Lessons from Earth's past
Climate models are invaluable tools for understanding Earth's climate system. But examination of the real world also provides insights into the role of greenhouse gases (carbon dioxide) in determining Earth's climate. Not only can much be learned by looking at the observational evidence fr...
| Published in: | Science |
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| Other Authors: | |
| Format: | Article in Journal/Newspaper |
| Language: | English |
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American Association for the Advancement of Science
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| Online Access: | https://doi.org/10.1126/science.1199380 http://n2t.net/ark:/85065/d71v5ff5 |
| Summary: | Climate models are invaluable tools for understanding Earth's climate system. But examination of the real world also provides insights into the role of greenhouse gases (carbon dioxide) in determining Earth's climate. Not only can much be learned by looking at the observational evidence from Earth's past, but such know ledge can provide context for future climate change. The atmospheric CO2 concentration currently is 390 parts per million by volume (ppmv), and continuing on a business-as-usual path of energy use based on fossil fuels will raise it to ∼900 to 1100 ppmv by the end of this century (see the first figure) (1). When was the last time the atmosphere contained ∼1000 ppmv of CO2? Recent reconstructions (2–4) of atmospheric CO2 concentrations through history indicate that it has been ∼30 to 100 million years since this concentration existed in the atmosphere (the range in time is due to uncertainty in proxy values of CO2). The data also reveal that the reduction of CO2 from this high level to the lower levels of the recent past took tens of millions of years. Through the burning of fossil fuels, the atmosphere will return to this concentration in a matter of a century. Thus, the rate of increase in atmospheric CO2 is unprecedented in Earth's history. What was Earth's climate like at the time of past elevated CO2? Consider one example when CO2 was ∼1000 ppmv at ∼35 million years ago (Ma) (2). Temperature data (5, 6) for this time period indicate that tropical to subtropical sea surface temperatures were in the range of 35° to 40°C (versus present-day temperatures of ∼30°C) and that sea surface temperatures at polar latitudes in the South Pacific were 20° to 25°C (versus modern temperatures of ∼5°C). The paleogeography of this time was not radically different from present-day geography, so it is difficult to argue that this difference could explain these large differences in temperature. Also, solar physics findings show that the Sun was less luminous by ∼0.4% at that time (7). Thus, an increase of CO2 from ... |
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