The Atlantic meridional overturning circulation (AMOC) is an important tipping element in the climate system, as it circulates water from north to south and back in a long cycle within the Atlantic Ocean. A future collapse would have severe impacts especially on the climate of the North Atlantic. This type of abrupt climate change was last experienced during the Dansgaard-Oeschger events in the last ice age, caused by the collapse and restoration of the AMOC. This resulted in mean temperature variations in the northern hemisphere of 10 – 15 °C within a decade.
The strength of the AMOC has only been monitored continuously since 2004, and these observations have shown that it is weakening, but longer records are needed to assess the extent. Based on model simulations by the Climate Model Intercomparison Project (CMIP), the Intergovernmental Panel on Climate Change (IPCC) suggests that a complete collapse is unlikely in the 21st century. However, concern that the climate is tipping into an undesirable state is growing as greenhouse gas concentrations rise.
Peter Ditlevsen and Susanne Ditlevsen, University of Copenhagen, Copenhagen, Denmark, have conducted a study using statistical significance and data-driven estimators to predict the timing of the tipping point. Based on their findings, they have projected that a collapse of the AMOC is likely to happen around mid-century, or potentially any time from 2025 onwards and no later than 2095, if we keep emitting greenhouse gases as we are now.
The researchers analyzed sea surface temperatures in the North Atlantic from 1870 to 2020 as a proxy for AMOC. They used these records because they extend much further back than direct AMOC measurements, providing more reliable information on temperature trends.
Although the researchers have developed sound statistical methods, they cannot rule out the possibility that a collapse is only partial and does not lead to a complete collapse of AMOC, as some models suggest. In addition, a high ramp speed, i.e., a high speed at which the critical value of the control parameter is approached, could increase the probability of a collapse.
The researchers summarize that overall it is a worrisome result and should require rapid and effective action to reduce global greenhouse gas emissions to avoid the steady change of the control parameter toward a collapse of the AMOC. That is, a reduction in temperature increase and freshwater input from ice melt in the North Atlantic. They advise monitoring the current and EWS by direct measurements.
- Warning of a forthcoming collapse of the Atlantic meridional overturning circulation,
Peter Ditlevsen, Susanne Ditlevsen,
Nat. Commun. 2023.