THE EMERGENCE of two category 5 hurricanes (Katrina and Rita) in a row over the Gulf of Mexico is a troubling occurrence. But for most tropical meteorologists, the truly astonishing "storm of the decade" occurred in March 2004. Hurricane Catarina — so named because it made landfall in the southern Brazilian state of Santa Catarina — was the first South Atlantic hurricane in recorded history.
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A crucial question is this: Was Catarina simply a rare event at the outlying edge of the normal bell curve of South Atlantic weather or was Catarina a threshold event, signaling some fundamental and abrupt change of state in the planet's climate system?
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But all the major components of global climate — air, water, ice and vegetation — are actually nonlinear: At certain thresholds they can switch from one state of organization to another, with catastrophic consequences for species too finely tuned to the old norms.
Until the early 1990s, however, it was generally believed that these major climate transitions took centuries if not millenniums to accomplish.
Now, thanks to the decoding of subtle signatures in ice cores and sea-bottom sediments, we know that global temperatures and ocean circulation can, under the right circumstances, change abruptly — in a decade or less.
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Where other researchers model the late 21st-century climate that our children will live with upon the precedents of the Altithermal (the hottest phase of the current Holocene period, 8,000 years ago) or the Eemian (the previous, even warmer interglacial episode, 120,000 years ago), growing numbers of geophysicists toy with the possibilities of runaway warming returning the Earth to the torrid chaos of the Paleocene-Eocene Thermal Maximum(PETM: 55 million years ago) when the extreme and rapid heating of the oceans led to massive extinctions.
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