rise, and see how different inputs to the
carbon cycle might affect concentrations of the greenhouse gas CO2.
The Carbon Cycle
– Step 1
1. What is the relationship between increased carbon in the ocean and increased carbon in the soil? How
else might carbon be transferred to soil?
– Step 2
2. What is the relationship between an increase in total carbon concentration (the smokestack) and
increased carbon in the ocean surface? How might this change marine life populations? What impact
could fifty years at this level of emissions have on marine fauna? On marine flora?
3. In addition to circulating through the carbon cycle, where else might excess carbon be found? In fifty
years, where would you be most likely to see excess carbon?
Curb Emissions
– Step 1
4. How has atmospheric carbon levels changed?
5. Without any fossil fuel consumption, which parts of the cycle have improved their carbon levels
in comparison to previous data? Which sections of the cycle have improved from the previous
levels you have recorded but still are increasing their carbon levels?
– Step 2
6. What effect does a high carbon level have on the deep ocean? Why might it be important to
keep an eye on the deep ocean carbon levels? What could that one number tell you about the
cycle as a whole?
7. Try reducing the level of fossil fuel percentage increase and decrease deforestation by 50%.
Predict what will happen to the atmospheric carbon levels and record it in your Data Table. Run
the simulation to test your hypothesis. Were you correct? Were you surprised by the result?
What about your result surprised you?
ASSIGNMENT: Watch the video for each unit using the link in the top box
“Video Index”. Provide a written summary for each individual video.
Read and study each Unit
4.) Ecosystems: Summarize the video
5.) Human Population Dynamics: Summarize the video
Do the complete lab and submit written components below.
Be very detailed and explain each LAB clearly, responding to each question in full.
Demographics Lab (Units 5, 13)
Baby boom. Overpopulation. Birth dearth. These terms all refer to human population growth, and can
conjure images of environmental and economic peril. Which are real issues, and should they matter to
Demographers like the US Census Bureau make population projections based on mathematical models.
In this lab you will explore a fully functional simulation, based on real demographic data. You will
examine important demographic trends through a series of guided lessons. After completing these
lessons you will understand the factors that control human population growth, recognize the seachange in human history that is the “demographic transition,” and gain a sense of how population
demographics has a very human impact in all areas of our habitable planet.

The Demographic Transition
– Step 1
1. How do you suppose living conditions differ between the country furthest along in the
demographic transition compared to the country earliest in the transition? How would living
conditions in these two countries affect both birth and death rates?
2. Think of three social factors that contribute to lower birth rates in the countries farther along.
How might these social conditions be encouraged to emerge in less developed countries?
– Step 2
3. How does the shape of the population pyramid differ from most developed to least developed country?
Population Momentum
– Step 1
4. How does an increase or decrease in the average childbearing age group change the population? Why
do “first world” countries tend to have older childbearing women than “third world” countries?
– Step 2
5. Did the pattern of population change match your prediction? If not, why not? Compare the final
population pyramid for Italy to the one you sketched of Nigeria. How do they compare, and why
are they similar or different?
6. How are Italy’s numbers different from Nigeria’s? What do you think accounts for the
ASSIGNMENT: Watch the video for each unit using the link in the top box
“Video Index”. Provide a written summary for each individual video.
Read and study each Unit
6.) Risk, Exposure and Health: Summarize Video
7.) Agriculture: Summarize Video
Do the complete lab and submit written components below.
Be very detailed and explain each LAB clearly, responding to each question in full.
Disease Lab (Unit 5, 6)
Recently, new diseases, such as SARS, and the potential for a pandemic avian flu have raised
international concerns about health. As populations grow (see the Demographics lab), especially in
densely packed urban areas, there is increased risk of disease transmission. This lab will allow you to
explore various types of diseases: “Kold” is similar to the common cold, “Impfluenza” resembles a
typical influenza outbreak, and “Red Death” represents a fast-spreading epidemic with a high mortality
rate (such as avian flu if it were to develop through human-to-human transmission). What factors come
into play in the spread of these diseases, and what can we do to counter them?

The Virgin Field
– Step 1
1. Do you get the exact same results each time? How do the results compare to each other and to
your prediction? What factors might contribute to susceptibility to the disease?
– Step 2
2. What could be done to prevent the spread of disease in a low population density? What kinds
of challenges would high population density present to these precautions?
– Step 1
3. Was your prediction correct? If not, why not?
4. Notice that Impfluenza, unlike Kold, has a death rate. How many people die, on average,
when you run the simulator on the virgin field?
ASSIGNMENT: Watch the video for each unit using the link in the top box
“Video Index”. Provide a written summary for each individual video.
Read and study each Unit
8.) Water Resources: Summarize the video
9.) Biodiversity Decline: Summarize the video
10.) Energy Challenges: Summarize the video
Do the complete lab and submit written components below.
Be very detailed and explain each LAB clearly, responding to each question in full.
Ecology Lab (Units 4, 7, 9, 13)
As you learned in Unit 4, ecosystems are a complex and delicate balancing game. The addition or
removal of any species affects many other species that might compete for or provide food. In this lab
you will get a chance to “build your own” ecosystem, and explore the effects of these

The Producers
– Challenge
– Step 1
1. Do you find one producer to be dominant? Why might one producer be dominant over
– Step 2
2. If the simulation included decomposers, how would your current results change?
Food Web
– Challenge
– Step 1
3. Was your prediction correct? How did you arrive at your prediction? What differences
were there between your prediction and the simulation?
4. What would happen to this imaginary ecosystem if the producers were to die out?
Step 2
5. Was your prediction correct? How did you arrive at your prediction? What differences were there
between your prediction and the simulation?
ASSIGNMENT: Watch the video for each unit using the link in the top box
“Video Index”. Provide a written summary for each individual video.
Read and study each Unit
11. Atmospheric Pollution: Summarize the video
12.) Earth’s Changing Climate: Summarize the video
13.) Looking Forward: Our Global Experiment: Summarize the video
Do the complete lab and submit written components below.
Be very detailed and explain each LAB clearly, responding to each question in full.
Energy Lab (Units 10, 12, 13)
In today’s world, with populations and economies booming, the demand for energy is rising. A portfolio
of different energy sources is used to meet this demand. Since there is no perfectly clean, safe, and
inexpensive source of energy, the composition of this portfolio involves tradeoffs of safety, cost, and-of
increasing concern-emissions of greenhouse gases such as CO2 (if you haven’t done the Carbon Cycle
lab yet, we recommend you start there). In this lab, your challenge is to try to meet the world’s
projected energy demand by choosing from the available energy sources while keeping atmospheric
CO2 under control and avoiding the particular limits and pitfalls associated with each energy source.
1. Lessons
a. Managing Resources
b. – Introduction
c. Adjust the settings as stated in this introduction
– Step 1
2. How close was your prediction to any of the simulation runs? Were your simulation runs similar? How
did they differ?
– Step 2
3. Was your prediction closer to the cheap and quick energy supply model or the eco–friendly model?
Which model met both needs best? Is there a feasible way of bringing this model to fruition in the
“real” world.
a. Energy Efficiency
b. – Introduction
c. – Step 1
4. Considering what you read in the text, how realistic was your prediction? What kinds of problems occur
in the simulation?
Step 2
5. What combination of parameters did you find for a “best case” scenario this time? Were they close to
your prediction? Why might your prediction have been off? What kinds of problems/issues/factors
come into play this time that weren’t present previously?
6. Based on this simulation, what would need to be changed in American people’s lives in order to meet
the parameters you’ve designed in this model?
7. Considering what you now know about carbon emission issues (see the Carbon Lab) and energy/fuel
sources, what might humans have to do in order to meet energy demands, maintain low CO2 emission
levels, and not cause further harm to the environment? What initial steps might we all take?
Unit 1 : Many Planets, One Earth
Astronomers have discovered dozens of planets orbiting
other stars, and space probes have explored many parts of
our solar system, but so far scientists have only discovered
one place in the universe where conditions are suitable for
complex life forms: Earth. In this unit, examine the unique
characteristics that make our planet habitable and learn how
these conditions were created.
Surfaces of Mars, Moon, Venus, Earth. Source:
1. Introduction
2. Many Planets, One Earth
3. Reading Geologic Records
4. Carbon Cycling and Earth’s Climate
5. Testing the Thermostat: Snowball Earth
6. Atmospheric Oxygen
7. Early Life: Single-Celled Organisms
8. The Cambrian Explosion and the Diversification of Animals
9. The Age of Mammals
10. Further Reading
Unit 1 : Many Planets, One Earth
1. Introduction
Earth’s long history tells a story of constant environmental change and of close connections between
physical and biological environments. It also demonstrates the robustness of life. Simple organisms
first appeared on Earth some 3.8 billion years ago, and complex life forms emerged approximately
2 billion years ago. Life on Earth has endured through many intense stresses, including ice ages,
warm episodes, high and low oxygen levels, mass extinctions, huge volcanic eruptions, and meteorite
impacts. Untold numbers of species have come and gone, but life has survived even the most
extreme fluxes.
To understand why Earth has been so conducive to life, we need to identify key conditions that make
it habitable and ask why they exist here but not on neighboring planets. This unit describes how
Earth’s carbon cycle regulates its climate and keeps surface temperatures within a habitable range.
It also examines another central factor: the rise of free oxygen in the atmosphere starting more than
2 billion years ago. Next we briefly survey the evolution of life on Earth from simple life forms through
the Cambrian explosion and the diversification of multicellular organisms—including, most recently,
humans. This unit also describes how scientists find evidence in today’s geologic records for events
that took place millions or even billions of years ago (Fig. 1).
Figure 1. Evidence of glaciation in seaside rocks, Massachusetts
Humans are latecomers in geologic time: when Earth’s history is mapped onto a 24-hour time scale,
we appear less than half a minute before the clock strikes midnight (footnote 1). But even though
Unit 1 : Many Planets, One Earth
humans have been present for a relatively short time, our actions are changing the environment in
many ways, which are addressed in units 5 through 13 of this course. Life on Earth will persist in
spite of these human impacts. But it remains to be seen how our species will manage broad-scale
challenges to our habitable planet, especially those that we create. As history shows, Earth has
maintained conditions over billions of years that are uniquely suitable for life on Earth, but those
conditions can fluctuate widely. Human impacts add to a natural level of ongoing environmental
2. Many Planets, One Earth
Our solar system formed from a solar nebula, or cloud of gas and dust, that collapsed and condensed
about 4.56 billion years ago. Most of this matter compacted together to form the sun, while the
remainder formed planets, asteroids, and smaller bodies. The outer planets, Jupiter, Saturn, Uranus,
and Neptune, condensed at cold temperatures far from the sun. Like the sun, they are made mostly
of hydrogen and helium. In contrast, the terrestrial planets, Mercury, Venus, Earth and Mars, formed
closer to the sun where temperatures were too high to allow hydrogen and helium to condense.
Instead they contain large amounts of iron, silicates (silicon and oxygen), magnesium, and other
heavier elements that condense at high temperatures.
The young Earth was anything but habitable. Radioactive elements decaying within its mass and
impacts from debris raining down from space generated intense heat—so strong that the first eon of
Earth’s history, from about 4.5 to 3.8 billion years ago, is named the Hadean after hades, the Greek
word for hell. Most original rock from this period was melted and recycled into Earth’s crust, so very
few samples remain from our planet’s formative phase. But by studying meteorites—stony or metallic
fragments up to 4.5 billion years old that fall to Earth from space—scientists can see what materials
were present when the solar system was formed and how similar materials may have been melted,
crystallized, and transformed as Earth took shape (Fig. 2).
Unit 1 : Many Planets, One Earth
Figure 2. The Willamette Meteorite, the largest ever found in the United States (15 tons)
© Denis Finnin, American Museum of Natural History.
About 4 billion years ago, conditions on Earth gradually began to moderate. The planet’s surface
cooled, allowing water vapor to condense in the atmosphere and fall back as rain. This early
hydrologic cycle promoted rock weathering, a key part of the carbon-silicate cycle that regulates
Earth’s climate (discussed in section 4). Evidence from ancient sediments indicates that oceans
existed on Earth as long ago as 3.5 billion years.
Conditions evolved very differently on adjoining planets. Venus, which has nearly the same size and
density as Earth and is only about 30 percent closer to the sun, is sometimes referred to as our “sister
planet.” Scientists once thought that conditions on Venus were much like those on Earth, just a little
bit warmer. But in reality Venus is a stifling inferno with an average surface temperature greater than
460°C (860°F). This superheated climate is produced by Venus’s dense atmosphere, which is about
100 times thicker than Earth’s atmosphere and is made up almost entirely of carbon dioxide (CO2)
(Fig. 3). As we will see in Unit 2, “Atmosphere,” CO2 is a greenhouse gas that traps heat reflected
back from planetary surfaces, warming the planet. To make conditions even more toxic, clouds on
Venus consists mainly of sulfuric acid droplets.
Paradoxically, if Venus had an atmosphere with the same composition as Earth’s, Venus would be
colder even though it is closer to the sun and receives approximately twice as much solar radiation as
Earth does. This is because Venus has a higher albedo (its surface is brighter than Earth’s surface),
so it reflects a larger fraction of incoming sunlight back to space. Venus is hot because its dense
atmosphere functions like a thick blanket and traps this outgoing radiation. An atmosphere with the
same makeup as Earth’s would function like a thinner blanket, allowing more radiation to escape back
to space (Fig. 3).
Unit 1 : Many Planets, One Earth
Figure 3. Comparison of Venus and Earth
Courtesy NASA/JPL-Caltech.
Mars is not much farther from the sun than Earth, but it is much colder. Clouds of ice and frozen CO2
(dry ice) drift over its surface. Frozen ice caps at the poles, which can be seen from Earth with a
telescope, reflect sunlight. Although Mars’s atmosphere consists mainly of CO2, it is 100 times thinner
than Earth’s atmosphere, so it provides only a small warming effect. Early in its history, the “Red
Planet” had an atmosphere dense and warm enough to sustain liquid water, and it may even have
had an ocean throughout its northern hemisphere. Today, however, all water on Mars is frozen.
Why is Venus so hot? Why is Mars so cold? And why has the Earth remained habitable instead of
phasing into a more extreme state like Mars or Venus? The key difference is that an active carbon
cycle has kept Earth’s temperature within a habitable range for the past 4 billion years, despite
changes in the brightness of the sun during that time. This process is described in detail in section 4,
“Carbon Cycling and Earth’s Climate.” Moderate surface temperatures on Earth have created other
important conditions for life, such as a hydrologic cycle that provides liquid water.
How unique are the conditions that allowed life to develop and diversify on Earth? Some scientists
contend that circumstances on Earth were extremely unusual and that complex life is very unlikely
to find such favorable conditions elsewhere in our universe, although simple life forms like microbes
may be very common (footnote 2). Other scientists believe that Earth’s history may not be the only
environment in which life could develop, and that other planets with very different sets of conditions
could foster complex life. What is generally agreed, however, is that no other planet in our solar
Unit 1 : Many Planets, One Earth
system has developed along the same geologic and biologic path as Earth. Life as we know it is a
direct result of specific conditions that appear thus far to be unique to our planet.
3. Reading Geologic Records
Scientists have divided Earth’s history into a series of time segments that are collectively referred
to as the geologic time scale (Figure 4). Each of these units is defined based on geologic and fossil
records, with divisions between the units marking some major change such as the appearance of
a new class of living creatures or a mass extinction. Geologic time phases become shorter as we
move forward from Earth’s formation toward the present day because records grow increasingly rich.
Newer rocks and fossils are better preserved than ancient deposits, so more information is available
to categorize recent phases in detail and pinpoint when they began and ended.
Figure 4. The geologic time scale
© United States Geological Survey.
Most of what we know about our planet’s history is based on studies of the stratigraphic record—
rock layers and fossil remains embedded in them. These rock records can provide insights into
questions such as how geological formations were created and exposed, what role was played by
living organisms, and how the compositions of oceans and the atmosphere have changed through
geologic time.
Unit 1 : Many Planets, One Earth
Scientists use stratigraphic records to determine two kinds of time scales. Relative time refers
to sequences—whether one incident occurred before, after, or at the same time as another. The
geologic time scale shown in Figure 4 reads upwards because it is based on observations from
sedimentary rocks, which accrete from the bottom up (wind and water lay down sediments, which
are then compacted and buried). However, the sedimentary record is discontinuous and incomplete
because plate tectonics are constantly reshaping Earth’s crust. As the large plates on our planet’s
surface move about, they split apart at some points and collide or grind horizontally past each other
at others. These movements leave physical marks: volcanic rocks intrude upward into sediment beds,
plate collisions cause folding and faulting, and erosion cuts the tops off of formations thrust up to the
Geologists have some basic rules for determining relative ages of rock layers. For example, older
beds lie below younger beds in undisturbed formations, an intruding rock is younger than the layers
it intrudes into, and faults are younger than the beds they cut across. In the geologic cross-section
shown in Figure 5, layers E, F, G, H, I, and J were deposited through sedimentation, then cut by
faults L and K, then covered by layers D, C, and B. A is a volcanic intrusion younger than the layers it
Figure 5. Sample geologic cross-section
Scientists also use fossil records to determine relative age. For example, since fish evolved before
mammals, a rock formation at site A that contains fish fossils is older than a formation at site B that
contains mammalian fossils. And environmental changes can leave telltale geologic imprints in rock
records. For example, when free oxygen began to accumulate in the atmosphere, certain types of
Unit 1 : Many Planets, One Earth
rocks appeared for the first time in sedimentary beds and others stopped forming (for more details,
see section 6, “Atmospheric Oxygen”). Researchers study mineral and fossil records together to trace
interactions between environmental changes and the evolution of living organisms.
Until the early twentieth century, researchers could only assign relative ages to geologic records.
More recently, the expanding field of nuclear physics has enabled scientists to calculate the absolute
age of rocks and fossils using radiometric dating, which measures the decay of radioactive isotopes
in rock samples. This approach has been used to determine the ages of rocks more than 3.5 billion
years old (footnote 3). Once they establish the age of multiple formations in a region, researchers can
correlate strata among those formations to develop a fuller record of the entire area’s geologic history
(Fig. 6).
Figure 6. Geologic history of southern California
© United States Geological Survey, Western Earth Surface Processes Team.
Our understanding of Earth’s history and the emergence of life draws on other scientific fields along
with geology and paleontology. Biologists trace genealogical relationships among organisms and
the expansion of biological diversity. And climate scientists analyze changes in Earth’s atmosphere,
temperature patterns, and geochemical cycles to determine why events such as ice ages and rapid
warming events occurred. All of these perspectives are relevant because, as we will see in the
following sections, organisms and the physical environment on Earth have developed together and
influenced each other’s evolution in many ways.
Unit 1 : Many Planets, One Earth
4. Carbon Cycling and Earth’s Climate
How did early Earth transition from a hell-like environment to temperatures more hospitable to life?
Early in the Archean (ancient) eon, about 3.8 billion years ago, the rain of meteors and rock bodies
from space ended, allowing our planet’s surface to cool and solidify. Water vapor in the atmosphere
condensed and fell as rain, creating oceans. These changes created the conditions for geochemical
cycling—flows of chemical substances between reservoirs in Earth’s atmosphere, hydrosphere (water
bodies), and lithosphere (the solid part of Earth’s crust).
At this time the sun was about 30 percent dimmer than it is today, so our planet received less solar
radiation. Earth’s surface should have been well below the freezing point of water, too cold for life to
exist, but evidence shows that liquid water was present and that simple life forms appeared as far
back as 3.5 billion years ago. This contradiction is known as the “faint young sun” paradox (Fig. 7).
The unexpected warmth came from greenhouse gases in Earth’s atmosphere, which retained enough
heat to keep the planet from freezing over.
Figure 7. The faint, young sun and temperatures on Earth
The Archean atmosphere was a mix of gases including nitrogen, water vapor, methane (CH4), and
CO2. (As discussed in section 6, “Atmospheric Oxygen,” free oxygen did not accumulate in the
atmosphere until more than two billion years after Earth was formed.) Volcanoes emitted CO2 as a
byproduct of heating within the Earth’s crust. But instead of developing a runaway greenhouse effect
Unit 1 : Many Planets, One Earth
like that on Venus, Earth’s temperatures remained within a moderate range because the carbon
cycle includes a natural sink—a process that removes excess carbon from the atmosphere. This sink
involves the weathering of silicate rocks, such as granites and basalts, that make up much of Earth’s
As illustrated in Figure 8, this process has four basic stages. First, rainfall scrubs CO2 out of the air,
producing carbonic acid (H2CO3), a weak acid. Next, this solution reacts on contact with silicate rocks
to release calcium and other cations and leave behind carbonate and biocarbonate ions dissolved in
the water. This solution is washed into the oceans by rivers, and then calcium carbonate (CaCO3),
also known as limestone, is precipitated in sediments. (Today most calcium carbonate precipitation
is caused by marine organisms, which use calcium carbonate to make their shells.) Over long time
scales, oceanic crust containing limestone sediments is forced downward into Earth’s mantle at points
where plates collide, a process called subduction. Eventually, the limestone heats up and turns the
limestone back into CO2, which travels back up to the surface with magma. Volcanic activity then
returns CO2 to the atmosphere.
Figure 8. The geochemical carbon cycle
© Snowball Earth.org.
Many climatic factors influence how quickly this process takes place. Warmer temperatures speed
up the chemical reactions that take place as rocks weather, and increased precipitation may
flush water more rapidly through soil and sedimentary rocks. This creates a negative feedback
relationship between rock weathering and climatic changes: when Earth’s climate warms or cools, the
Unit 1 : Many Planets, One Earth
system responds in ways that moderate the temperature change and push conditions back toward
equilibrium, essentially creating a natural thermostat.
For example, when the climate warms, weathering rates accelerate and convert an increasing
fraction of atmospheric CO2 to calcium carbonate, which is buried on the ocean floor. Atmospheric
concentrations of CO2 decline, modifying the greenhouse effect and cooling Earth’s surface. In the
opposite instance, when the climate cools weathering slows down but volcanic outgassing of CO2
continues, so atmospheric CO2 levels rise and warm the climate.
This balance between CO2 outgassing from volcanoes and CO2 conversion to calcium carbonate
through silicate weathering has kept the Earth’s climate stable through most of its history. Because
this feedback takes a very long time, typically hundreds of thousands of years, it cannot smooth out
all the fluctuations like a thermostat in one’s home. As a result, our planet’s climate has fluctuated
dramatically, but it has never gone to permanent extremes like those seen on Mars and Venus.
Why is Venus a runaway greenhouse? Venus has no water on its surface, so it has no medium
to dissolve CO2, form carbonic acid, and react with silicate rocks. As a result volcanism on Venus
continues to emit CO2 without any carbon sink, so it accumulates in the atmosphere. Mars may have
had such a cycle early in its history, but major volcanism stopped on Mars more than 3 billion years
ago, so the planet eventually cooled as CO2 escaped from the atmosphere. On Earth, plate tectonics
provide continuing supplies of the key ingredients for the carbon-silicate cycle: CO2, liquid water, and
plenty of rock.
5. Testing the Thermostat: Snowball Earth
We can see how durable Earth’s silicate weathering “thermostat” is by looking at some of the
most extreme climate episodes on our planet’s history: severe glaciations that occurred during the
Proterozoic era. The first “Snowball Earth” phase is estimated to have occurred about 2.3 billion
years ago, followed by several more between about 750 and 580 million years ago. Proponents of the
Snowball Earth theory believe that Earth became so cold during several glacial cycles in this period
that it essentially froze over from the equator to the poles for spans of ten million years or more. But
ultimately, they contend, the carbon-silicate cycle freed Earth from this deep-freeze state.
If Earth has a natural thermostat, how could it become cold enough for the entire planet to freeze
over? One possible cause is continental drift. Researchers believe that around 750 million years
ago, most of the continents may have been clustered in the tropics following the breakup of the
supercontinent Rodinia (Fig. 9), and that such a configuration would have had pronounced effects on
Earth’s climate (footnote 4).
Unit 1 : Many Planets, One Earth
Figure 9. The breakup of Rodinia
© Snowball Earth.org.
Had the continents been located closer to the poles as they are today, ice sheets would have
developed at high latitudes as the planet cooled. Ice cover would prevent the rocks beneath from
weathering, thus slowing the rate at which carbon was removed from the atmosphere and allowing
CO2 from volcanic eruptions to build up in the atmosphere. As a result, Earth’s surface temperature
would warm.
But if continents were clustered at low latitudes, Earth’s land masses would have remained ice-free
for a long time even as ice sheets built up in the polar oceans and reflected a growing fraction of solar
energy back to space. Because most continental area was in the tropics, the weathering reactions
would have continued even as the Earth became colder and colder. Once sea ice reached past about
30 degrees latitude, Snowball Earth scholars believe that a runaway ice-albedo effect occurred: ice
reflected so much incoming solar energy back to space, cooling Earth’s surface and causing still more
ice to form, that the effect became unstoppable. Ice quickly engulfed the planet and oceans froze to
an average depth of more than one kilometer.
The first scientists who imagined a Snowball Earth believed that such a sequence must have been
impossible, because it would have cooled Earth so much that the planet would never have warmed
up. Now, however, scientists believe that Earth’s carbon cycle saved the planet from permanent
deep-freeze. How would a Snowball Earth thaw? The answer stems from the carbon-cycle thermostat
discussed earlier.
Unit 1 : Many Planets, One Earth
Even if the surface of the Earth was completely frozen, volcanoes powered by heat from the planet’s
interior would continue to vent CO2. However, very little water would evaporate from the surface
of a frozen Earth, so there would be no rainfall to wash CO2 out of the atmosphere. Over roughly
10 million years, normal volcanic activity would raise atmospheric CO2 concentrations by a factor
of 1,000, triggering an extreme warming cycle (Fig. 10). As global ice cover melted, rising surface
temperatures would generate intense evaporation and rainfall. This process would once again
accelerate rock weathering, ultimately drawing atmospheric CO2 levels back down to normal ranges.
Figure 10. The geochemical carbon cycle on a Snowball Earth
© Snowball Earth.org.
Many geologic indicators support the snowball glaciation scenario. Glacial deposits (special types of
sediments known to be deposited only by glaciers or icebergs) are found all around the world at two
separate times in Earth’s history: once around 700 million years ago and then again around 2,200
million years ago. In both cases some of these glacial deposits have magnetic signatures that show
that they were formed very close to the equator, supporting an extreme glacial episode.
Another important line of evidence is the existence of special iron-rich rocks, called iron formations,
that otherwise are seen only very early in Earth’s history when scientists believe that atmospheric
oxygen was much lower. In the presence of oxygen, iron exists as “ferric” iron (Fe ), a form that is
very insoluble in water (there is less than one part per billion of iron dissolved in seawater today).
However, before oxygen accumulated in the atmosphere, iron would have existed in a reduced
state, called “ferrous” iron (Fe ), which is readily dissolved in seawater. Geologists believe that iron
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formations were produced when iron concentrations in the deep ocean were very high but some
oxygen existed in the surface ocean and atmosphere. Mixing of iron up from the deep ocean into the
more oxidized ocean would cause the chemical precipitation of iron, producing iron formations.
Geologists do not find iron formations after about 1.8 billion years ago, once oxygen levels in the
atmosphere and ocean were high enough to remove almost all of the dissolved iron. However, iron
formations are found once again around 700 million years ago within snowball glacial deposits. The
explanation appears to be that during a Snowball Earth episode sea ice formed over most of the
ocean’s surface, making it difficult for oxygen to mix into the water. Over millions of years iron then
built up in seawater until the ice started to melt. Then atmospheric oxygen could mix with the ocean
once again, and all the iron was deposited in these unusual iron formations (Fig. 11).
Figure 11. Banded iron formation from Ontario, Canada
© Denis Finnin, American Museum of Natural History.
The Snowball Earth is still a controversial hypothesis. Some scientists argue that the evidence is
not sufficient to prove that Earth really did freeze over down to the equator. But the hypothesis is
supported by more and more unusual geological observations from this time, and also carries some
interesting implications for the evolution of life.
Unit 1 : Many Planets, One Earth
How could life survive a snowball episode? Paradoxically, scientists theorize that these deep freezes
may have indirectly spurred the development of complex life forms. The most complex life forms on
Earth at the time of the Neoproterozoic glaciations were primitive algae and protozoa. Most of these
existing organisms were undoubtedly wiped out by glacial episodes. But recent findings have shown
that some microscopic organisms can flourish in extremely challenging conditions—for example,
within the channels inside floating sea ice and around vents on the ocean floor where superheated
water fountains up from Earth’s mantle. These environments may have been the last reservoirs of
life during Snowball Earth phases. Even a small amount of geothermal heat near any of the tens of
thousands of natural hot springs that exist on Earth would have been sufficient to create small holes
in the ice. And those holes would have been wonderful refuges where life could persist.
Organisms adaptable enough to survive in isolated environments would have been capable of rapid
genetic evolution in a short time. The last hypothesized Snowball Earth episode ended just a few
million years before the Cambrian explosion, an extraordinary diversification of live that took place
from 575 to 525 million years ago (discussed in section 8, “Multi-Celled Organisms and the Cambrian
Explosion”). It is possible, although not proven, that the intense selective pressures of snowball
glaciations may have fostered life forms that were highly adaptable and ready to expand quickly once
conditions on Earth’s surface moderated.
6. Atmospheric Oxygen
A stable climate is only one key requirement for the complex life forms that populate Earth today.
Multi-cellular organisms also need a ready supply of oxygen for respiration. Today oxygen makes
up about 20 percent of Earth’s atmosphere, but for the first two billion years after Earth formed, its
atmosphere was anoxic (oxygen-free). About 2.3 billion years ago, oxygen increased from a trace gas
to perhaps one percent of Earth’s atmosphere. Another jump took place about 600 million years ago,
paving the way for multi-cellular life forms to expand during the Cambrian Explosion.
Oxygen is a highly reactive gas that combines readily with other elements like hydrogen, carbon,
and iron. Many metals react directly with oxygen in the air to form metal oxides. For example, rust
is an oxide that forms when iron reacts with oxygen in the presence of water. This process is called
oxidation, a term for reactions in which a substance loses electrons and become more positively
charged. In this case, iron loses electrons to oxygen (Fig. 12). What little free oxygen was produced
in Earth’s atmosphere during the Archean eon would have quickly reacted with other gases or with
minerals in surface rock formations, leaving none available for respiration.
Unit 1 : Many Planets, One Earth
Figure 12. Oxidation of iron to form rust
Geologists trace the rise of atmospheric oxygen by looking for oxidation products in ancient rock
formations. We know that very little oxygen was present during the Archean eon because sulfide
minerals like pyrite (fool’s gold), which normally oxidize and are destroyed in today’s surface
environment, are found in river deposits dating from that time. Other Archean rocks contain banded
iron formations (BIFs)—the sedimentary beds described in section 5 that record periods when waters
contained high concentrations of iron. These formations tell us that ancient oceans were rich in iron,
creating a large sink that consumed any available free oxygen.
Scientists agree that atmospheric oxygen levels increased about 2.3 billion years ago to a level
that may have constituted about 1 percent of the atmosphere. One indicator is the presence of rock
deposits called red beds, which started to form about 2.2 billion years ago and are familiar to travelers
who have visited canyons in Arizona or Utah. These strata of reddish sedimentary rock, which formed
from soils rich in iron oxides, are basically the opposite of BIFs: they indicate that enough oxygen had
accumulated in the atmosphere to oxidize iron present in soil. If the atmosphere had still been anoxic,
iron in these soils would have remained in solution and would have been washed away by rainfall and
river flows. Other evidence comes from changes in sulfur isotope ratios in rocks, which indicate that
about 2.4 billion years ago sulfur chemistry changed in ways consistent with increasing atmospheric
Why did oxygen levels rise? Cyanobacteria, the first organisms capable of producing oxygen through
photosynthesis, emerged well before the first step up in atmospheric oxygen concentrations, perhaps
as early as 2.7 billion years ago. Their oxygen output helped to fill up the chemical sinks, such as iron
Unit 1 : Many Planets, One Earth
in soils, that removed oxygen from the air. But plant photosynthesis alone would not have provided
enough oxygen to account for this increase, because heterotrophs (organisms that are not able to
make their own food) respire oxygen and use it to metabolize organic material. If all new plant growth
is consumed by animals that feed on living plants and decomposers that break down dead plant
material, carbon and oxygen cycle in what is essentially a closed loop and net atmospheric oxygen
levels remain unchanged (Fig. 13).
Figure 13. Cycling of carbon and oxygen
However, material can leak out of this loop and alter carbon-oxygen balances. If organic matter
produced by photosynthesis is buried in sediments before it decomposes (for example, dead trees
may fall into a lake and sink into the lake bottom), it is no longer available for respiration. The
oxygen that decomposers would have consumed as they broke it down goes unused, increasing
atmospheric oxygen concentrations. Many researchers theorize that this process caused the initial
rise in atmospheric oxygen.
Some scientists suspect that atmospheric oxygen increased again about 600 million years ago to
levels closer to the composition of our modern atmosphere. The main evidence is simply that many
different groups of organisms suddenly became much larger at this time. Biologists argue that it is
difficult for large, multicellular animals to exist if oxygen levels are extremely low, as such animals
cannot survive without a fairly high amount of oxygen. However, scientists are still not sure what
caused a jump in oxygen at this time.
Unit 1 : Many Planets, One Earth
One clue may be the strange association of jumps in atmospheric oxygen with snowball glaciations.
Indeed, the jumps in atmospheric oxygen at 2.3 billion years ago and 600 million years ago do seem
to be associated with Snowball Earth episodes (Fig. 14). However, scientists are still unsure exactly
what the connection might be between the extreme ice ages and changes in the oxygen content of
the atmosphere.
Figure 14. Atmospheric oxygen levels over geological time
© Snowball Earth.org.
Why have atmospheric oxygen levels stayed relatively stable since this second jump? As discussed
above, the carbon-oxygen cycle is a closed system that keeps levels of both elements fairly constant.
The system contains a powerful negative feedback mechanism, based on the fact that most
animals need oxygen for respiration. If atmospheric oxygen levels rose substantially today, marine
zooplankton would eat and respire organic matter produced by algae in the ocean at an increased
rate, so a lower fraction of organic matter would be buried, canceling the effect. Falling oxygen levels
would reduce feeding and respiration by zooplankton, so more of the organic matter produced by
algae would end up in sediments and oxygen would rise again. Fluctuations in either direction thus
generate changes that push oxygen levels back toward a steady state.
Forest fires also help to keep oxygen levels steady through a negative feedback. Combustion is a
rapid oxidation reaction, so increasing the amount of available oxygen will promote a bigger reaction.
Rising atmospheric oxygen levels would make forest fires more common, but these fires would
consume large amounts of oxygen, driving concentrations back downward.
Unit 1 : Many Planets, One Earth
7. Early Life: Single-Celled Organisms
We do not know exactly when life first appeared on Earth. There is clear evidence that life existed
at least 3 billion years ago, and the oldest sediments that have been discovered—rocks formed up
to 3.8 billion years ago—bear marks that some scientists believe could have been left by primitive
microorganisms. If this is true it tells us that life originated soon after the early period of bombardment
by meteorites, on an Earth that was probably much warmer than today and had only traces of oxygen
in its atmosphere.
For the first billion years or so, life on Earth consisted of bacteria and archaea, microscopic
organisms that represent two of the three genealogical branches on the Tree of Life (Fig. 15). Both
groups are prokaryotes (single-celled organisms without nuclei). Archaea were recognized as a
unique domain of life in the 1970s, based on some distinctive chemical and genetic features. The
order in which branches radiate from the Tree of Life shows the sequence in which organisms
evolved. Reading from the bottom up in the same way in which a tree grows and branches outward,
we can see that eucarya (multi-celled animals) were the last major group to diverge and that animals
are among the newest subgroups within this domain.
Figure 15. The universal Tree of Life
© National Aeronautics and Space Administration.
Life on Earth existed for many millions of years without atmospheric oxygen. The lowest groups on
the Tree of Life, including thermatogales and nearly all of the archaea, are anaerobic organisms
that cannot tolerate oxygen. Instead they use hydrogen, sulfur, or other chemicals to harvest energy
Unit 1 : Many Planets, One Earth
through chemical reactions. These reactions are key elements of many chemical cycles on Earth,
including the carbon, sulfur, and nitrogen cycles. “Prokaryotic metabolisms form the fundamental
ecological circuitry of life,” writes paleontologist Andrew Knoll. “Bacteria, not mammals, underpin the
efficient and long-term functioning of the biosphere” (footnote 5).
Some bacteria and archaea are extremophiles—organisms that thrive in highly saline, acidic, or
alkaline conditions or other extreme environments, such as the hot water around hydrothermal vents
in the ocean floor. Early life forms’ tolerances and anaerobic metabolisms indicate that they evolved
in very different conditions from today’s environment.
Microorganisms are still part of Earth’s chemical cycles, but most of the energy that flows through our
biosphere today comes from photosynthetic plants that use light to produce organic material. When
did photosynthesis begin? Archaean rocks from western Australia that have been dated at 3.5 billion
years old contain organic material and fossils of early cyanobacteria, the first photosynthetic bacteria
(footnote 6). These simple organisms jump-started the oxygen revolution by producing the first traces
of free oxygen through photosynthesis: Knoll calls them “the working-class heroes of the Precambrian
Earth” (footnote 7).
Cyanobacteria are widely found in tidal flats, where the organic carbon that they produced was
buried, increasing atmospheric oxygen concentrations. Mats of cyanobacteria and other microbes
trapped and bound sediments, forming wavy structures called stromatolites (layered rocks) that mark
the presence of microbial colonies (Fig. 16).
Figure 16. Stromatolites at Hamelin Pool, Shark Bay, Australia
© National Aeronautics and Space Administration, JSC Astrobiology Institute.
Unit 1 : Many Planets, One Earth
The third domain of life, eukaryotes, are organisms with one or more complex cells. A eukaryotic cell
contains a nucleus surrounded by a membrane that holds the cell’s genetic material. Eukaryotic cells
also contain organelles—sub-components that carry out specialized functions such as assembling
proteins or digesting food. In plant and eukaryotic algae cells, chloroplasts carry out photosynthesis.
These organelles developed through a process called endosymbiosis in which cyanobacteria took
up residence inside host cells and carried out photosynthesis there. Mitochondria, the organelles
that conduct cellular respiration (converting energy into usable forms) in eukaryotic cells, are also
descended from cyanobacteria.
The first eukaryotic cells evolved sometime between 1.7 and 2.5 billion years ago, perhaps coincident
with the rise in atmospheric oxygen around 2.3 billion years ago. As the atmosphere and the oceans
became increasingly oxygenated, organisms that used oxygen spread and eventually came to
dominate Earth’s biosphere. Chemosynthetic organisms remained common but retreated into
sediments, swamps, and other anaerobic environments.
Throughout the Proterozoic era, from about 2.3 billion years ago until around 575 million years
ago, life on Earth was mostly single-celled and small. Earth’s biota consisted of bacteria, archaea,
and eukaryotic algae. Food webs began to develop, with amoebas feeding on bacteria and algae.
Earth’s land surfaces remained harsh and largely barren because the planet had not yet developed
a protective ozone layer (this screen formed later as free oxygen increased in the atmosphere), so
it was bombarded by intense ultraviolet radiation. However, even shallow ocean waters shielded
microorganisms from damaging solar rays, so most life at this time was aquatic.
As discussed in sections 5 and 6, global glaciations occurred around 2.3 billion years ago and
again around 600 million years ago. Many scientists have sought to determine whether there is a
connection between these episodes and the emergence of new life forms around the same times.
For example, one Snowball Earth episode about 635 million years ago is closely associated with the
emergence of multicellularity in microscopic animals (footnote 8). However, no causal relationship
has been proved.
8. The Cambrian Explosion and the Diversification of Animals
The first evidence of multicellular animals appears in fossils from the late Proterozoic era, about 575
million years ago, after the last snowball glaciation. These impressions were made by soft-bodied
organisms such as worms, jellyfish, sea pens, and polyps similar to modern sea anemones (Fig. 17).
In contrast to the microorganisms that dominated the Proterozoic era, many of these fossils are at
least several centimeters long, and some measure up to a meter across.
Unit 1 : Many Planets, One Earth
Figure 17. Fossils of Kimberella (thought to be a jellyfish)
Courtesy Wikimedia Commons. GNU Free Documentation License.
Shortly after this time, starting about 540 million years ago, something extraordinary happened: the
incredible diversification of complex life known as the Cambrian Explosion. Within 50 million years
every major animal phylum known in fossil records quickly appeared. The Cambrian Explosion can be
thought of as multicellular animals’ “big bang”—an incredible radiation of complexity.
What triggered the Cambrian Explosion? Scientists have pointed to many factors. For example, the
development of predation probably spurred the evolution of shells and armor, while the growing
complexity of ecological relationships created distinct roles for many sizes and types of organisms.
Rising atmospheric and oceanic oxygen levels promoted the development of larger animals, which
need more oxygen than small ones in order to move blood throughout their bodies. And some
scientists believe that a mass extinction at the end of the Proterozoic era created a favorable
environment for new life forms to evolve and spread.
Following the Cambrian Explosion, life diversified in several large jumps that took place over three
eras: Paleozoic, Mesozoic, and Cenozoic (referring back to Fig. 4, the Cambrian period was the
first slice of the Paleozoic era). Together these eras make up the Phanerozoic eon, a name derived
from the Greek for “visible life.” The Phanerozoic, which runs from 540 million years ago to the
present, has also been a tumultuous phase in the evolution of life on Earth, with mass extinctions at
the boundaries between each of its three geologic eras. Figure 18 shows the scale of historic mass
extinctions as reflected in marine fossil records.
Unit 1 : Many Planets, One Earth
Figure 18. Marine genus biodiversity
© Wikimedia Commons. Courtesy Dragons Flight. GNU Free Documentation License.
Early in the Paleozoic most of Earth’s fauna lived in the sea. Many Cambrian organisms developed
hard body parts like shells and bones, so fossil records became much more abundant and diverse.
The Burgess Shale, rock beds in British Columbia made famous in paleontologist Stephen Jay
Gould’s book Wonderful Life, are ancient reef beds in the Canadian Rockies of British Columbia that
are filled with fossil deposits from the mid-Cambrian period (footnote 9).
Land plants emerged between about 500 and 400 million years ago. Once established, they
stabilized soil against erosion and accelerated the weathering of rock by releasing chemicals from
their roots. Since faster weathering pulls increased amounts of carbon out of the atmosphere, plants
reduced the greenhouse effect and cooled Earth’s surface so dramatically that they are thought to
have helped cause several ice ages and mass extinctions during the late Devonian period, about 375
million years ago. By creating shade, they also provided habitat for the first amphibians to move from
water to land.
The most severe of all mass extinctions took place at the end of the Paleozoic era at the Permian/
Triassic boundary, wiping out an estimated 80 to 85 percent of all living species. Scientists still do
not understand what caused this crisis. Geologic records indicate that deep seas became anoxic,
which suggest that something interfered with normal ocean mixing, and that Earth’s climate suddenly
became much warmer and drier. Possible causes for these developments include massive volcanic
eruptions or a melting of methane hydrate deposits (huge reservoirs of solidified methane), both of
which could have sharply increased the greenhouse effect.
Unit 1 : Many Planets, One Earth
The Mesozoic era, spanning the Triassic, Jurassic, and Cretaceous periods, was the era of reptiles,
which colonized land and air more thoroughly than the amphibians that preceded them out of the
water. Dinosaurs evolved in the Triassic, about 215 million years ago, and became the largest and
most dominant animals on Earth for the next 150 million years. This period also saw the emergence
of modern land plants, including the first angiosperms (flowering plants); small mammals; and the
first birds, which evolved from dinosaurs. Figure 19 shows a model of a fossilized Archaeopteryx, a
transitional species from the Jurassic period with both avian and dinosaur features.
Figure 19. Model of Archaeopteryx fossil
© Wikimedia Commons. CeCILL license.
Another mass extinction at the end of the Mesozoic, 65 million years ago, killed all of the dinosaurs
except for birds, along with many other animals. For many years scientists thought that climate
change caused this extinction, but in 1980 physicist Louis Alvarez, his son, Walter, a geologist, and
other colleagues published a theory that a huge meteorite had hit Earth, causing impacts like shock
waves, severe atmospheric disturbances, and a global cloud of dust that would have drastically
cooled the planet. Their most important evidence was widespread deposits of iridium—a metal that is
Unit 1 : Many Planets, One Earth
extremely rare in Earth’s crust but that falls to Earth in meteorites—in sediments from the so-called KT (Cretaceous-Tertiary) boundary layer.
Further evidence discovered since 1980 supports the meteor theory, which is now widely accepted.
A crater has been identified at Chicxulub, in Mexico’s Yucatan peninsula, that could have been
caused by a meteorite big enough to supply the excess iridium, and grains of shocked quartz from the
Chicxulub region have been found in sediments thousands of kilometers from the site that date to the
K-T boundary era.
9. The Age of Mammals
The first mammals on Earth were rodent-sized animals that evolved in the shadow of dinosaurs
during the Jurassic and Triassic periods. After the K-T boundary extinction eliminated dinosaurs as
predators and competitors, mammals radiated widely. Most of the modern mammal orders, from
bats to large types like primates and whales, appeared within about 10 million years after dinosaurs
died out. Because mammals could maintain a relatively constant internal temperature in hot or cold
environments, they were able to adapt to temperature changes more readily than cold-blooded
animals like reptiles, amphibians, and fish. This characteristic helped them to populate a wide range
of environments.
Another important ecological shift was the spread of angiosperms (flowering plants), which
diversified and became the dominant form of land plants. Unlike earlier plants like ferns and conifers,
angiosperms’ seeds were enclosed within a structure (the flower) that protected developing embryos.
Their flower petals and fruits, which grew from plants’ fertilized ovaries, attracted animals, birds, and
insects that helped plants spread by redistributing pollen and seeds. These advantages enabled
angiosperms to spread into more diverse habitats than earlier types of plants.
Earth’s climate continued to fluctuate during the Cenozoic, posing challenges for these new life forms.
After an abrupt warming about 55 million years ago, the planet entered a pronounced cooling phase
that continued up to the modern era. One major cause was the ongoing breakup of Gondwanaland,
a supercontinent that contained most of the land masses in today’s southern hemisphere, including
Africa, South America, Australia, and India (Fig. 20).
Unit 1 : Many Planets, One Earth
Figure 20. Gondwanaland
© United States Geological Survey.
Once these fragments started to separate about 160 million years ago, ocean currents formed around
Antarctica. Water trapped in these currents circulated around the pole and became colder and colder.
As a result, Antarctica cooled and developed a permanent ice cover, which in turn cooled global
atmospheric and ocean temperatures. Climates became dryer, with grasslands and arid habitat
spreading into many regions that previously had been forested.
Continued cooling through the Oligocene and Miocene eras, from about 35 million to 5 million years
ago, culminated in our planet’s most recent ice age: a series of glacial advances and retreats during
the Pleistocene era, starting about 3.2 million years ago (Fig. 21). During the last glacial maximum,
about 20,000 years ago, ice sheets covered most of Canada and extended into what is now New
England and the upper Midwestern states.
Unit 1 : Many Planets, One Earth
Figure 21. Cenozoic cooling
© Global Warming Art. GNU License/www.globalwarmingart.com.
Human evolution occurred roughly in parallel with the modern ice age and was markedly influenced
by geologic and climate factors. Early hominids (members of the biological family of the great apes)
radiated from earlier apes in Africa between 5 and 8 million years ago. Humans’ closest ancestor,
Australopithecus, was shorter than modern man and is thought to have spent much of its time living
in trees. The human genus, Homo, which evolved about 2.5 million years ago, had a larger brain,
used hand tools, and ate a diet heavier in meat than Australopithecus. In sum, Homo was better
adapted for life on the ground in a cooler, drier climate where forests were contracting and grasslands
were expanding.
By 1.9 million years ago, Homo erectus had migrated from Africa to China and Eurasia, perhaps
driven partly by climate shifts and resulting changes to local environments. Homo sapiens, the
modern human species, is believed to have evolved in Africa about 200,000 years ago. Homo
sapiens gradually migrated outward from Africa, following dry land migration routes that were
exposed as sea levels fell during glacial expansions. By about 40,000 years ago Homo sapiens had
settled Europe, and around 10,000 years ago man reached North America. Today, archaeologists,
anthropologists, and geneticists are working to develop more precise maps and histories of the
human migration out of Africa, using mitochondrial DNA (maternally inherited genetic material) to
assess when various areas were settled.
Unit 1 : Many Planets, One Earth
Early in their history, humans found ways to manipulate and affect their environment. Mass
extinctions of large mammals, such as mammoths and saber-toothed cats, occurred in North
and South America, Europe, and Australia roughly when humans arrived in these areas. Some
researchers believe that over-hunting, alone or in combination with climate change, may have been
the cause. After humans depleted wildlife, they went on to domesticate animals, clear forests, and
develop agriculture, with steadily expanding impacts on their surroundings that are addressed in units
5 through 12 of this text.
10. Further Reading
University of California Museum of Paleontology, Web Geological Time Machine, http://
www.ucmp.berkeley.edu/help/timeform.html. An era-by-era guide through geologic time using
stratigraphic and fossil records.
Science Education Resource Center, Carleton College, “Microbial Life in Extreme Environments,”
http://serc.carleton.edu/microbelife/extreme/index.html. An online compendium of information about
extreme environments and the microbes that live in them.
James Shreve, “Human Journey, Human Origins,” National Geographic, March
2006, http://www7.nationalgeographic.com/ngm/0603/feature2/index.html?
fs=www3.nationalgeographic.com&fs=plasma.nationalgeographic.com. An overview of what DNA
evidence tells us about human migration out of Africa, with additional online resources.
1. American Museum of Natural History, “Our Dynamic Planet: Rock Around the Clock,” http://
2. Peter D. Ward and Donald Brownlee, Rare Earth: Why Complex Life Is Uncommon in the
Universe (New York: Springer-Verlag, 2000).
3. U.S. Geological Survey, “Radiometric Time Scale,” http://pubs.usgs.gov/gip/geotime/
radiometric.html, and “The Age of the Earth,” http://geology.wr.usgs.gov/parks/gtime/ageofearth.html.
4. Paul F. Hoffman and Daniel P. Schrag, “Snowball Earth,” Scientific American, January 2000, pp.
5. Andrew Knoll, Life on a Young Planet: The First Three Billion Years of Evolution on Earth
(Princeton University Press, 2003), p. 23.
6. University of California Museum of Paleontology, “Cyanobacteria: Fossil Record,” http://
7. Knoll, Life on a Young Planet, p. 42.
Unit 1 : Many Planets, One Earth
8. “Did the Snowball Earth Kick-Start Complex Life?”, http://www.snowballearth.org/kick-start.html.
9. Stephen Jay Gould, Wonderful Life: The Burgess Shale and the Nature of History (New York:
Norton, 1990).
albedo : The fraction of electromagnetic radiation reflected after striking a surface.
archaea : A major division of microorganisms. Like bacteria, Archaea are single-celled organisms
lacking nuclei and are therefore prokaryotes, classified as belonging to kingdom Monera in the
traditional five-kingdom taxonomy.
bacteria : Microscopic organisms whose single cells have neither a membrane-bounded nucleus nor
other membrane-bounded organelles like mitochondria and chloroplasts.
Cambrian explosion : Between about 570 and 530 million years ago, when a burst of diversification
occurred, with the eventual appearance of the lineages of almost all animals living today.
cation : An ion with a positive charge.
cyanobacteria : A phylum of Bacteria that obtain their energy through photosynthesis. They are often
referred to as blue-green algae, although they are in fact prokaryotes, not algae.
eukaryotes : A single-celled or multicellular organism whose cells contain a distinct membrane-bound
extremophiles : Microorganisms belonging to the domains Bacteria and Archaea that can live and
thrive in environments with extreme conditions such as high or low temperatures and pH levels, high
salt concentrations, and high pressure.
geochemical cycling : Flows of chemical substances between reservoirs in Earth’s atmosphere,
hydrosphere (water bodies), and lithosphere (the solid part of Earth’s crust).
heterotrophs : An organism that requires organic substrates to get its carbon for growth and
negative feedback : When part of a system’s output, inverted, feeds into the system’s input; generally
with the result that fluctuations are weakened.
oxidation : An array of reactions involving several different types of chemical conversions: (1) loss of
electrons by a chemical, (2) combination of oxygen and another chemical, (3) removal of hydrogen
atoms from organic compounds during biological metabolism, (4) burning of some material, (5)
biological metabolism that results in the decomposition of organic material, (6) metabolic conversions
in toxic materials in biological organism, (7) stabilization of organic pollutants during wastewater
treatment, (8) conversion of plant matter to compost, (9) decomposition of pollutants or toxins that
contaminate the environment.
Unit 1 : Many Planets, One Earth
phylum : The largest generally accepted groupings of animals and other living things with certain
evolutionary traits.
plate tectonics : A concept stating that the crust of the Earth is composed of crustal plates moving on
the molten material below.
prokaryotes : Organisms without a cell nucleus, or any other membrane-bound organelles. Most are
unicellular, but some prokaryotes are multicellular. The prokaryotes are divided into two domains: the
bacteria and the archaea.
radiometric dating : A technique used to date materials based on a knowledge of the decay rates
of naturally occurring isotopes, and the current abundances. It is the principal source of information
about the age of the Earth and a significant source of information about rates of evolutionary change.
Snowball Earth : Hypothesis that proposes that the Earth was entirely covered by ice in part of the
Cryogenian period of the Proterozoic eon, and perhaps at other times in the history of Earth
stratigraphic record : Sequences of rock layers. Correlating the sequences of rock layers in different
areas enables scientists to trace a particular geologic event to a particular period.
subduction : The process in which one plate is pushed downward beneath another plate into the
underlying mantle when plates move towards each other.
Unit 1 : Many Planets, One Earth
Unit 2 : Atmosphere
The atmosphere is a critical system that helps to regulate
Earth’s climate and distribute heat around the globe. In
this unit, discover the fundamental processes that cause
atmospheric circulation and create climate zones and
weather patterns, and learn how carbon cycling between
atmosphere, land, and ocean reservoirs helps to regulate
Earth’s climate.
Utah sky.
1. Introduction
2. The Structure of the Atmosphere
3. Radiative Balance and the Natural Greenhouse Effect
4. Major Greenhouse Gases
5. Vertical Motion in the Atmosphere
6. Atmospheric Circulation Patterns
7. Climate, Weather, and Storms
8. The Global Carbon Cycle
9. Feedbacks in the Atmosphere
10. Further Reading
Unit 2 : Atmosphere
1. Introduction
Earth’s atmosphere is a critical system for life on our planet. Together with the oceans, the
atmosphere shapes Earth’s climate and weather patterns and makes some regions more habitable
than others. But Earth’s climate is not static. How variable is it, and how quickly does it change? What
physical factors control climate, and how do they interact with one another?
To see how and why climate fluctuates, we need to learn about the basic characteristics of the
atmosphere and some physical concepts that help us understand weather and climate. This unit
describes the structure of the atmosphere and examines some of its key functions, including
screening out harmful solar radiation, warming Earth through the natural greenhouse effect, and
cycling carbon. It then summarizes how physical processes shape the distributions of pressures and
temperatures on Earth to create climate zones, weather patterns, and storms, creating conditions
suitable for life around the planet.
The atmosphere is a complex system in which physical and chemical reactions are constantly taking
place. Many atmospheric processes take place in a state of dynamic balance—for example, there
is an average balance between the heat input to, and output from, the atmosphere. This condition
is akin to a leaky bucket sitting under a faucet: when the tap is turned on and water flows into the
bucket, the water level will rise toward a steady state where inflow from the tap equals outflow
through the leaks. Once this condition is attained, the water level will remain steady even though
water is constantly flowing in and out of the bucket.
Similarly, Earth’s climate system maintains a dynamic balance between solar energy entering
and radiant energy leaving the atmosphere. Levels of oxygen in the atmosphere are regulated
by a dynamic balance in the natural carbon cycle between processes that emit oxygen through
photosynthesis and others that consume oxygen, such as respiration. The strength of atmospheric
circulation is also controlled by a dynamic balance. Some parts of the planet receive more energy
from the sun than others, and this uneven heating creates wind motions that act to move heat from
warm to cold regions. (The process by which differential heating triggers atmospheric motion is
discussed below in Section 5, “Vertical Motion in the Atmosphere.”)
Today human actions are altering key dynamic balances in the atmosphere. Most importantly,
humans are increasing greenhouse gas levels in the troposphere, which raises Earth’s surface
temperature by increasing the amount of heat radiated from the atmosphere back to the ground.
The broad impacts of global warming are discussed in Unit 12, “Earth’s Changing Climate,” but it
should be noted here that climate change will alter factors that are key determinants of environmental
conditions upon which ecosystems depend. As the following sections will show, changing global
surface temperatures and precipitation patterns will have major impacts on Earth’s climate and
Unit 2 : Atmosphere
2. The Structure of the Atmosphere
The atmosphere is composed of nitrogen, oxygen, argon, water vapor, and a number of trace gases
(Table 1). This composition has remained relatively constant throughout much of Earth’s history.
Chemical reactions maintain the ratios of major constituents of the atmosphere to each other. For
example, oxygen is released into the atmosphere by photosynthesis and consumed by respiration.
The concentration of oxygen in the atmosphere is maintained by a balance between these two
Photosynthesis: CO2 + H2O + light → CH2O” + O2
Respiration: CH2O + O2 → CO2 + H2O + energy
“CH2O” denotes the average composition of organic matter.
Many gases play critical roles in the atmosphere even though they are present in relatively low
concentrations. Important substances that will receive further attention throughout this text include
tropospheric ozone, which is addressed in Unit 11, “Atmospheric Pollution,” and greenhouse gases
(CO2, methane, N2O, and chlorofluorocarbons), which are discussed in Section 4 of this unit and in
Unit 12, “Earth’s Changing Climate.”
Table 1. Atmospheric gas composition (average). Concentrations of gases shown in
color are rising due to human activities.
Nitrogen (N)
Oxygen (O)
Water (HO)
Argon (Ar)
Carbon Dioxide (CO2)
Mole fraction
0.04 to < 5x10 ; 4x10 — strat 0.0093 -6 370x10 (date: 2000) Neon (Ne) 18.2x10 Ozone (O3) 0.02x10 to 10x10 Helium (He) 5.2x10 Methane (CH4) 1.7x10 Krypton (Kr) 1.1x10 Hydrogen (H) 0.55x10 Unit 2 : Atmosphere -6 -6 -6 -6 -6 -6 -3- -6 www.learner.org Gas Mole fraction -6 Nitrous Oxide (N2O) 0.32x10 Carbon Monoxide (CO) 0.03x10 to 0.3x10 Chlorofluorocarbons 3.0x10 Carbonyl Sulfide (COS) 0.1x10 -6 -6 -9 -9 Earth's atmosphere extends more than 560 kilometers (348 miles) above the planet's surface and is divided into four layers, each of which has distinct thermal, chemical, and physical properties (Fig. 1). Figure 1. Structure of the atmosphere © 2006. Steven C. Wofsy, Abbott Lawrence Rotch Professor of Atmospheric and Environmental Science, lecture notes. Almost all weather occurs in the troposphere, the lowest layer of the atmosphere, which extends from the surface up to 8 to 16 kilometers above Earth's surface (lowest toward the poles, highest in the tropics). Earth's surface captures solar radiation and warms the troposphere from below, creating rising air currents that generate vertical mixing patterns and weather systems, as detailed further below. Temperatures decrease by about 6.5°C with each kilometer of altitude. At the top Unit 2 : Atmosphere -4- www.learner.org of the troposphere is the tropopause, a layer of cold air (about -60°C), which forms the top of the troposphere and creates a "cold trap" that causes atmospheric water vapor to condense. The next atmospheric layer, the stratosphere, extends upward from the tropopause to 50 kilometers. In the stratosphere temperatures increase with altitude because of absorption of sunlight by stratospheric ozone. (About 90 percent of the ozone in the atmosphere is found in the stratosphere.) The stratosphere contains only a small amount of water vapor (only about one percent of total atmospheric water vapor) due to the "cold trap" and the tropopause, and vertical air motion in this layer is very slow. The stratopause, where temperatures peak at about -3°C, marks the top of the stratosphere. In the third atmospheric layer, the mesosphere, temperatures once again fall with increasing altitude, to a low of about -93°C at an altitude of 85 kilometers. Above this level, in the thermosphere, temperatures again warm with altitude, rising higher than 1700°C. The atmosphere exerts pressure at the surface equal to the weight of the overlying air. Figure 1 also shows that atmospheric pressure declines exponentially with altitude—a fact familiar to everyone who has felt pressure changes in their ears while flying in an airplane or climbed a mountain and struggled to breathe at high levels. At sea level, average atmospheric pressure is 1013 millibars, corresponding to a mass of 10,000 kg (10 tons) per square meter or a weight of 100,000 Newtons per square meter (14.7 pounds per square inch) for a column of air from the surface to the top of the atmosphere. Pressure falls with increasing altitude because the weight of the overlying air decreases. It falls exponentially because air is compressible, so most of the mass of the atmosphere is compressed into its lowest layers. About half of the mass of the atmosphere lies in the lowest 5.5 kilometers (the summit of Mt. Everest at 8850 m extends above about roughly two-thirds of the atmosphere), and 99 percent is within the lowest 30 kilometers. 3. Radiative Balance and the Natural Greenhouse Effect Earth's surface temperature has been remarkably constant over geologic time. Even the dramatic cooling that occurred during the most recent ice age represented a change of only 3°C in the global average surface temperature, occurring over thousands of years. Seasonal changes in temperature, although large in a particular place, correspond to very tiny changes in global mean temperature. Why have temperatures held so steady? Earth exchanges energy with its environment primarily through transfers of electromagnetic radiation. At any time our planet is simultaneously absorbing energy from the sun and radiating energy back into space. The temperature remains stable over long periods of time because the planet radiates energy back to space at a rate that closely balances the energy input it receives from the sun (i.e., the planet is close to being in radiative energy balance). Earth receives energy from the sun in the form of solar radiation—radiation with varying wavelengths along the electromagnetic spectrum. The sun emits strongly in the visible light range, but it also Unit 2 : Atmosphere -5- www.learner.org produces ultraviolet and infrared radiation. The earth radiates heat back to space mostly at much longer wavelengths than solar radiation (Fig. 2). Figure 2. The electromagnetic spectrum © Yochanan Kushnir. When visible solar radiation reaches Earth, it may be absorbed by clouds, the atmosphere, or the planet's surface. Once absorbed it is transformed into heat energy, which raises Earth's surface temperature. However, not all solar radiation intercepted by the Earth is absorbed. The fraction of incoming solar radiation that is reflected back to space constitutes Earth's albedo, as shown below in Figure 3. Unit 2 : Atmosphere -6- www.learner.org Figure 3. Earth-atmosphere energy balance Any form of matter emits radiation if its temperature is above absolute zero (zero degrees Kelvin). Incoming solar radiation warms Earth, and the planet emits infrared radiation back to outer space. Note that Earth emits radiation at a longer wavelength—i.e., a lower energy level—than the sun (Fig. 2). This difference occurs because the total energy flux from an object varies with the fourth power of the object's absolute temperature, and the sun is much hotter than the Earth. Some outgoing infrared energy emitted from the Earth is trapped in the atmosphere and prevented from escaping to space, through a natural process called the "greenhouse effect." The most abundant gases in the atmosphere—nitrogen, oxygen, and argon—neither absorb nor emit terrestrial or solar radiation. But clouds, water vapor, and some relatively rare greenhouse gases (GHGs) such as carbon dioxide, methane, and nitrous oxide in the atmosphere can absorb long-wave radiation (terrestrial radiation, see Figure 2). Molecules that can absorb radiation of a particular wavelength can also emit that radiation, so GHGs in the atmosphere therefore will radiate energy both to space and back towards Earth. This back-radiation warms the planet's surface. In Figure 3, 100 units of solar radiation are intercepted by the Earth each second. On average 30 units are reflected, 5 by the surface and 25 by clouds. Energy balance is achieved by Earth's emission of 70 units of infrared ("terrestrial") radiation to space. The earth's surface is warmed directly by only 45 units of solar energy, with almost twice as much energy (88 units) received from thermal radiation due to greenhouse gases and clouds in the atmosphere. Energy is removed from the Unit 2 : Atmosphere -7- www.learner.org surface by radiation of infrared energy back to the atmosphere and space (88 units) and by other processes such as evaporation of water and direct heat transfer (29 units). Note that the amount of heat received by the surface is actually much larger (3x) than the amount the surface receives in solar radiation, due to the natural greenhouse effect. The result is a surface temperature on average around 15°C (60°F), as compared to temperatures colder than âÂ#Â#18°C (0°F) if there were no greenhouse effect. 4. Major Greenhouse Gases Many GHGs, including water vapor (the most important), ozone, carbon dioxide, methane, and nitrous oxide, are naturally present in the atmosphere. Other GHGs are synthetic chemicals that are emitted only as a result of human activity. Anthropogenic (human) activities are significantly increasing atmospheric concentrations of many GHGs. • Carbon dioxide (CO2), the most significant GHG directly affected by anthropogenic activity, is the product of the oxidation of carbon in organic matter, either through combustion of carbon-based fuels or the decay of biomass. Natural CO sources include volcanic eruptions, respiration of organic matter in natural ecosystems, natural fires, and exchange of dissolved CO with the oceans. The main anthropogenic sources are (a) fossil fuel combustion and (b) deforestation and land use changes (such as converting agricultural land or forests to urban development), which release stored organic matter and reduce the ability of natural ecosystems to store carbon. • Methane (CH4) is produced by anaerobic decay of organic material in landfills, wetlands, and rice fields; enteric fermentation in the digestive tracts of ruminant animals such as cattle, goats, and sheep; manure management; wastewater treatment; fossil fuel combustion; and leaks from natural gas transportation and distribution systems and abandoned coal mines. • Nitrous oxide (N2O) is produced by fertilizer use, animal waste management, fossil fuel combustion, and industrial activities. Unit 2 : Atmosphere -8- www.learner.org • Hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) are synthetic chemicals that are used in a variety of industrial production processes such as semiconductor manufacturing. PFCs are also produced as a by-product of aluminum smelting. Both groups of chemicals are finding increasing use as substitutes for ozone-depleting chlorofluorocarbons (CFCs), which are being phased out under the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer. HFCs and PFCs are replacing CFCs in applications such as refrigeration and foam-blowing for insulation. When atmospheric GHG concentrations increase, Earth temporarily traps infrared radiation more efficiently, so the natural radiative balance is disturbed until its surface temperature rises to restore equilibrium between incoming and outgoing radiation. It takes many decades for the full effect of greenhouse gases to be realized in higher surface temperatures, because the oceans have a huge capacity to store heat. They must be gradually warmed by excess infrared radiation from the atmosphere. Figure 4 illustrates the relative contributions from man-made emissions of various GHGs to climate change. Unit 2 : Atmosphere -9- www.learner.org Figure 4. Importance of human-produced greenhouse gases Courtesy Marian Koshland Science Museum of the National Academy of Sciences http:// www.koshland-science-museum.org. As we will see in section 8, "The Global Carbon Cycle," CO2 emitted from combustion of fossil fuel cycles between the atmosphere and land and ocean "sinks" (carbon storage reservoirs), which are absorbing a large fraction of anthropogenic carbon emissions. Ultimately, though, there are limits to the amount of carbon that these sinks can absorb. These sinks are more likely to delay than to prevent human actions from altering Earth's radiative balance. Higher surface temperatures on Earth will have profound impacts on our planet's weather and climate. Before we consider those impacts, however, we need to understand how variables such as pressure, temperature, and moisture combine to create air currents, drive normal atmospheric circulation patterns, and create the overall climate. Unit 2 : Atmosphere -10- www.learner.org 5. Vertical Motion in the Atmosphere To see how atmospheric motion generates climate and weather systems, it is useful to start by picturing an air parcel—a block of air with uniform temperature and pressure throughout. The air parcel can change over time by rising, falling, or emitting or absorbing heat. If the pressure of the surrounding environment changes, the pressure of the parcel changes, but it does not exchange heat or chemicals with the surroundings and therefore may behave differently from its surroundings. Many weather patterns start with rising air. According to the Perfect Gas Law, if an air parcel stays at a constant pressure, increasing its temperature will cause it to expand and become less dense. Contrary to popular belief, however, air does not rise simply because it is warm. Rather, air that becomes warmer than its surroundings—for example, if it is warmed by heat radiating from the Earth's surface—becomes buoyant and floats on top of cooler, denser air in the same way that oil floats on top of water. This buoyant motion is called convection. Water is also a key factor in weather and climate. As shown in Table 1, two to three percent of the atmosphere typically consists of water vapor. Weather forecasts often report the relative humidity, which compares the amount of water vapor in the air to the maximum amount that air can hold at that temperature. When relative humidity reaches 100 percent, the air has reached saturation pressure and cannot absorb any more water vapor. (High humidity levels make people feel uncomfortable because little or none of their sweat can evaporate and draw heat from their skin.) As air warms, the amount of water vapor that it can hold rises exponentially. Consequently, atmospheric water vapor concentrations are highest in warm regions and decrease toward the poles (Fig. 5). Unit 2 : Atmosphere -11- www.learner.org Figure 5. Mean distribution of atmospheric water vapor above Earth's surface, 1988– 1999 Courtesy Cooperative Institute for Research in the Atmosphere, Colorado State University. Atmospheric water vapor contributes to weather patterns in several ways. First, adding water vapor to the air reduces its density, so adding moisture to dry air may make it become buoyant and rise. Secondly, moist air carries latent energy, the potential for condensation of water vapor to heat the air. Liquid water absorbs energy when it evaporates, so when this water vapor condenses, energy is released and warms the surrounding environment. (As we will see in the next section, thunderstorms and hurricanes draw energy from the release of latent heat.) The dew point, another key weather variable, denotes the temperature to which air would have to cool to reach 100 percent relative humidity. When an air parcel cools to its dew point, water vapor begins condensing and forming cloud droplets or ice crystals, which may ultimately grow large enough to fall as rain or snow. When a rising air parcel expands it pushes away the surrounding atmosphere, and in doing this work it expends energy. If heat is not added or removed as this hypothetical parcel moves—a scenario called an adiabatic process—the only source of energy is the motion of molecules in the air parcel, and therefore the parcel will cool as it rises. (Recall from Figure 1 that that in the troposphere, temperature falls 6.5°C on average with each kilometer of altitude. The actual decrease under realworld conditions, which may vary from region to region, is called the atmospheric lapse rate.) Unit 2 : Atmosphere -12- www.learner.org A dry air parcel (one whose relative humidity is less than 100 percent) cools by 9.8°C for each thousand meters that it rises, a constant decrease called the dry adiabatic lapse rate. However, if the parcel cools enough that its relative humidity reaches 100 percent, water starts to condense and form cloud droplets. This condensation process releases latent heat into the parcel, so the parcel cools at a lower rate as it moves upward, called the moist adiabatic lapse rate. Atmospheric conditions can be stable or unstable, depending on how quickly the temperature of the environment declines with altitude. An unstable atmosphere is more likely to produce clouds and storms than a stable atmosphere. If atmospheric temperature decreases with altitude faster than the dry adiabatic lapse rate (i.e., by more than 9.8°C per kilometer), the atmosphere is unstable: rising air masses will be warmer and less dense than the surrounding air, so they experience buoyancy and will continue to rise and form clouds that can generate storms. If temperature falls more gradually with altitude than the dry adiabatic lapse rate but more steeply than the wet adiabatic lapse rate, the atmosphere is conditionally unstable. In this case, air masses may rise and form clouds if they contain enough water vapor to warm them as they expand (Fig. 6), but they have to get a fairly strong push upwards to start the condensation process (up to 4000 meters in the figure). If temperature falls with altitude more slowly than the moist adiabatic lapse rate, the atmosphere is stable: rising air masses will become cooler and denser than the surrounding atmosphere and sink back down to where they started. Figure 6. Conditional instability Unit 2 : Atmosphere -13- www.learner.org Convection is not the only process that lifts air from lower to higher altitudes. When winds run into mountains and are forced upward the air cools, often forming clouds over windward slopes and the crests of hills. Convergence occurs when air masses run together, pushing air upward, as happens often in the tropics and in warm summer conditions in midlatitudes, generating thunderstorms. And when warm and cold air fronts collide, the denser cold air slides underneath the warm air layer and lifts it. In each case, if warm air is lifted high enough to reach its dew point, clouds will form. If lifting forces are strong, the system will produce tall, towering clouds that can generate intense rain or snow storms. As discussed in section 9, "Feedbacks in the Atmosphere," clouds are important factors in Earth's energy balance. Their net impact is hard to measure and model because different types of clouds have different impacts on climate. Low-altitude clouds emit and absorb infrared radiation much as the ground does, so they are roughly the same temperature as Earth's surface and thus do not increase atmospheric temperatures. However, they have a cooling effect because they reflect a portion of incoming solar radiation back into space, increasing Earth's albedo and reducing the total input of solar energy to the planet's surface. In contrast, high-altitude clouds tend to be thinner, so they do not reflect significant levels of incoming solar radiation. However, since they reside in a higher, cooler area of the atmosphere, they efficiently absorb outgoing thermal radiation and warm the atmosphere, and they radiate heat back to the surface from a part of the atmosphere that would otherwise not contribute to the greenhouse effect. 6. Atmospheric Circulation Patterns Atmospheric circulation is set up when mass moves in the atmosphere. This motion may be vertical, as when warm air rises and becomes buoyant. It can also be horizontal: wind is created by air moving from high pressure areas, where air is densely compressed, to low-pressure areas, where air is less dense, although horizontal winds follow curved trajectories due to the rotation of the earth (see below). Atmospheric forces cause the air to move, modifying the difference in pressure. On a weather map, pressure differences are demarcated by parallel lines called isobars that show changes in pressure, usually in increments of 2 to 4 millibars. Sea breezes show how vertical and horizontal movements combine to modify temperature and pressure at a local level. During the day coastal land regions heat up more than the sea because land warms more quickly than water. Air over the land is thus warmed and rises, increasing pressure in the atmosphere above the surface, where it starts to cool and form clouds. It then flows at altitude from the area of high pressure over land to lower pressure over the sea. Because there is then less mass over the land and more over the sea, pressure at the surface is higher at sea, so air flows in from the sea to the land. At night, when land cools more quickly than the ocean, the cycle is reversed (Fig. 7). Unit 2 : Atmosphere -14- www.learner.org Figure 7. Sea breeze Adapted from graphic by National Oceanic Atmospheric Administration, Jet Stream. The sea breezes in this example flow directly between two points, but many larger weather systems follow less-direct courses. Their paths are not random, however. Winds that move over very long distances appear to curve because of the Coriolis force, an apparent force caused by Earth's rotation. This phenomenon occurs because all points on the planet's surface rotate once around Earth's axis every 24 hours, but different points move at different speeds: air at a point on the equator rotates at 1,700 kilometers per hour, compared to 850 kilometers per hour for a point that lies at 60 degrees latitude, closer to Earth's spin axis. Because Earth spins, objects on its surface have angular momentum, or energy of motion, which defines how a rotating object moves around a reference point. An object's angular momentum is the product of its mass, its velocity, and its distance from the reference point (its radius). Angular momentum is conserved as an object moves on the Earth, so if its radius of spin decreases (as it moves from low latitude to high latitude), its velocity must increase. This relationship is what makes figure skaters rotate faster when they pull their arms in close to their bodies during spins. The same process affects a parcel of air moving north from the Equator toward the pole: its radius of spin around Earth decreases as it moves closer to Earth's axis of rotation, so its rate of spin increases. The parcel's angular velocity is greater than the angular velocity of Earth's surface at the higher latitude, so it deflects to the right of its original trajectory relative to the planet's surface (Fig. 8). In the Southern hemisphere, the parcel would appear to deflect to the left. Unit 2 : Atmosphere -15- www.learner.org Figure 8. Coriolis force © 2006. Steven C. Wofsy, Abbott Lawrence Rotch Professor of Atmospheric and Environmental Science, lecture notes. This effect was discovered by French scientist Gustave-Gaspard Coriolis, who sought to explain why shots fired from long-range cannons were falling wide to the right of their targets. The Coriolis force only affects masses that travel over long distances, so it is not apparent in local weather patterns such as sea breezes. Nor, contrary to an oft-repeated misbelief, does it make water draining from a sink or toilet rotate in one direction in the Northern Hemisphere and the other direction in the Southern Hemisphere. But the Coriolis force makes winds appear to blow almost parallel to isobars, rather than directly across them from high to low pressure. The Coriolis force makes the winds in low-pressure weather systems such as hurricanes rotate (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere), curving into spirals. Air initially starts to move through the atmosphere under the influence of pressure gradients that push it from high pressure to low pressure areas. As it travels, the Coriolis force starts to bend its course. The motion tends toward a state called geostrophic flow, where the pressure gradient force and the Coriolis force exactly balance each other. At this point the air parcel is no longer moving from a high-pressure to a low-pressure zone. Instead, it follows a course parallel to the isobars. In Figure 9, the air parcel is in geostrophic flow at point A3. Unit 2 : Atmosphere -16- www.learner.org Figure 9. Geostrophic flow © 2006. Steven C. Wofsy, Abbott Lawrence Rotch Professor of Atmospheric and Environmental Science, lecture notes. When a low-pressure region develops in... Purchase answer to see full attachment

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