CROSSING A CLIMATE THRESHHOLD
Ocean Currents Changed the Climate
Carbon dioxide gas erupted from mid-ocean volcanoes, changing the pattern
of ocean circulation.
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