CENTURY-SCALE EFFECTS OF INCREASED ATMOSPHERIC CO2 ON THE OCEAN-ATMOSPHERE SYSTEM

SEVERAL studies have addressed the likely effects of CO2-induced clima te change over the coming decades1-10, but the longer-term effects hav e received less attention. Yet these effects could be very significant , as persistent increases in global mean temperatures may ultimately i nfluence the large-scale processes in the coupled ocean-atmosphere sys tem that are thought to play a central part in determining global clim ate. The thermohaline circulation is one such process - Broecker has a rgued11 that it may have undergone abrupt changes in response to risin g temperatures and ice-sheet melting at the end of the last glacial pe riod. Here we use a coupled ocean-atmosphere climate model to study th e evolution of the world's climate over the next few centuries, driven by doubling and quadrupling of the concentration of atmospheric CO2. We find that the global mean surface air temperature increases by abou t 3.5 and 7-degrees-C, respectively, over 500 years, and that sea-leve l rise owing to thermal expansion alone is about 1 and 2 m respectivel y (ice-sheet melting could make these values much larger). The thermal and dynamical structure of the oceans changes markedly in the quadrup led-CO2 climate - in particular, the ocean settles into a new stable s tate in which the thermohaline circulation has ceased entirely and the thermocline deepens substantially. These changes prevent the ventilat ion of the deep ocean and could have a profound impact on the carbon c ycle and biogeochemistry of the coupled system.