Major Uncertainties in the Earth System: Using the Present to Help Predict the Future

Radiative forcing of climate over the next century may change by as much as it did during 100 centuries of deglaciation, with potentially catastrophic results for socioeconomic systems. Predicting these changes is difficult because both human and biogeophysical systems that control the forcing are themselves difficult to predict. Earth system models that simulate a broader range of climate forcing are now becoming available, but some of the most influential processes are much less constrained by observations than meteorological and physical ocean processes that have traditionally been included, so uncertainties in future climate (even for a given level of fossil fuel combustion) remains high. Three particular areas of significant uncertainty that have emerged are (1) the strength of positive feedback among circulation, drought, and forest dieback in the tropics; (2) the degree of saturation of terrestrial carbon sinks in the middle latitudes; and (3) the vulnerability of frozen organic matter in high northern latitudes. Earth system models include these effects, but lack strong observational support. Clever use of field campaigns, manipulative experiments, and diagnostic modeling is required to improve confidence in future radiative forcing, which is now dominated by these poorly-constrained mechanisms.