Konsulentrapport om Golfstrømmen - engelsk abstract

The circulation of the Atlantic Ocean differs significantly from the circulation of the other ocean basins by showing an intense overturning whereby warm water flows northward at the surface, cools and subducts at high latitudes returning southward as cold water at great depth. Associated with the overturning is a large transport of heat believed to be essential for the mild climate of northern Europe and Scandinavia. Possible changes in the Atlantic circulation could have severe regional as well as global climatic changes. Accordingly, the future changes in the Atlantic circulation have been subject to scientific investigations as well as intensive public debate. The present report gives an overview of the problem in a climate change perspective for the 21st century and reviews the latest research in an attempt to reach an up-to-date conclusion.

The response of the Atlantic ocean circulation to the global warming is most likely a gradual decline in the intensity of the overturning. This will be associated with a reduction in oceanic heat supply to the Arctic and sub-Arctic area which on its own acts to slow down the rate of global warming in the northern hemisphere. Nevertheless, almost all areas on Earth will experience an atmospheric warming by the end of the century.

We know that climate of the past has varied much more dramatically than the relatively slow warming trend that can be projected to result  from human emissions of greenhouse gases. At glacial times, the climate of the northern hemisphere went through a series of abrupt changes. Within a few decades or possibly less, atmospheric temperatures changed with an amplitude comparable to half the full glacial to interglacial difference. The most often advocated  explanation for such abrupt climate variability is changes in the circulation of the North Atlantic Ocean. The risk that global warming might lead to similar abrupt changes cannot be ruled out. Recent research, however, support the scientific view that abrupt climate change within the 21st century is a highly unlikely response of the climate system to global warming. Furthermore, the impact on European climate of a possible collapse in the Atlantic ocean circulation and associated heat transport is likely quite different and weaker in the warmed climate of the future than in the colder climates of the past.

The risk of and implication of a complete shut-down of the thermohaline circulation is however not fully understood and hence further computations using coupled atmosphere-ocean models are recommended in order to improve the understanding of the problem.