Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 4atmospheric greenhouse gases and aerosols,to make the best possible projections for thefuture behavior of the AMOC. Such a predictionsystem could serve as a warning systemfor an abrupt change in the AMOC.8.4 Projections of Future Changes inRadiative Forcing and Related ImpactsOne of the motivating factors for the study ofAMOC behavior is the possibility of abruptchange in the future driven by increasing greenhousegas concentrations. In order to evaluatethe likelihood of such an abrupt change, itis crucial to have available the best possibleprojections for future changes in radiativeforcing, especially those changes in radiativeforcing due to human activity. This includesnot only greenhouse gases, which tend to bewell mixed and long lived in the atmosphere,but also aerosols, which tend to be shorter livedwith more localized spatial patterns. Thus,realistic projections of aerosol concentrationsand their climatic effects are crucial for AMOCprojections.One of the important controls on the AMOC isthe freshwater flux into the Atlantic, includingthe inflow of freshwater from rivers surroundingthe Arctic. For example, observations (Petersonet al., 2002) have shown an increase duringthe 20th century of Eurasian river dischargeinto the Arctic. For the prediction of AMOCchanges it is crucial to have complete observationsof changes in the high-latitude hydrologiccycle, including precipitation, evaporation, andriver discharge, as well as water released intothe Atlantic from the Greenland ice sheet andfrom glaciers. This topic is discussed moreextensively in Chapter 2.162