Recent weakening of the system of ocean currents known as the Atlantic meridional overturning circulation (AMOC) is analysed in two papers published in this week’s Nature. The AMOC has a major impact on our climate, redistributing heat and influencing the carbon cycle, but it has been unknown whether its apparent decline in recent decades could reflect long-term natural variability.
David Thornalley and colleagues present palaeo-oceanographic evidence that deep convection in the Labrador Sea and the AMOC have been unusually weak since the end of the Little Ice Age, at about 1850, compared with the previous 1,500 years. The authors propose that the end of the Little Ice Age was associated with the release of freshwater from the Arctic and Nordic seas that triggered this alteration of the AMOC. However, non-AMOC influences on the proxies used in the analysis and the proxies’ variable sensitivity to the AMOC make it difficult to determine whether this transition took place as an abrupt shift towards the end of the Little Ice Age or more gradually over the past 150 years.
In a second paper, Levke Caesar and colleagues combine an ensemble of global climate models with global sea-surface temperature datasets to identify a ‘fingerprint’ of an AMOC slowdown of about 3 sverdrups (15%) since the mid-twentieth century. The fingerprint, which is most pronounced during winter and spring, comprises a cooling in the subpolar Atlantic Ocean caused by reduced heat transport and a warming in the Gulf Stream region due to a northward shift of the mean Gulf Stream path.
The studies differ in the timing of the AMOC slowdown, likely reflecting the many nuances in the expression of the AMOC. Yet, writes Summer Praetorius in an accompanying News & Views article, “it is - at least scientifically - reassuring to see that the present two studies converge on the conclusion that the modern AMOC is in a relatively weak state.” However, in the context of future climate-change scenarios, “it is perhaps less reassuring, because a weakened AMOC might lead to considerable changes in climate and precipitation patterns throughout the Northern Hemisphere,” she concludes.