Global climatic impacts of a collapse of the Atlantic thermohaline circulation

Part of the uncertainty in predictions by climate models results from limited knowledge of the stability of the thermohaline circulation of the ocean. Here we provide estimates of the response of pre-industrial surface climate variables should the thermohaline circulation in the Atlantic Ocean collapse. For this we have used HadCM3, an ocean-atmosphere general circulation model that is run without flux adjustments. In this model a temporary collapse was forced by applying a strong initial freshening to the top layers of the North Atlantic. In the first five decades after the collapse surface air temperature response is dominated by cooling of much of the Northern Hemisphere (locally up to 8degreesC, 1-2degreesC on average) and weak warming of the Southern Hemisphere (locally up to 1degreesC, 0.2degreesC on average). Response is strongest around the North Atlantic but significant changes occur over the entire globe and highlight rapid teleconnections. Precipitation is reduced over large parts of the Northern Hemisphere. A southward shift of the Intertropical Convergence Zone over the Atlantic and eastern Pacific creates changes in precipitation that are particularly large in South America and Africa. Colder and drier conditions in much of the Northern Hemisphere reduce soil moisture and net primary productivity of the terrestrial vegetation. This is only partly compensated by more productivity in the Southern Hemisphere. The total global net primary productivity by the vegetation decreases by 5%. It should be noted, however, that in this version of the model the vegetation distribution cannot change, and atmospheric carbon levels are also fixed. After about 100 years the model's thermohaline circulation has largely recovered, and most climatic anomalies disappear.