Box 4.1 Abrupt Climate Change in the Ancient Hot House

About 55 million years ago, the earth underwent an abrupt climate change known as the Late Paleocene Thermal Maximum (LPTM), recently renamed the Paleocene-Eocene Thermal Maximum (PETM) (Kennett and Stott, 1991; Dickens et al., 1995; Dickens, 1999; Norris and Röhl, 1999; Bains et al., 1999). Against the backdrop of an already warm climate with reduced pole-equator temperature contrasts, bottom water temperatures increased by 4-6°C (Thomas and Shackleton, 1996), and high-latitude surface temperatures by 4-8°C, over 10-20 kyr (Norris and Röhl, 1999). Thirty to fifty per cent of benthic foraminifera went extinct (Thomas and Shackleton, 1996). The suite of dramatic global changes inferred for the LPTM includes increased aridity in subtropical latitudes and increased high-latitude precipitation (Robert and Maillot, 1990; Schmitz et al., 2001).

At the onset of the LPTM warming, marine and terrestrial carbon isotope values exhibit a negative shift of at least 2.5 per mil (Norris and Röhl, 1999). The only known sources for this quantity and composition of carbon today are the vast reserves of natural gas hydrate in oceanic, deep lake and polar sediments and the free methane gas trapped beneath hydrate deposits. Methane hydrate is a solid complex of methane and water that is stable only at low temperatures and high pressures, such as found in sediment of the mid-depth and deep ocean. The carbon isotope signature during the LPTM is indicative of massive destabilization of marine methane hydrates (Dickens et al., 1995); it is estimated that 1,200-2,000 gigatons of methane gas were released.

system and of the mechanisms involved in abrupt climate change, this committee concludes that human activities could trigger abrupt climate change. Impacts cannot be predicted because current knowledge is limited, but might include changes in coupled modes of atmospheric-ocean behavior, the occurrence of droughts, and the vigor of thermohaline circulation (THC) in the North Atlantic. More research is needed to better understand the relationship between human influences on climate, especially global warming, and possible abrupt climate change.

CHANGES IN NORTH ATLANTIC THERMOHALINE CIRCULATION

A question of great societal relevance is whether the North Atlantic THC will remain stable under the global warming expected for the next few



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