Book 2.indb - US Climate Change Science Program

4CHAPTERAbrupt Climate ChangeThe Potential for Abrupt Changein the Atlantic MeridionalOverturning CirculationLead Author: Thomas L. Delworth,* NOAA Geophysical FluidDynamics LaboratoryContributing Authors: Peter U. Clark,* Department of Geosciences,Oregon State UniversityMarika Holland, National Center for Atmospheric ResearchWilliam E. johns, Rosenstiel School of Marine and AtmosphericScience, University of MiamiTill kuhlbrodt, Department of Meteorology, NCAS-Climate,University of Reading, United Kingdomjean Lynch-Stieglitz, School of Earth and Atmospheric Sciences,Georgia Institute of TechnologyCarrie Morrill,* Cooperative Institute for Research in EnvironmentalSciences, University of Colorado, and NOAA National Climatic DataCenterRichard Seager,* Lamont-Doherty Earth Observatory, ColumbiaUniversityAndrew j. Weaver,* School of Earth and Ocean Sciences, Universityof Victoria, CanadaRong Zhang, NOAA Geophysical Fluid Dynamics Laboratory* SAP 3.4 Federal Advisory Committee memberkEy FINDINGSThe Atlantic Meridional Overturning Circulation (AMOC) is an important component of the Earth’s climatesystem, characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and asouthward flow of colder water in the deep Atlantic. This ocean circulation system transports a substantialamount of heat from the Tropics and Southern Hemisphere toward the North Atlantic, where the heat istransferred to the atmosphere. Changes in this circulation have a profound impact on the global climatesystem, as indicated by paleoclimate records. These include, for example, changes in African and Indianmonsoon rainfall, atmospheric circulation of relevance to hurricanes, and climate over North America andWestern Europe. In this chapter, we have assessed what we know about the AMOC and the likelihood offuture changes in the AMOC in response to increasing greenhouse gases, including the possibility of abruptchange. We have five primary findings:• It is very likely that the strength of the AMOC will decrease over the course of the 21st century inresponse to increasing greenhouse gases, with a best estimate decrease of 25–30%.• Even with the projected moderate AMOC weakening, it is still very likely that on multidecadal tocentury time scales a warming trend will occur over most of the European region downstream of theNorth Atlantic Current in response to increasing greenhouse gases, as well as over North America.• No current comprehensive climate model projects that the AMOC will abruptly weaken or collapse inthe 21st century. We therefore conclude that such an event is very unlikely. Further, an abrupt collapseof the AMOC would require either a sensitivity of the AMOC to forcing that is far greater than currentmodels suggest or a forcing that greatly exceeds even the most aggressive of current projections (suchas extremely rapid melting of the Greenland ice sheet). However, we cannot completely exclude eitherpossibility.117