Century-scale effects of increased atmospheric C0₂ on the ocean-atmosphere system

SEVERAL studies have addressed the likely effects of CO₂-induced climate change over the coming decades^1-10, but the longer-term effects have received less attention. Yet these effects could be very significant, as persistent increases in global mean temperatures may ultimately influence the large-scale processes in the coupled ocean-atmosphere system that are thought to play a central part in determining global climate. The thermohaline circulation is one such process - Broecker has argued¹¹ that it may have undergone abrupt changes in response to rising temperatures and ice-sheet melting at the end of the last glacial period. Here we use a coupled ocean-atmosphere climate model to study the 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 about 3.5 and 7 °C, respectively, over 500 years, and that sea-level rise owing to thermal expansion alone is about 1 and 2 m respectively (ice-sheet melting could make these values much larger). The thermal and dynamical structure of the oceans changes markedly in the quadrupled-CO₂ climate - in particular, the ocean settles into a new stable state in which the thermohaline circulation has ceased entirely and the thermocline deepens substantially. These changes prevent the ventilation of the deep ocean and could have a profound impact on the carbon cycle and biogeochemistry of the coupled system.