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Lessons from the Geologic Past
Conspicuous though we now are, Homo sapiens have
emerged only within the last few hundreds of thousands of
years, a flash of an instant in the planet's 5-billion-year history.
The results of our actions are merely superimposed on natural
variations in the earth's network of oceanic, atmospheric, and
biological systems.
As scientists try to predict the future global environment,
they constantly confront an enormous obstacle: incomplete un-
derstanding of how physical, chemical, and biological processes
affect each other and shape the planet today. What is known
about the past, however, demonstrates that climate and the
fortunes of earth's inhabitants have been intertwined since life
began on earth, and that relatively small changes can have large
and unexpected consequences.
Michael McElroy, chairman of the Department of Earth and
Planetary Sciences at Harvard University, notes that if earth
scientists could work in the laboratory, "We would have hy-
potheses, we would do experiments, we would manipulate the
experiment and we would learn about processes by the conven-
tional iteration of theory and experiment. But as geophysicists
20
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LESSONS FROM THE GEOLOGIC PAST
21
or geochemists, we do not have that luxury. We cannot manip-
ulate the earth. Our laboratory is the past. If we imaginatively
attempt to understand the changes that took place, in the long
term we will have our best chance to predict and to guide our
future." In particular, several phases in earth's history shed light
on what may be in store with future changes in climate.
FORMATION OF THE OZONE SHIELD
Our lessons from the past begin with a development that
occurred more than ~ billion years ago. Early aquatic organ-
isms, blue-green algae, began to use energy from the sun to
split molecules of water and carbon dioxide and then recom-
b~ne them into organic compounds and oxygen the process
known as photosynthesis. Oxygen was used up as organic car-
bon was converted to carbon dioxide, but not all of it. In a
fateful development, oxygen-poisonous to organisms then-
began to accumulate in the atmosphere, touching off a massive
ecological disaster for primordial organisms. As oxygen built
up, the carbon dioxide content began to cirop. High in the at-
mosphere, some molecules of oxygen (02) were split as they
absorbed energy from ultraviolet rays and formed single atoms
of oxygen. When these recombined with oxygen, ozone (03)
molecules formed, which are very effective absorbers of ultra-
violet rays emitted by the sun. The ozone formed a protective
shield around the earth, eliminating the threat of irradiation by
ultraviolet light. With this development, the land was fit for
more complex life. The organisms that first began to cast off
the oxygen did not survive the switch to an oxygenated atmo-
sphere, but some others did, and made the pivotal transition
from water to land.
PANGAEA
A paroxysmal change that occurred about 300 million years
ago can help us understand more about our climate today. At
that time, when the age of dinosaurs was just beginning, the
movement of the earth's crustal plates caused the two major
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22
THE EARTH AS A SYSTEM
PANGAEA
1 Do Myr
Locations of continents for various times during me past several hundred million
years. (Adapted from N. Calder. 1983. Timescale: An Atlas of the Fourth Dimension.
Viking Press, New York.)
continents at the time Laurasia in the north and Gondwana-
land in the south to mass together for one unique, relatively
brief period of less than 100 million years into a megacontinent
called Pangaea, or "all lands."
What was it like on Pangaea? The earth in the age of Pan-
gaea, of course, was very different from the earth today, and
from the earth in the preceding and following geologic times,
when the continents were not so closely joined. Nevertheless,
scientists use this ancient natural experiment to help understand
how the distribution of lands and oceans affects climate.
John E. Kutzbach and Robert G. Gallunore, both of the
Center for Climatic Research at the University of Wisconsin at
Madison, are using a general circulation climate mocle! to cal-
culate the climate of Pangaea. Results from their model, along
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LESSONS FROM THE GEOLOGIC PAST
23
with evidence from the fossil and geologic record, support ear-
lier speculations that the megacontinent was beset by large-scale
monsoon conditions in both summer and winter and that these
seasons were typified, respectively, by extreme heat and cold.
The continental interior was hot and dry, with monthly average
temperatures in summer of 35°C, or well over 100°F. (Since these
were average temperatures, many days were probably as warm
as almost 50°C, or 120°F.) it was relatively humid in the polar
regions and along the coasts of the vast, continental-scale em-
bayment called the Tethys Ocean, where monsoon winds were
strong; the tropics, except along the coasts, were dry. These
computer simulations of the climate are supported by geologic
evidence of the location of plant and animal fossils and mineral
deposits; the degree of agreement between model and geologic
data is an indication of scientists' growing understanding of
how the climate system works. Although the Pangaean world
bore little semblance to ours, the global average temperature
was only about 5°C (or 9°F) higher than at present.
Before Pangaea formed, the diversity and abundance of life
on our planet rose, but as the continents converged into a sin-
gle mass, the greatest extinction of all time occurred. By some
estimates, more than half of all families and three quarters of all
species became extinct. Scientists believe that perhaps this great
dying between the Permian and Triassic periods was somehow
related to the marked changes in climate that accompanied the
development of Pangaea. One possibility is that most organisms
could not adapt to the extreme fluctuations in temperature and
moisture between summer and winter. Today, only Siberia and
northern Canada experience as wide a range of seasonal vari-
ation. Other possible explanations abound: Did the Pangaean
world have too few unique habitats? Did some sort of catastro-
phe occur? Did changes in the ocean currents, temperature, or
salinity disrupt the global climate or the chemical and biological
balances? Or did the atmospheric concentrations of oxygen and
carbon dioxide perhaps create conditions that the plants and
organisms living then simply could not tolerate?
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24
THE EARTH AS A SYSTEM
THE ICE AGES
Another significant and telling event in the earth's history
was the long slide of climate from warm to cold, beginning
about 100 million years ago, when the climate was still much
warmer than present and the level of carbon dioxide may have
been 10 times greater than it is today. What natural processes
could have produced such a high level of carbon dioxide in the
earth's atmosphere? It is possible that widespread volcanism
had infused the atmosphere with carbon dioxide. The continents
dispersed as Pangaea broke apart, and volcanism may have
been much more active than today along the mid-ocean ridges
where the seafloor was forming and on the continental margins
where the seafloor was being subducted. As continents drifted
toward their current locations, the seafloor spread apart at a
slower rate. Many researchers believe that, consequently, there
was less volcanic activity and related carbon dioxide emissions,
which led to a cooler climate.
This cooling trend that began almost 100 million years ago,
coupled with continued movements of continental plates, led to
the growth of huge ice sheets on Antarctica and Greenland. Co-
incidentally, the North American plate buckled and the Rocky
Mountains began to rise. Halfway around the world, the Indian
continent collided with the Eurasian continent, giving rise to the
Tibetan Plateau, a process still under way. This phase of moun-
tain building may have contributed to conditions conducive to
further glaciation of North America and Eurasia in the past
million to 2 million years.
To earth scientists, ice ages are in many ways the flip side of
the warming that may be in store for us now. In the past decade,
great progress has been made toward understanding the cause
of the glacial cycles during the most recent ~ million to 2 million
years of earth history. While researchers have attempted to
explain the causes of ice ages for more than a century, recently
developed scientific tools are yielding major new findings.
Some of the most important new information comes from
cores carefully extracted by drilling deep into the ice caps of
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LESSONS FROM THE GEOLOGIC PAST
25
Greenland and Antarctica. By analyzing the gas in bubbles
trapped in the ice, scientists have learned that the atmosphere
during the ice ages was quite different from that of today and
that the concentrations of greenhouse gases and dust in the
atmosphere have undergone wide fluctuations. While several
ice cores have been drilled in ice caps around the world, one
more than 2 kilometers (~.2 molest long is particularly valuable
because it covers more than an entire glacial cycle. This core,
recovered from a drillhole at Vostok in Antarctica, contains the
record of 160,000 years of climate history, from the present warm
period, or "interglacial," through the most recent 100,000-year-
long ice age, through a previous warm period and back into an
. .
even ear 1er ice age.
The Vostok ice core is being analyzed by Claude Lorius and
colleagues at the Laboratory of Glaciology and Geophysics of
the Environment in St. Martin d'Heres and at the Laboratory of
Isotopic Geochemistry in Gif sur Yvette, both in France. While
researchers in recent years had already learned that carbon diox-
ide levels during the most recent ice age were lower than they
are during today's interglacial, the French group reports an even
stronger relationship between this greenhouse gas and tempera-
ture: Atmospheric greenhouse gases and climate generally shift
in lockstep throughout the glacial cycle. As the earth moves into
an interglacial period, for instance, temperatures rise, and so do
concentrations of carbon dioxide. During the deepest part of the
ice age, temperatures plummet, and so does carbon dioxide, to
perhaps 60 percent of that during the interglacial periods. But
researchers do not yet know which is cause and which is effect.
Ice cores are far from the only tool providing insights into
the earth's climatic history. Researchers glean clues from other
sources such as fossilized pollen grains, annual growth rings
of trees, records of changing sea level in coral reefs, and even
fossilized middens, the junk piles left by packets. Cores of
sediment extracted from the floor of the deep sea are especially
useful because their chemical composition and the warm- or
cold-water fossils they contain reflect changes in ocean temper-
ature and the volume of the polar ice caps.
1, ,
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THE EARTH AS A SYSTEM
Information from these diverse sources adds up to a picture
of growing complexity but increasing clarity. Scientists have
established that ice ages are almost certainly triggered by rel-
atively small changes in the amount of sunlight reaching the
earth at different latitudes and seasons. These small changes in
sunlight, only a few percent, are caused by three orbital effects:
slight changes in the earth's elliptical orbit around the sun from
nearly circular to more elliptical, over a cycle of about 100,000
years; shifts in the degree at which the earth's axis is tilted, over
a cycle of about 40,000 years; and wobbling of the axis itself,
over a cycle of about 20,000 years. When the orbital conditions
result in less sunlight in summer, the climate cools, ice may
gradually accumulate into mountains of ice over 2 miles tall,
and, because water is transferred to the ice caps, sea level may
drop by several hundred feet. When the orbital conditions yield
more sunlight in summer, the climate warms, the ice melts, and
sea level rises.
While orbital changes may trigger the glacial cycles, the
shifts in sunlight are not great enough in themselves to force cli-
mate change of ice age magnitude. Oceans, with their vast
capacity for storing heat and carbon, also may play a crit-
ical role in causing climate change. Lorius and colleagues
suggest that two kinds of ocean fluctuations-"deep changes
possibly driven by sea level and surface changes driven by
atmospheric circulation"~rive the carbon dioxide variations
between glacial and nonglacial tunes.
Not only physics but also chemistry and biology may be
catalysts in the climate cycle. Plankton and other photosyn-
thetic microorganisms living In the ocean may help regulate
the world's climate. These microbes absorb carbon dioxide in
the process of photosynthesis. According to one scenario, the
plankton may flourish, or "bloom," as polar ice caps grow and
nutrients in the water become more concentrated or as ocean
currents change, bringing nutrient-rich bottom waters to the
surface. The explosion in plankton concentrations would mean
more photosynthesis. Carbon dioxide levels in the ocean would
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LESSONS FROM THE GEOLOGIC PAST
27
drop, and consequently more wouIcl be pulled from the atmo-
sphere, cooling the earth.
Plankton may also be involved in another mechanism, along
with the clouds that form over the ocean and at any given time
cover 30 percent of the world. Researchers studying both arctic
and antarctic ice say that the concentration of sulfate particles
varies with temperature as temperatures drop, sulfate concen-
trations rise. One possible reason is that some kinds of plankton
excrete a sulfur compound called dimethyIsulfide (DMS). When
dimethylsulfide diffuses from the ocean to the atmosphere, it
oxidizes into sulfate particles, which act as condensation nuclei
for water droplets that form marine stratus clouds over the open
ocean. As the plankton bloom, the number of nuclei increases.
With more nuclei, more incoming solar radiation is reflected
back to space by the clouds, lowering the water's surface tem-
perature and cooling the earth.
THE CURRENT WARM PHASE
The geologic record shows that the latest act of the glacial
drama opened as the most recent glaciation began to wind down
about 1S,000 years ago. As has been the pattern, the cold period
lasted about 100,000 years; the present balmy climate is a brief
warm spell in a typically icy cycle. This most recent switch
from a glacial to a warm phase is of special interest to scientists
grappling to make sense of the complexities of modern climate
because the amount of carbon dioxide that has accumulated in
the atmosphere from when the melting began to the present
is roughly equal to the amount of greenhouse gases projected
to build up from the present to about the middle of the next
century.
To determine what happened when the most recent glacia-
tion ended and why, a number of academic institutions have
pooled their efforts through the Cooperative Holocene Map-
ping Project (COHMAP). As Kutzbach explains, they use both
geologic data and general circulation climate models to identify
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28
THE EARTH AS A SYSTEM
1.5 °C
I I I L e s s
LITTLE ICE
A G E ,'-
r ~l
~J\: I
1 1 1 1 1 1
900 1 100 1300 1500 1700 1900
Y E A R
9 1 a ci a I
ore
glacial
Estimates of the changes in temperature in Europe over the past 1000 years.
(Reprinted, by permission, from J. Imbrie and K. P. Imbrie. 1986. Ice Ages: Solving
the Mystery. Harvard University Press, Cambridge, Mass. Copyright (A 1986 by John
Imbrie and Katherine Palmer Imbrie.)
and evaluate the causes and mechanisms of this most recent
change from glacial to the current interglacial. Although the ex-
act sequence of events is unclear, COHMAP results suggest that
the orbitally caused increase of summer sunlight, the rise in car-
bon dioxide, and the melting of glacial ice all began about 1S,000
years ago. The global warming trend over the past 13,000 years
has been about 5°C (or 9°F). In high latitudes, such as Canada,
the warming since IS,000 years ago has been much greater-
10°, 20°, even 30°C. Arctic permafrost and sea ice receded and
sea level rose. Spruce forests migrated from the central United
States to southern Canada, and lake beds clried up in California
and Nevada. These and other environmental changes influ-
encecl human communities throughout the world. This most
recent global warming of about 5°C occurred over a period of
many millennia. In contrast, the projected future warming from
human-producec! greenhouse gases may occur within a century
or so, in other words, perhaps 10 to 50 times faster.
Results from COHMAP also show that climate changes in
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LESSONS FROM THE GEOLOGIC PAST
29
the mid-latitudes and the tropics likewise seem to be linked
to small changes in the earth's orbit. As radiation in summer
increased, the land became hotter, creating greater contrast be-
tween ocean and land temperatures. This produced strong sum-
mer monsoons from 12,000 to 6,000 years ago in the Northern
Hemisphere tropics and subtropics, and drought in the interi-
ors of North America and central Asia. Thus, at a time when
the American desert basins were dry, lakes flooded much of
North Africa that is today covered by the shifting sands of the
Sahara Desert. These lakes formed in shallow depressions of
the desert floor. In parts of North Africa and the Middle East,
climatic conditions became more favorable for the agricultural
revolution then under way.
Agreement between mode! results and the geologic record
of this relatively recent global change reassures scientists that
they are now beginning to understand what happens in the
climate system.
The climate has also varied in recent centuries and decades.
Although these changes were not as dramatic as those of earlier
times, nor as large as those expected in the next century, their
beginnings and endings are accurately known. From this, scien-
tists know that climate can change abruptly and that the changes
can be large enough to have regional impacts. The golden age of
the Anasazi Indians on Mesa Verde in the southwestern United
States, for instance, may have been cut short by overpopulation
and overuse of land, coupled with the persistent drought that
began suddenly in the late thirteenth century. This drought be-
gan about the time that Europe was gripped by a cold snap,
known as the Little Ice Age, that persisted until the nineteenth
century. In the 1930s dry conditions led to the North American
Dust Bowl. Whether these variations in climate were caused by
changes in the amount of sunlight or in the frequency of vol-
canic eruptions or by subtle internal oscillations of atmosphere
and ocean is not known.
As evident from these examples of natural climate variabil-
ity, it is difficult to recognize the initial phases of human-caused
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30
THE EARTH AS A SYSTEM
climate change. Nonetheless, it is clear that throughout the his-
tory of life on earth, the fortunes of earth's inhabitants have been
inextricably tied to variations in climate. To be sure, photosyn-
thesizing algae helped create and maintain the conditions that
allowed life to persist without pause for more than 3.5 billion
years. We, on the other hand, have produced conditions that
could push the earth to the brink of climate change at a rate
unprecedented In the planet's history.
It is highly probable that life will survive. After all, life has
survived all the past changes of climate. But it may not be life
as we know it now. Will plant and animal communities respond
quickly enough to the projected environmental change, or will
the uneven pace of adjustment literally tear communities apart?
Will humans be able to adapt as planetary conditions change?
The answer may lie in the planet's past, and in understanding
the complex, interdependent components that make up the earth
system.
Representative terms from entire chapter:
geologic past
