Book 2.indb - US Climate Change Science Program

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