One Earth, One Future Our Changing Global Environment (1990) / Chapter Skim
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3. Lessons from the Geologic Past
3. Lessons from the Geologic Past
Pages 20-30
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From page 20... ...
As scientists try to predict the future global environment, they constantly confront an enormous obstacle: incomplete understanding 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.
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From page 21... ...
FORMATION OF THE OZONE SHIELD Our lessons from the past begin with a development that occurred more than ~ billion years ago. Early aquatic organisms, blue-green algae, began to use energy from the sun to split molecules of water and carbon dioxide and then recomb~ne them into organic compounds and oxygen the process known as photosynthesis.
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From page 22... ...
The earth in the age of Pangaea, 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.
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From page 23... ...
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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From page 24... ...
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.
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From page 25... ...
While researchers in recent years had already learned that carbon dioxide 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 temperature: 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.
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From page 26... ...
Oceans, with their vast capacity for storing heat and carbon, also may play a critical 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.
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From page 27... ...
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 glaciation ended and why, a number of academic institutions have pooled their efforts through the Cooperative Holocene Mapping Project (COHMAP)
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From page 28... ...
and evaluate the causes and mechanisms of this most recent change from glacial to the current interglacial. Although the exact sequence of events is unclear, COHMAP results suggest that the orbitally caused increase of summer sunlight, the rise in carbon dioxide, and the melting of glacial ice all began about 1S,000 years ago.
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From page 29... ...
This produced strong summer monsoons from 12,000 to 6,000 years ago in the Northern Hemisphere tropics and subtropics, and drought in the interiors 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.
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From page 30... ...
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?
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