Risk of breakdown of Atlantic Ocean circulation

The so-called thermohaline circulation (THC) in the Atlantic Ocean transports vast amounts of heat towards high northern latitudes, contributing to the relatively mild climate in northwestern Europe. A cessation of the circulation was shown to have a number of drastic consequences: sea level rise in the North Atlantic by up to 1m (also see press release April 4, 2005), disruption of marine ecosystems and global changes of precipitation patterns. “Such a dramatic change in ocean circulation would disturb our entire climate system”, says Anders Levermann, PIK physicist and co-author of the study.

Even if the global climate only warms by another 2°C, a majority of the experts think that a shutdown of the THC could be triggered by the end of the century. With a warming of 4ºC – a likely outcome of unabated greenhouse gas emissions according to the IPCC’s latest report – two thirds of the experts assess the probability of triggering a THC collapse as being between 10% and 60%. “Given the potentially dangerous consequences, such a risk is too high to be ignored”, says Kirsten Zickfeld, who led the study.

The elicited probabilities are generally higher than those published in recent modelling studies. “An advantage of the elicitation technique relative to classical modelling studies is that it considers information from sources other than modelling, such as results from observations, past climate changes in Earth’s history, and the scientist’s assessment of the skill of climate models,” says Zickfeld.

A collapse of the conveyor-like Atlantic circulation is merely one of a number of identified possible “tipping points” of the climate system. These are phenomena that are likely to occur abruptly and/or irreversibly as a result of global warming, implying large-scale consequences for ecological and human systems. Examples of other tipping points include the melting of Arctic sea ice as well as the Greenland ice sheet, or a major change in the Indian summer monsoon (also see press release August 15, 2005).

Original article:
Zickfeld, K., Levermann, A., Morgan, M.G., Kuhlbrodt, T., Rahmstorf, S., Keith, D.W., 2007: Expert judgements on the response of the Atlantic meridional overturning circulation to climate change, Climatic Change, 82, 235-265.

The article can be viewed on the internet:

Dr. Kirsten Zickfeld, e-mail: zickfeld@ocean.seos.uvic.ca, phone: + 1 - 250 - 472 40 08

Prof. Dr. Anders Levermann, e-mail: anders.levermann@pik-potsdam.de, phone + 49 - 331 - 288 - 25 60

Press office:

Uta Pohlmann, e-mail: info (at) pik-potsdam.de, phone + 49 - 331 - 288 - 25 07