The role of the thermohaline circulation in abrupt climate change

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The possibility of a reduced Atlantic thermohaline circulation in response to increases in greenhouse-gas concentrations has been demonstrated in a number of simulations with general circulation models of the coupled ocean±atmosphere system. But it remains difficult to assess the likelihood of future changes in the thermohaline circulation, mainly owing to poorly constrained model parameterizations and uncertainties in the response of the climate system to greenhouse warming. Analyses of past abrupt climate changes help to solve these problems. Data and models both suggest that abrupt climate change during the last glaciation originated through changes in the Atlantic thermohaline circulation in response to small changes in the hydrological cycle. Atmospheric and oceanic responses to these changes were then transmitted globally through a number of feedbacks. The palaeoclimate data and the model results also indicate that the stability of the thermohaline circulation depends on the mean climate state.

The ocean affects climate through its high heat capacity relative to the surrounding land, thereby moderating daily, seasonal and interannual temperature fluctuations, and through its ability to transport heat from one location to another. In the North Atlantic, differential solar heating between high and low latitudes tends to accelerate surface waters polewards whereas freshwater input to high latitudes together with low-latitude evaporation tend to brake this flow. Today, the former thermal forcing dominates the latter haline (freshwater) forcing and the meridional overturning in the Atlantic drives surface waters northward, while deep water that forms in the Nordic Seas flows southward as North Atlantic Deep Water (NADW). This thermohaline circulation (THC) is responsible for much of the total oceanic poleward heat transport in the Atlantic. […]

[…] Here we refer to an abrupt change as a persistent transition of climate (over subcontinental scale) that occurs on the timescale of decades. Although understanding the mechanisms behind abrupt climate transitions in the past is interesting in its own right, there is a pressing need to gain insight into the likelihood of their future occurrence. Most, but not all, coupled GCM (Global Climate Model) projections of the twenty-first century climate show a reduction in the strength of the Atlantic overturning circulation with increasing concentrations of greenhouse gases. If the warming is strong enough and sustained long enough, a complete collapse cannot be excluded. The successful simulation of past abrupt events that are found in the palaeoclimate record is the only test of model fidelity in estimating the possibility of large ocean±atmosphere reorganizations when projecting future climate change.

Peter U. CLARK, Nicklas G. PISIAS, Thomas F. STOCKER & Andrew J. WEAVER

NATURE vol. 415

21 Février 2002

© 2002 Macmillan Magazines Ltd

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