Book 2.indb - US Climate Change Science Program

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The U.S. Climate Change Science Program Chapter 4We currently lack along-term, sustainedobserving systemfor the AMOC.of Greenland as an upper-bound estimate ofpotential external freshwater forcing from themelting of the Greenland ice sheet. Under theSRES A1B scenario they, too, only found aweakening of the AMOC with a subsequentrecovery in its strength. They concluded thatGreenland ice sheet melting would not causeabrupt climate change in the 21st century.Based on our analysis, we conclude that it isvery likely that the strength of the AMOC willdecrease over the course of the 21st century.Both weighted and unweighted multimodelensemble averages under an SRES A1B futureemission scenario suggest a best estimateof 25–30% reduction in the overall AMOCstrength. Associated with this reduction is thepossible cessation of LSW water formation. Inmodels where the AMOC weakens, warmingstill occurs downstream over Europe due tothe radiative forcing associated with increasinggreenhouse gases (Gregory et al., 2005; Stoufferet al., 2006). No model under idealized (1%/yearor 2%/year increase) or SRES scenario forcingexhibits an abrupt collapse of the AMOCduring the 21st century, even accounting forestimates of accelerated Greenland ice sheetmelting. We conclude that it is very unlikely thatthe AMOC will undergo an abrupt transitionduring the course of the 21st century. Basedon available model simulations and sensitivityanalyses, estimates of maximum Greenland icesheet melting rates, and our understanding ofmechanisms of abrupt climate change from thepaleoclimate record, we further conclude it isunlikely that the AMOC will collapse beyondthe end of the 21st century as a consequence ofglobal warming, although the possibility cannotbe entirely excluded.8. What Are the Observationaland ModelingRequirements NecessaryTo Understand the OverturningCirculation andEvaluate Future Change?It has been shown in this chapter that theAMOC plays a vital role in the climate system.In order to more confidently predict futurechanges—especially the possibility of abruptchange—we need to better understand theAMOC and the mechanisms governing itsvariability and sensitivity to forcing changes.Improved understanding of the AMOC comesat the interface between observational andtheoretical studies. In that context, theories canbe tested, oftentimes using numerical models,against the best available observational data.The observational data can come from themodern era or from proxy indicators of pastclimates.We describe in this section a suite of activitiesthat are necessary to increase our understandingof the AMOC and to more confidently predictits future behavior. While the activities arenoted in separate categories, the true advancesin understanding—leading to a predictivecapability—come in the synthesis of the variousactivities described below, particularly inthe synthesis of modeling and observationalanalyses.8.1 Sustained Modern ObservingSystemWe currently lack a long-term, sustainedobserving system for the AMOC. Without thisin place, our ability to detect and predict futurechanges of the AMOC—and their impacts—isvery limited. The RAPID project may beviewed as a prototype for such an observingsystem. The following set of activities istherefore needed:• Research to delineate what would constitutean efficient, robust observational networkfor the AMOC. This could include studies inwhich model results are sampled accordingto differing observational networks, therebyevaluating the utility of those networks forobserving the AMOC and guiding the developmentof new observational networks andthe enhancement of existing observationalnetworks.• Sustained deployment over decades of theobservational network identified above torobustly measure the AMOC. This wouldlikely include observations of key processesinvolved in deep water formation in theLabrador and Norwegian Seas, and theircommunication with the rest of the Atlantic(e.g., U.S. CLIVAR AMOC Planning Team,2007).160
Abrupt Climate Change• Focused observational programs as part ofprocess studies to improve understandingof physical processes of importance to theAMOC, such as ocean-atmosphere coupling,mixing processes, and deep overflows. Theseshould lead to improved representation ofsuch processes in numerical models.8.2 Acquisition and Interpretation ofPaleoclimate DataWhile the above stresses current observations,much can be learned from the study of ancientclimates that provide insight into the past behaviorof the AMOC. We need to develop paleoclimatedatasets that allow robust, quantitativereconstructions of past ocean circulations andtheir climatic impacts. Therefore, the followingset of activities is needed:• Acquisition and analysis of high-resolutionrecords from the Holocene that can provideinsight on decadal to centennial time scalesof AMOC-related climate variability. This isan important baseline against which to judgefuture change.• Acquisition and analysis of paleoclimaterecords to document past changes in theAMOC, including both glacial and nonglacialconditions. These will provide a morerobust measure of the response of the AMOCto changing radiative forcing and will allownew tests of models. Our confidencein predictions of future AMOC changes isenhanced to the extent that models faithfullysimulate such past AMOC changes.• More detailed assessment of the past relationshipbetween AMOC and climate, especiallythe role of AMOC changes in abrupt climatechange.• Acquisition and analysis of paleoclimaterecords that can provide improved estimatesof past changes in meltwater forcing.This information can lead to improvedunderstanding of the AMOC response tofreshwater input and can help to betterconstrain models.8.3 Improvement and Use of ModelsModels provide our best tools for predicting futurechanges in the AMOC and are an importantpathway toward increasing our understandingof the AMOC, its variability, and its sensitivityto change. Such insights are limited, however,by the fidelity of the models employed. There isan urgent need both to (1) improve the modelswe use and (2) use models in innovative waysto increase our understanding of the AMOC.Therefore, the following set of activities isneeded:• Development of models with increasedresolution in order to more faithfully representthe small-scale processes that areimportant for the AMOC. The models usedfor the IPCC AR4 assessment had oceanicresolution on the order of 50–100 km in thehorizontal, with 30–50 levels in the vertical.In reality, processes with spatial scales ofseveral kilometers (or less) are importantfor the AMOC.• Development of models with improvednumerics and physics, especially those thatappear to influence the AMOC. In particular,there is a need for improved representationof small-scale processes that significantlyimpact the AMOC. For example, overflowsof dense water over sills in the North Atlanticare an important feature for the AMOC, andtheir representation in models needs to beimproved.• Development of advanced models of landbasedice sheets, and their incorporation inclimate models. This is particulary crucialin light of uncertainties in the interactionbetween the AMOC and land-based icesheets on long time scales.• Design and execution of innovative numericalexperiments in order to (1) shed light onthe mechanisms governing variability andchange of the AMOC, (2) estimate theinherent predictability of the AMOC, and(3) develop methods to realize that predictability.The use of multimodel ensembles isparticularly important.• Development and use of improved data assimilationsystems for providing estimatesof the current and past states of the AMOC,as well as initial conditions for prediction ofthe future evolution of the AMOC.• Development of prototype prediction systemsfor the AMOC. These predictionsystems will start from the observed state ofthe AMOC and use the best possible models,together with projections of future changes inModels provideour best tools forpredicting futurechanges in theAMOC and are animportant pathwaytoward increasingour understandingof the AMOC, itsvariability, and itssensitivity tochange.161
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TABLE OF CONTENTSAuthor Team for th
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The U.S. Climate Change Science Pro
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PREFACEReport Motivation and Guidan
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1CHAPTERAbrupt Climate ChangeIntrod
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Abrupt Climate Change-35Arctic temp
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Abrupt Climate ChangeFigure 1.2. Po
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Abrupt Climate Changewell below sea
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Abrupt Climate Changeheterogeneous
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Abrupt Climate Changeapart from dro
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Abrupt Climate Change6. Abrupt Chan
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Abrupt Climate Changeintermediate c
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Abrupt Climate Changeper square met
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Abrupt Climate Changeis the norther
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Abrupt Climate ChangeRegardless how
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Abrupt Climate Changecentrations, a
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Abrupt Climate ChangeBox 2.1. Glaci
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Abrupt Climate Changetime. Degree-d
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Abrupt Climate Changetion, and accu
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Abrupt Climate ChangeBox 2.2. Mass
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Abrupt Climate Changeof the glacier
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Abrupt Climate ChangeFigure 2.6. Ra
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Abrupt Climate Changewater) or, mor
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Abrupt Climate ChangeFigure 2.7. Re
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Abrupt Climate Changeresults in a l
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Abrupt Climate Changeis not providi
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Abrupt Climate Changemeasurements c
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Abrupt Climate Changemore than 10,0
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Abrupt Climate Changeocean models h
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Abrupt Climate Changeto atmosphere-
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3CHAPTERHydrological Variability an
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Abrupt Climate Change• The integr
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Abrupt Climate Changethe soil (King
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Abrupt Climate Changeon (Kaplan et
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Abrupt Climate Changeorbital-time-s
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Abrupt Climate ChangeFigure 3.4. (t
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Abrupt Climate Changemodels forced
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Abrupt Climate ChangeBox 3.2. Waves
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Abrupt Climate Changebecause (1) th
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Abrupt Climate ChangeHarrison et al
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Abrupt Climate Changethat even the
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Abrupt Climate Changethat seen afte
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Abrupt Climate Changeentire period
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Abrupt Climate Changelong-lived gre
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Abrupt Climate Changeplanetary-scal
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Abrupt Climate ChangeBox 3.3. Paleo
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Abrupt Climate Changerelated to the
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Abrupt Climate Changeinto the North
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Abrupt Climate ChangeThe summertime
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Abrupt Climate Change(Anderson et a
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Abrupt Climate ChangeHere the model
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U.S. Climate Change Science Program
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