CROSSING A CLIMATE THRESHHOLD

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The Climate Suddenly Warmed

Last year, paleontologist Paul Koch of Princeton and paleoceanographer Jim Zachos of the University of California, Santa Cruz, traced a sudden rise in temperature onto land with an odd collection of clues including fossil teeth of land mammals and tiny marine shells from the Antarctic ocean. The short-lived hot spell, they concluded, warmed habitats enough for mammals to cross a polar land bridge from either Asia or Europe into North America.

Ominously, this long-ago sauna suggests that a slowly-changing climate can jump suddenly out of control. Many scientists have thought the ancient temperature spike-only about 100,000 years long-resulted from crossing some kind of critical threshold, but the cause remained a puzzle. Recently, a group of researchers has proposed a new mechanism: The gradual warming preceding the hot spell may have included a release of methane gas from ocean sediment. That methane, they think, helped fuel the sudden rise in temperature at the end of the Paleocene epoch, one of the hottest greenhouse warmings in Earth history.

In the past century, humans have begun to create their own greenhouse warming. A miasma of exhaust, spewed from smokestacks and tailpipes, lingers in the atmosphere and traps heat like an insulating blanket. The world is going to get warmer, says Zachos. "In fact, it already has." Now, his research raises disturbing new questions: Will climate go haywire and precipitate another heat wave? Have we pushed it too far?

First, though, a quick trip back to the world of Paleocene mammals...