NOAA GFDL - Geophysical Fluid Dynamics LaboratoryGFDL - Geophysical Fluid Dynamics Laboratory

A weakened AMOC may prolong greenhouse gas induced Mediterranean drying even with significant and rapid climate change mitigation

August 22nd, 2022

Key Findings

  • Multiple large ensembles of climate simulations are used to explore 21st century climate response to greenhouse gases, including the response to significant reductions in greenhouse gas emissions after 2040. A decline in winter precipitation over the Mediterranean is a robust response to global warming across climate models.
  • The drying trend persists even with substantial reductions in greenhouse gas emissions after 2040. This seemingly contradictory behavior is related to changes in Atlantic Ocean circulation.
  • The Atlantic Meridional Overturning Circulation weakens in response to increasing greenhouse gases, and stays in a weakened state through the year 2100 even with greenhouse gas reductions, leading directly to the continued decline of Mediterranean winter precipitation.
  • Some impacts of climate change may not be quickly reversed even with substantial reductions in greenhouse gas emissions.

Thomas L. Delworth, William F. Cooke, Vaishali Naik, David Paynter, and Liping Zhang. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.2116655119

Multiple large ensembles of climate simulations are used to explore 21st century climate response to greenhouse gases, including the response to significant reductions in greenhouse gas emissions after 2040. The authors explore the extent to which future climate changes could be reversed by reduced emissions of greenhouse gases.

A decline in winter precipitation over the Mediterranean is a robust response to global warming across climate models, with significant impacts on agriculture and water resources. This study finds that in response to greenhouse gas reductions, the Mediterranean summer rainfall decline is reversed, but the winter rainfall decline continues. The winter Mediterranean drying trend persists even with substantial reductions in greenhouse gas emissions after 2040. This seemingly contradictory behavior is related to changes in Atlantic Ocean circulation. The Atlantic Meridional Overturning Circulation (AMOC) weakens in response to increasing greenhouse gases and stays in a weakened state through the year 2100, even with greenhouse gas reductions. The persistently weakened AMOC impacts atmospheric circulation in climatically important ways, leading to prolonged Mediterranean drying in the winter. This highlights the risk that some important climate changes may not be easily reversed even with substantial reductions in greenhouse gas emissions.

The authors use large ensembles of simulations with GFDL’s SPEAR high-resolution coupled climate model to better distinguish the response of the system to changing radiative forcing from natural variability. This research shows that some elements of the climate system respond rapidly and directly to changes in greenhouse gas concentrations, such as global mean temperature, global mean precipitation, and Arctic sea ice. Other elements of the climate system have more complex behavior, with long delay times and possible threshold-like behavior. Some impacts of climate change may not be easily reversible, even with substantial reductions in greenhouse gas emissions. This study provides information on possible future climates and whether climate changes may be reversed with emissions reductions.

Figure: This figure shows an index of the strength of the Atlantic Meridional Overturning Circulation (AMOC), units are Sverdrups (Sv; 1 Sv = 106 m3s-1). This indicates how much water (in cubic meters per second) is flowing northward in the upper layers of the North Atlantic at (a) 45oN and (b) 26oN. The colored lines show various model simulations, while the magenta line in (b) shows observations at 26oN. Downward trends show weakening of the AMOC. The thick solid lines indicate ensemble means, while the shaded regions indicate the range of the ensemble members. The SSP534OS simulation shows a declining and persistently weakened AMOC, despite substantial reductions in greenhouse emissions after 2040. This persistently weak AMOC prolongs the Mediterranean drying.