4 Rahmstorf

NATURE | VOL 419 | 12 SEPTEMBER 2002 | www.nature.com/nature 207 The world ocean is one of the main constituents of the climate system and affects climate in a multitude of ways. Its sheer size is obvious: 71% of the Earth is covered by ocean, so that most of the solar radiation received at the Earth’s surface goes into the ocean and warms the surface waters. As a result of its heat capacity and circulation, the ocean has the ability to both store and redistribute this heat before it is released to the atmosphere (much of it in form of latent heat, that is, water vapour) or radiated back into space. The heat storage effect is most apparent on the seasonal timescale. The mid-latitude temperature range between summer and winter is typically around 8 7C over the ocean and at the coast, whereas this range is up to several tens of degrees in the continental interiors (see Figure 2.1 in ref. 1, which contours the observed seasonal temperature range). A corresponding figure for the temperature deviation from the zonal mean (Fig. 1 in ref. 2) gives an indication of the effect of ocean heat transport on surface temperatures, with warm anomalies over the three main regions of deepwater formation of the world ocean: the northern North Atlantic, the Ross Sea and the Weddell Sea. These are key areas for the thermohaline circulation of the world ocean (see Box 1), where surface waters after releasing heat to the atmosphere reach a critical density and sink. Clearly not all deviations from zonal mean conditions are due to ocean circulation, but the magnitude of the warm anomaly over the northern North Atlantic (~10 7C) is in agreement with estimates and simulations of climate models of the effect of ocean heat transport (Fig. 1). In addition to its heat storage and transport effects, the ocean can influence the Earth’s heat budget by its sea-ice cover, which changes the planetary albedo and can thus affect the steady-state global-mean temperature. Sea ice also acts as an effective thermal blanket, insulating the ocean from the overlying atmosphere. This is so effective that in a typical ice-covered sea more than half of the air–sea heat Ocean circulation and climate during the past 120,000 years