CROSSING A CLIMATE THRESHHOLD

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Ocean Currents Changed the Climate

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Carbon dioxide gas erupted from mid-ocean volcanoes, changing the pattern of ocean circulation.

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Underwater, the effect of the sudden warming was even more profound. The surface of oceans in the Southern Hemisphere warmed from 15 degrees Celsius to 20 degrees Celsius, almost room temperature. While that may not seem like much of a change to humans, the shift was deadly for many creatures in the sea. Over 10,000 years, more species of deep water foraminifera went extinct than at any time in the last 65 million years. But the implications transcend fossil teeth and tiny shells.

Zachos thinks the rapid warming represents a brief period when the usual rules of climate suddenly changed. For tens of millions of years, the Paleocene world had warmed slowly as mid-ocean volcanoes disgorged carbon dioxide gas. Once the world warmed past a critical threshold, something triggered a quick hot spell. The idea is speculative and the mechanisms mysterious: "We don't understand very well how these types of transitions work," he admits.

One idea hinges on the delicate balance of ocean currents. The currents act like giant conveyor belts, transporting heat and gases around the world. Today, the currents are driven mainly by temperature. Cold water rich in dissolved oxygen sinks in the polar regions and flows to the tropics. But the gradual warming of the poles, Zachos suggests, rearranged the oceanic conveyor belts. Once the poles lost their chill, warm waters made salty by evaporation sank sluggishly into the depths.

Now some researchers suggest that another kind of gas may have played a role...