3.2
Towards Attributing Greenhouse Gas Induced Changes In The Water Cycle
Gabriele Hegerl, Duke University, Durham, NC
Studies of the global surface temperature record robustly detect anthropogenic influences on global and large-scale temperature. Changes in the water cycle may have greater impacts than changes in temperature. However, climate models are less skillful in simulating variability and change in rainfall, and patterns of model-simulated changes in annual precipitation are quite model dependent, making external influence on observed precipitation changes difficult to detect. Nevertheless, recently observed zonally averaged rainfall changes are reasonably consistent with multi-model simulations of 20th century rainfall changes. Recent results indicate that evidence for a detectable anthropogenic signal in zonal mean changes in rainfall may be emerging.
Greenhouse warming influences the water cycle by a number of mechanisms. Firstly, a warmer atmosphere holds more moisture; therefore changes in temperature are expected to lead to a global scale increase in rainfall. Warmer temperatures are expected to more strongly influence heavy precipitation than mean precipitation. Observed spatial patterns of change in heavy precipitation events seem to show similarity with changes expected due to a greenhouse warming. However, difficulties in detecting such changes are associated with the different scales represented by station-based data and model simulated rainfall extremes. Also, for physical reasons, global annual mean precipitation responds more strongly to forcing by shortwave radiation than by longwave radiation, consistent with some evidence for a detectable response to volcanic eruptions in global land rainfall. A second mechanism is increased drought with increasing evaporation due to higher temperatures. There is evidence for attributable changes in temperature extremes, although the hottest days of the year seem to show relatively small changes. A further mechanism arises from changes in the atmospheric circulation. Global mean sea level pressure shows detectable anthropogenic changes associated with strengthening of the Northern and Southern Annular modes.
Recorded presentationSession 3, Presidential Forum on Bridging the Studies of Weather and Climate: Part III
Monday, 15 January 2007, 4:00 PM-5:30 PM, Ballroom C2
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