Mar 23, 2015

Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation

by
Stefan Rahmstorf, Jason E. Box, Georg Feulner, Michael E. Mann, Alexander Robinson, Scott Rutherford & Erik J. Schaffernicht
,
Nature Climate Change

What is new is that we have used proxy reconstructions of large-scale surface temperature...previously published by one of us (study co-author Mike Mann) that extend back to 900 AD...to estimate the circulation (AMOC) intensity over the entire last 1100 years...This shows that despite the substantial uncertainties in the proxy reconstruction, the weakness of the flow after 1975 is unique in more than a thousand years, with at least 99 per cent probability. This strongly suggests that the weak overturning is not due to natural variability but rather a result of global warming.

Lead author, Stefan Rahmstorf

  • States that possible changes in Atlantic meridional overturning circulation (AMOC) provide a key source of uncertainty regarding future climate change
  • States that maps of temperature trends over the twentieth century show a conspicuous region of cooling in the northern Atlantic
  • Presents multiple lines of evidence suggesting that this cooling may be due to a reduction in the AMOC over the twentieth century and particularly after 1970
  • States that since 1990 the AMOC seems to have partly recovered—this time evolution is consistently suggested by an AMOC index based on sea surface temperatures, by the hemispheric temperature difference, by coral-based proxies and by oceanic measurements
  • Discusses a possible contribution of the melting of the Greenland Ice Sheet to the slowdown
  • Uses a multi-proxy temperature reconstruction for the AMOC index which suggests that the AMOC weakness after 1975 is an unprecedented event in the past millennium (p > 0.99)
  • Research suggests melting of Greenland in the coming decades could contribute to further weakening of the AMOC