The cause of the LPTM warming is unknown (Dickens, 1999) but has been speculated to be increased volcanism (Thomas and Shackleton, 1996) or low-latitude deepwater formation (Bains et al., 1999). Recently, Bice and Marotzke (in press) presented results from an ocean general circulation model indicating that a sudden switch of deepwater formation from high southern to high northern latitudes could have led to mid-depth and deep-ocean warming of around 5°C. The switch was caused by a slow increase in the atmospheric water cycle, as expected under increasing temperatures (Manabe and Bryan, 1985; Manabe and Stouffer, 1993), and consistent with LPTM sedimentary evidence (Robert and Maillot, 1990; Schmitz et al., 2001). The mid-depth warming displayed by the model could destabilize large volumes of methane hydrate in the depth range of 1,000-2,000m over much of the world ocean. The THC switch seen in the model of Bice and Marotzke (2001) and the inferred subsequent methane release are an abrupt climate change, according to the definition given in Chapter 1. When the freshwater forcing is reduced to pre-LPTM values, deepwater is again formed in the Southern Hemisphere, with a hysteresis characterized by case (b) in Figure 3.1.
The climate during the LPTM may well be a valid past analogue of the greenhouse world expected for the next several centuries (Dickens, 1999), despite the different continental configuration. The results of Bice and Marotzke (2001) indicate more severe potential consequences of a drastic change in THC, as might occur in a future greenhouse world (Manabe and Stouffer, 1993), than previously assumed.
centuries. A possible shutdown of the THC would not induce a new glacial period, as press reports suggested; however, it clearly would involve massive changes both in the ocean (major circulation regimes, upwelling and sinking regions, distribution of seasonal sea ice, ecological systems, sea level) and in the atmosphere (land-sea temperature contrast, storm paths, hydrological cycle, extreme events). The most pronounced changes are expected in regions that are today most affected by the influence of the North Atlantic THC (e.g., Scandinavia and Greenland).
Current knowledge of the evolution of the THC is summarized in the Third Assessment Report of the Intergovernmental Panel on Climate Change (2001b). Several comprehensive coupled climate models were run with a scenario of increasing greenhouse gas forcing for the next 100 years. Most models show a reduction in the THC in response to the forcing (Plate 7). This is due to enhanced warming of the sea surface in the high latitudes and