Seeking Clues About Sea Level From Fossil Beaches - NYTimes.com
The earth has warmed up many times, for purely natural reasons, and those episodes often featured huge shifts of climate, partial collapse of the polar ice sheets and substantial increases in sea level.
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Skeptics who play down the importance of global warming like to note that these past changes occurred with no human intervention. They argue that the climate is ever-changing, yet humans or their predecessors managed to prosper.
The geologic record does offer startling examples of the instability of the planet. Whale bones can be dug up in the Sahara. The summit of Mount Everest is a chunk of ancient seafloor.
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“Absolutely, unequivocally, nature has changed before,” said Richard B. Alley, a leading climate scientist at Pennsylvania State University. “But it looks like we’re going to do something bigger and faster than nature ever has.”
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Carbon dioxide, the main greenhouse gas, appears to have played a crucial role. When changes in the orbit caused the earth to cool, scientists say, a large amount of carbon dioxide entered the ocean, reducing the heat-trapping properties of the atmosphere and thus amplifying the cooling. Conversely, when the shifts in sunlight led to initial warming, carbon dioxide emerged from the ocean and helped speed the end of the previous ice age.
Based on this record, scientists like Dr. Alley describe carbon dioxide as the master control knob of the earth’s climate. A large body of scientific evidence shows that the current increase in the gas is being caused by human activity, meaning that people are essentially twisting the earth’s thermostat hard to the right.
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“At every point, as our knowledge increases,” Dr. Raymo said, “we’ve always discovered that the climate system is more sensitive than we thought it could be, not less.”
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