A low-order model for the response of the Atlantic thermohaline circulation to climate change
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Concern has been expressed that anthropogenic climate change may lead to a slowdown or even collapse of the Atlantic thermohaline circulation (THC). Because of the possibly severe consequences that such an event could have on the northern North Atlantic and northwestern Europe, integrated assessment models (IAMs) are needed to explore the associated political and socioeconomic implications. State-of-the-art climate models representing the THC are, however, often too complex to be incorporated into an integrated assessment framework. In this paper we present a low-order model of the Atlantic THC which meets the main requirements of IAMs: it (1) is physically based, (2) is computationally highly efficient, (3) allows for comprehensive uncertainty analysis and (4) can be linked to globally aggregated climate models that are mostly used in IAMs. The model is an interhemispheric extension of the seminal Stommel model. Its parameters are determined by a least-squares fit to the output of a coupled climate model of intermediate complexity. Results of a number of transient global warming simulations indicate that the model is able to reproduce many features of the behaviour of coupled ocean–atmosphere circulation models such as the sensitivity of the THC to the amount, regional distribution and rate of climate change.
KeywordsThermohaline circulation Box model Climate change Sensitivity analysis
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We wish to thank Andrey Ganopolski for performing the experiments with CLIMBER-2 and for giving helpful advice. We are grateful to Till Kuhlbrodt for suggestions for the revision of the manuscript, and to Alexa Griesel, Marisa Montoya and Miguel Morales Maqueda for fruitful discussions. Thanks are also due to two anonymous reviewers for their helpful comments. Financial support for the first author was provided by the German Federal Ministry for Education, Science, Research and Technology (under grant no. 01LD0016).