New study: Weakening of North Atlantic current can be prevented by reducing carbon emissions

Nansen Center CEO Sebastian Mernild participates in an international study incorporating a comprehensive assessment of Greenland Ice Sheet melting suggesting that the freshwater influx could weaken the Atlantic Meridional Overturning Circulation (AMOC) over the next three centuries. Continued melting of the Greenland Ice Sheet could have a significant impact on the AMOC, a system of surface and deep ocean currents – including the Gulf Stream – in the Atlantic Ocean that keeps upper North America and Europe temperate. The published study indicates that if carbon emissions continue unabated, there is a 44 percent likelihood of a collapse of the AMOC system by the year 2300.

The paper Fate of the Atlantic Meridional Overturning Circulation – Strong decline under continued warming and Greenland melting by Bakker et al. are today published in the journal Geophysical Research Letters. Previous studies and international assessment reports, e.g. those from the Intergovernmental Panel on Climate Change, have not or only in a very simplified way considered the melting of the Greenland Ice Sheet (GrIS) on the AMOC and do not provide any information on the uncertainties in the fate of the AMOC.

The current study, using eight state-of-the-science global climate models, incorporates a realistic assessment of the ice sheet melting and shows a definite weakening of the AMOC system. However, the weakening can be partially mitigated by a decline in the atmospheric CO2 emissions. The study also suggests that the freshwater influx from melting of the Greenland Ice Sheet will have less of an impact on the AMOC than the impact of the overall global warming, rising sea surface temperatures, and intensification of the water cycle leading to increased precipitation and evaporation. The Atlantic Meridional Overturning Circulation brings warm waters up from the tropics and transports cooler water to the south. Accordingly, a weakening of the system could mean that the North Atlantic would not warm as rapidly or thoroughly as it does now, affecting the regional climate in North America and northern Europe, including Norway. The circulation in the Atlantic is also important for preserving ocean ecosystems, affecting oceanic transport nutrient.

The results show that Greenland melt leads to an additional weakening of the AMOC, but its effect is smaller than that of warming and intensification of the water cycle. By years 2090-2100, the AMOC weakens by 18% in an intermediate greenhouse-gas mitigation scenario (RCP4.5), and by 37% under continued high emissions (RCP8.5). Afterwards, the AMOC stabilizes in the former, but continues to decline in the latter to -74% by 2290-2300, with a 44% likelihood of an AMOC collapse.

The results indicate that human greenhouse gas emissions and global warming will cause further AMOC decline during the next centuries and that melting of the Greenland Ice Sheet makes the AMOC more likely to collapse. However, an AMOC collapse can be avoided by CO2 mitigation if global warming stays below 4-5°C. These results are in contrast to the recent modeling study by Hansen et al. (2016), which has been reported widely in the press and suggested earlier and more dramatic effects of Greenland melt on the AMOC because of their use of much larger, and in our opinion unrealistic, meltwater fluxes from the Greenland Ice Sheet.

 

The article has been reviewed in EOS - Earth and Space Science News in Major Ocean Circulation Pattern at Risk from Greenland Ice Melt by Emily Underwood.

 

A popular summary in Danish Golfstrømmens skæbne i et varmere klima is published in Aktuel Videnskap, Denmark.

 

Citation: Bakker, P., A. Schmittner, J. T. M. Lenaerts, A. Abe‐Ouchi, D. Bi, M. R. van den Broeke, W.‐L. Chan, A. Hu, R. L. Beadling, S. J. Marsland, S. H. Mernild, O. A. Saenko, D. Swingedouw, A. Sullivan, J. Yin (2016); Fate of the Atlantic Meridional Overturning Circulation – Strong decline under continued warming and Greenland melting, Geophys. Res. Lett., 43, doi:10.1002/2016GL070457.

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