Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangeEXECUTIVE SUMMARYLead Authors: Peter U. Clark,* Department of Geosciences,Oregon State UniversityAndrew j. Weaver,* School of Earth and Ocean Sciences,University of Victoria, CanadaContributing Authors: Edward Brook,* Department ofGeosciences, Oregon State UniversityEdward R. Cook,* Lamont-Doherty Earth Observatory,Columbia UniversityThomas L. Delworth,* NOAA Geophysical Fluid DynamicsLaboratorykonrad Steffen,* Cooperative Institute for Research inEnvironmental Sciences, University of Colorado* SAP 3.4 Federal Advisory Committee memberMAIN RESULTS AND FINDINGSFor this Synthesis and Assessment Report,abrupt climate change is defined as:A large-scale change in the climatesystem that takes place over a fewdecades or less, persists (or is anticipatedto persist) for at least a few decades,and causes substantial disruptions inhuman and natural systems.This report considers progress in understandingfour types of abrupt change in the paleoclimaticrecord that stand out as being so rapid and largein their impact that if they were to recur, theywould pose clear risks to society in terms ofour ability to adapt: (1) rapid change in glaciers,ice sheets, and hence sea level; (2) widespreadand sustained changes to the hydrologic cycle;(3) abrupt change in the northward flow ofwarm, salty water in the upper layers of theAtlantic Ocean associated with the AtlanticMeridional Overturning Circulation (AMOC);and (4) rapid release to the atmosphere of methanetrapped in permafrost and on continentalmargins. While these four types of changepose clear risks to human and natural systems,this report does not focus on specific effectson these systems as a result of abrupt change.This report reflects the significant progress inunderstanding abrupt climate change that hasbeen made since the report by the NationalResearch Council in 2002 on this topic, andthis report provides considerably greater detailand insight on these issues than did the 2007Intergovernmental Panel on Climate Change(IPCC) Fourth Assessment Report (AR4).New paleoclimatic reconstructions have beendeveloped that provide greater understandingof patterns and mechanisms of past abrupt climatechange in the ocean and on land, and newobservations are further revealing unanticipatedrapid dynamic changes of modern glaciers, icesheets, and ice shelves as well as processes thatare contributing to these changes. This reportreviews this progress. A summary and explanationof the main results is presented first,followed by an overview of the types of abruptclimate change considered in this report. Thesubsequent chapters then address each of thesetypes of abrupt climate change, including asynthesis of the current state of knowledge andan assessment of the likelihood that one of theseabrupt changes may occur in response to humaninfluences on the climate system. Throughoutthis report we have adopted the IPCC terminologyin our expert assessment of the likelihoodof a particular outcome or result. The termvirtually certain implies a >99% probability;extremely likely: >95% probability; very likely:>90% probability; likely: >66% probability;more likely than not: >50% probability; aboutas likely as not: 33%–66% probability; unlikely: