Abstract

A version of the NCAR Community Climate Model (CCM) has been used to carry out a study of climate sensitivity to the size and distribution of continental ice sheets by comparing two perpetual season, fixed sea surface temperature (SST), winter simulations of the climate at a glacial maximum. The two simulations differ only in the size and configuration of the continental ice sheet boundary conditions. These conditions represent minimum and maximum ice sheet extent for a glacial maximum as presently constrained by the geologic evidence. All other boundary conditions are as specified by CLIMAP (1981) for winter surface conditions during the Last Glacial Maximum (LGM). The ice sheets specified by CLIMAP (1981), used by many previous researchers to simulate ice age climate conditions, are intermediate in size between the minimum and maximum ice sheets used for the simulations described here. The maximum case simulation resulted in unexpected positive temperature anomalies over the Northern Hemisphere midlatitudes as well as a more baroclinic atmosphere with stronger Northern Hemisphere thermal gradients, jet streams and storm tracks shifted 5° to 10° poleward. A more distinct split jet stream flow is observed over the maximum case Laurentide ice sheet, and jet stream intensity decreases over the maximum case North Pacific while increasing over the maximum case North Atlantic.

Abstract

A regional coupled soil–vegetation–atmosphere model is used to study changes and interactions between climate and the ecosystem in East Asia due to increased atmospheric CO₂. The largest simulated climate changes are due to the radiative influence of CO₂, modified slightly by vegetation feedbacks. Annual precipitation increases by about 20% in coastal areas of northern China and in central China, but only by 8% in southern China. The strongest warming of up to 4°C occurs in summer in northern China. Generally, the climate tends to be warmer and wetter under doubled CO₂ except for inland areas of northern China, where it becomes warmer and drier. Most of the changes discussed in this paper are associated with changes in the East Asian monsoon, which is intensified under doubled CO₂.

The largest changes and feedbacks between vegetation and climate occur in northern China. In some coastal and central areas around 40°N, temperate deciduous forests expand northward, replacing grassland due to warmer and wetter climate. Evergreen taiga retreats in the coastal northeast, causing extra cooling feedback due to less snow masking. The largest changes occur in extensive inland regions northward of 40°N, where deserts and shrub land expand due to warmer and drier conditions, and water supply is a critical factor for vegetation. These northern inland ecosystems experience considerable degradation and desertification, indicating a marked sensitivity and vulnerability to climatic change.

In support of the U.S. National Assessment of the Potential Consequences of Climate Variability and Change, climate scenarios were prepared to serve as the basis for evaluating the vulnerability of environmental and societal systems to changes projected for the twenty-first century. Since publication of the results of the assessment at the end of 2000, the National Research Council's report Climate Change Science: An Analysis of Some Key Questions, and the U.S. government's U.S. Climate Action Report—2002 have both relied on the assessment's findings. Because of the importance of these findings, it is important to directly address questions regarding the representativeness and usefulness of the model-based projections on which the findings were based. In particular, criticisms have focused on whether the climate models that were relied upon adequately represented twentieth-century conditions and whether their projections of conditions for the twenty-first century were outliers. Reexamination of the approach used in developing and evaluating the climate scenarios indicates that the results from the two primary climate modeling groups that were relied upon allowed the generation of climate scenarios that span much of the range of possible future climatic conditions projected by the larger set of model simulations, which was compiled for the IPCCs Third Assessment Report. With the set of models showing increasing agreement in their simulations of twentieth-century trends in climate and of projected changes in climate on subcontinental to continental scales, the climate scenarios that were generated seem likely to provide a plausible representation of the types of climatic conditions that could be experienced during the twenty-first century. Warming, reduced snow cover, and more intense heavy precipitation events were projected by all models, suggesting such changes are quite likely. However, significant differences remain in the projection of changes in precipitation and of the regional departures in climate from the larger-scale patterns. For this reason, evaluating potential impacts using climate scenarios based on models exhibiting different regional responses is a necessary step to ensuring a representative analysis. Utilizing an even more encompassing set of scenarios in the future could help move from mainly qualitative toward more certain and quantitative conclusions.