08 September 2017

Fate of the Atlantic Meridional Overturning Circulation: Strong decline under continued warming and Greenland melting

Science

The most recent IPCC assessment report concludes that the Atlantic Meridional Overturning Circulation (AMOC) could weaken substantially, but is very unlikely to collapse in the 21st century. However, the assessment largely neglected Greenland Ice Sheet (GrIS) mass loss, lacked a comprehensive uncertainty assessment, and was limited to the 21st century. Here, in a community effort, improved estimates of GrIS mass loss are included in multi-centennial projections using state-of-the-science climate models, and an AMOC-emulator provides probabilistic uncertainty assessments. We find that GrIS melting affects AMOC projections, but its effect is smaller than that of warming and intensification of the atmospheric hydrological cycle. By years 2090-2100, the AMOC weakens by 18% (3-34% with 90% probability) in RCP4.5 and by 37% (15-65%) in RCP8.5. Afterwards, it stabilizes in RCP4.5, but continues to decline in RCP8.5 to -74% (+4-100%) by 2290-2300, with a 44% chance of AMOC collapse. This result suggests that an AMOC collapse can be avoided by CO2 mitigation.

Impact

The most recent IPCC assessment report concludes that the Atlantic Meridional Overturning Circulation (AMOC) could weaken substantially, but is very unlikely to collapse in the 21st century. However, the assessment largely neglected Greenland Ice Sheet (GrIS) mass loss, lacked a comprehensive uncertainty assessment, and was limited to the 21st century. Here, in a community effort, improved estimates of GrIS mass loss are included in multi-centennial projections using state-of-the-science climate models, and an AMOC-emulator provides probabilistic uncertainty assessments. We find that GrIS melting affects AMOC projections, but its effect is smaller than that of warming and intensification of the atmospheric hydrological cycle. By years 2090-2100, the AMOC weakens by 18% (3-34% with 90% probability) in RCP4.5 and by 37% (15-65%) in RCP8.5. Afterwards, it stabilizes in RCP4.5, but continues to decline in RCP8.5 to -74% (+4-100%) by 2290-2300, with a 44% chance of AMOC collapse. This result suggests that an AMOC collapse can be avoided by CO2 mitigation.

Summary

The most recent IPCC assessment report concludes that the Atlantic Meridional Overturning Circulation (AMOC) could weaken substantially, but is very unlikely to collapse in the 21st century. However, the assessment largely neglected Greenland Ice Sheet (GrIS) mass loss, lacked a comprehensive uncertainty assessment, and was limited to the 21st century. Here, in a community effort, improved estimates of GrIS mass loss are included in multi-centennial projections using state-of-the-science climate models, and an AMOC-emulator provides probabilistic uncertainty assessments. We find that GrIS melting affects AMOC projections, but its effect is smaller than that of warming and intensification of the atmospheric hydrological cycle. By years 2090-2100, the AMOC weakens by 18% (3-34% with 90% probability) in RCP4.5 and by 37% (15-65%) in RCP8.5. Afterwards, it stabilizes in RCP4.5, but continues to decline in RCP8.5 to -74% (+4-100%) by 2290-2300, with a 44% chance of AMOC collapse. This result suggests that an AMOC collapse can be avoided by CO2 mitigation.

Contact
Aixue Hu
National Center for Atmospheric Research (NCAR)
Publications
Bi, D. "Fate of the Atlantic Meridional Overturning Circulation: Strong decline under continued warming and Greenland melting." Geophysical Research Letters 43, 12,252-12,260 (2017). [10.1002/2016gl070457].