Global Change and Our Common Future Papers from a Forum (1989) / Chapter Skim
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6. Historical Perspectives: Climatic Changes Throughout the Millennia
6. Historical Perspectives: Climatic Changes Throughout the Millennia
Pages 50-61
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From page 50... ...
The first example of global change is taken from more than a billion years ago. Early, innovative forms of life, blue-green algae, used the energy from the sun to split molecules of water and carbon dioxide and then recombined them differently to form organic compounds and oxygen -- a process we call photosynthesis.
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From page 51... ...
The drifting and rifting of our restless continents created a grand sequence of global changes over hundreds of millions of years. Figure 6.1 shows the continents' locations from 600 million to 100 million years ago.
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From page 52... ...
This agreement of the simulated climate with geologic data is important. It shows that we are beginning to understand, and model, the processes of early global change.
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From page 53... ...
perhaps ocean currents were different, or perhaps the amounts of oxygen and carbon dioxide were different. We do not know what caused the great extinction, but we know that it happened.
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From page 54... ...
In other words, this ancient global change may also have linked changes of carbon dioxide and climate, but with falling levels of greenhouse gases and temperature. If the amount of carbon dioxide doubles in the next century, it will possibly mark a return to the higher carbon dioxide levels, and higher temperature levels, of several million years ago.
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From page 55... ...
. ,3 ~ ,N,~ SURFACE PRESENT TYPE PRECIPITATION O ICE SHEET ~ DATA ANNUAL f~ YEAR ROUND ~ I`IOOEL ANNUAL ~ SEA ICE _ < 1000 mm pit WINTER ONLY Ed S" 1" O LAND O OCEAN The Earths Orbit June 2: POLLEN OAK SPRUCE FORAMS ATLANTIC I `: r``har ?
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From page 56... ...
Now we are making the moves; we are causing global changes. What happened when the recent glacial age ended?
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From page 57... ...
About 18,000 years ago, spruce forests were located in the central United States south of the ice sheet (Figure 6.7~. As the ice melted and the climate warmed, the spruce forest moved to the Great Lakes area 9,000 years ago and is currently located in southern and northwestern Canada.
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From page 58... ...
The fair agreement between model results and the geologic record of this global change gives us confidence that we are beginning to have a crude predictive capability for understanding what happened and why. A fifth and last set of examples of global change comes from the present millennium.
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From page 59... ...
based on geologic and paleoecologic evidence (top panel) and climate model simulations of enhanced monsoonal circulations (bottom panel)
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From page 60... ...
It complicates our task of recognizing the initial phases of human-caused climate change -- witness the discussions of the hot, dry summer of 1988. To sum up, I have highlighted several examples of global change from the past: the dawn of life on our planet, the restless rifting of the continents, the waxing and waning of ice ages and monsoons, and the droughts and cold spells of recent centuries.
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From page 61... ...
1988. Climatic changes of the last 18,000 years: Observations and model simulations.
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