Book 2.indb - US Climate Change Science Program

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PREFACEReport Motivation and Guidance for Using thisSynthesis and Assessment ReportLead Authors:John P. McGeehin, USGSJohn A. Barron, USGSDavid M. Anderson, NOAADavid J. Verardo, NSFA primary objective of the U.S. Climate Change ScienceProgram (CCSP) is to provide the best possible,up-to-date scientific information to support publicdiscussion and government and private sector decisionmakingon key climate-related issues. To help meet thisobjective, the CCSP has identified a set of 21 synthesisand assessment products (SAP) to address its highestpriority research, observation, and decision-supportneeds. This SAP (3.4) focuses on abrupt climate changeevents where key aspects of the climate system changefaster than the responsible forcings would suggest and/or faster than society can respond to those changes.This report addresses Goal 3 of the CCSP StrategicPlan: Reduce uncertainty in projections of how theEarth’s climate and related systems may change inthe future. The report (1) summarizes the currentknowledge of key climate parameters that could changeabruptly in the near future, potentially within years todecades and (2) provides scientific information on thesetopics for decision support. As such, the SAP is aimedat both the decision-making audience and the expertscientific and stakeholder community.BackgroundPast records of climate and environmental change derivedfrom archives such as tree rings, ice cores, corals,and sediments indicate that global and regional climatehas experienced repeated abrupt changes, many occurringover a time span of decades or less. Abrupt climatechanges might have a natural cause (such as volcanicaerosol forcing), an anthropogenic cause (such as increasingcarbon dioxide in the atmosphere), or mightbe unforced (related to internal climate variability).Regardless of the cause, abrupt climate change presentspotential risks for society that are poorly understood.An improved ability to understand and model futureabrupt climate change is essential to provide decisionmakerswith the information they need to plan for thesepotentially significant changes.The National Research Council (NRC) report“Abrupt Climate Change” (Alley et al., 2002)provides an excellent treatise on this topic. Additionally,the Intergovernmental Panel on ClimateChange Fourth Assessment Report (IPCC AR4)(IPCC, 2007) addresses many of the same topicsassociated with abrupt climate change. This SAPpicks up where the NRC report and the IPCC AR4leave off, updating the state and strength of existingknowledge, both from the paleoclimate and historicalrecords, as well as from model predictions forfuture change.Focus of this Synthesis and AssessmentProductThe content of this report follows a prospectus thatwas developed by the SAP Product Advisory Group,made up of the co-authors of this preface. The prospectusis available from the CCSP website (http://www.climatescience.gov).SAP 3.4 considers four types of change documentedin the paleoclimate record that stand out as being sorapid and large in their impact that they pose clearrisks to society in terms of our ability to adapt.They are supported by sufficient evidence in currentresearch indicating that abrupt changes could occurin the future. These four topics, each addressed as achapter in this report, are1. Rapid Changes in Glaciers and Ice Sheetsand their Impacts on Sea Level;2. Hydrological Variability and Change;3. Potential for Abrupt Change in theAtlantic Meridional OverturningCirculation (AMOC); and4. Potential for Abrupt Changes inAtmospheric Methane.VIII
Abrupt Climate ChangeThe following questions are considered in this report:Rapid Changes in Glaciers and Ice Sheets and theirImpacts on Sea Level• What is the paleoclimate evidence regarding ratesof rapid ice sheet melting?• What are the recent rates and trends in ice sheetmass balance?• What will be the impact on sea level if the recentlyobserved rapid rates of melting continue?• What is needed to model the mechanical processesthat accelerate ice loss?Hydrological Variability and Change• What is our present understanding of the causes ofmajor drought and hydrological change, including therole of the oceans or other natural or nongreenhousegasanthropogenic effects as well as land-use changes?(Note that this question is posed to facilitate an assessmentof what is known about natural causes forhydrological change as opposed to anthropogeniccauses, such as increased greenhouse gases. Theauthors also address anthropogenic influences, includinggreenhouse gases, as a potential source ofhydrological change, in the past, present, and future.)• What is our present understanding of the duration,extent, and causes of megadroughts of the past2,000 years?• What states of oceanic/atmospheric conditions andthe strength of land-atmosphere coupling are likely tohave been responsible for sustained megadroughts?• How might such a state affect the climate in regionsnot affected by drought? (For example, enhancedfloods or hurricanes in other regions.)• What will be the change in the state of natural variabilityof the ocean and atmosphere that will signalthe abrupt transition to a megadrought?Potential for Abrupt Change in the Atlantic MeridionalOverturning Circulation• What are the factors that control the overturningcirculation?• How well do the current ocean general circulationmodels (and coupled atmosphere-ocean models)simulate the overturning circulation?• What is the present state of the MOC?• What is the evidence for change in the overturningcirculation in the past?• What are the global and regional impacts of achange in the overturning circulation?• What factors that influence the overturning circulationare likely to change in the future, and what is theprobability that the overturning circulation will change?• What are the observational and modeling requirementsrequired to understand the overturning circulationand evaluate future change?Potential for Abrupt Changes in Atmospheric Methane• What is the volume of methane stored in terrestrialand marine sources and how much of it is likely to bereleased in various climate change scenarios?• What is the impact on the climate system of therelease of varying quantities of methane over varyingintervals of time?• What is the evidence in the past for abrupt climatechange caused by massive methane release?• How much methane is likely to be released bythawing of the topmost layer (3 meters) of permafrost?Is thawing at greater depths likely to occur?• What conditions (in terms of sea-level rise andwarming of bottom waters) would allow methanerelease from hydrates in sea-floor sediments?• What are the observational and modeling requirementsnecessary to understand methane storage andits release under various future scenarios of abruptclimate change?Each section of this report is structured to answer thesequestions in the manner that best suits the topic. Questionsare addressed either specifically as individual sections orsubsections of a chapter, or through a broader, more systematicdiscussion of the topic. Additional subject matteris presented in a chapter, beyond what is asked for in theprospectus, where the authors feel that this information isnecessary to effectively treat the topic.It is important to note that the CCSP Synthesis and AssessmentProducts are scientific documents that are intended tobe of use not only to scientists but to the American publicand to decisionmakers within the United States. As such,the geographic focus of the Abrupt Climate Change SAP isUnited States, and by extension, North American climate.Other regional examples of abrupt climate change are discussedwhen the authors feel that the information servesas an important analog to past, present, or future NorthAmerican climate.Suggestions for Reading, Using, andNavigating this ReportThis report is composed of four main chapters that correspondto the major climate themes indicated above. Thereis also an introductory chapter that provides an extensiveoverview of the information from the other four chapters, aswell as additional background information. The ExecutiveSummary further distills the information, with a focus onthe key findings and recommendations from each chapter.IX 11
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3CHAPTERHydrological Variability an
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202REFERENCESChapter 1 ReferencesAl
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U.S. Climate Change Science Program
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