21st Conference on Climate Variability and Change


CCSP 3.3: Causes of Observed Changes in Extremes and Projections of Future Changes

William J. Gutowski Jr., Iowa State University, Ames, IA; and G. Hegerl, G. J. Holland, T. R. Knutson, L. O. Mearns, R. J. Stouffer, P. J. Webster, M. F. Wehner, and F. W. Zwiers

Changes in some weather and climate extremes are attributable to human-induced emissions of greenhouse gases. Human-induced warming has likely caused much of the average temperature increase in North America over the past 50 years. This affects changes in temperature extremes.

Heavy precipitation events averaged over North America have increased over the past 50 years, consistent with the observed increases in atmospheric water vapor, which have been associated with human-induced increases in greenhouse gases.

It is very likely that the human-induced increase in greenhouse gases has contributed to the increase in sea surface temperatures in the hurricane formation regions. Over the past 50 years there has been a strong statistical connection between tropical Atlantic sea surface temperatures and Atlantic hurricane activity as measured by the Power Dissipation Index (which combines storm intensity, duration, and frequency). This evidence suggests a human contribution to recent hurricane activity. However, a confident assessment of human influence on hurricanes will require further studies using models and observations, with emphasis on distinguishing natural from human-induced changes in hurricane activity through their influence on factors such as historical sea surface temperatures, wind shear, and atmospheric vertical stability.

Future changes in extreme temperatures will generally follow changes in average temperature: abnormally hot days and nights and heat waves are very likely to become more frequent; cold days and cold nights are very likely to become much less frequent; the number of days with frost is very likely to decrease.

Droughts are likely to become more frequent and severe in some regions as higher air temperatures increase the potential for evaporation. Over most regions, precipitation is likely to be less frequent but more intense, and precipitation extremes are very likely to increase.

For North Atlantic and North Pacific hurricanes and typhoons, it is likely that hurricane/typhoon wind speeds and core rainfall rates will increase in response to human-caused warming. Analyses of model simulations suggest that for each 1C increase in tropical sea surface temperatures, hurricane surface wind speeds will increase by 1 to 8% and core rainfall rates by 6 to 18%. Frequency changes are currently too uncertain for confident projections. The spatial distribution of hurricanes /typhoons will likely change. Storm surge levels are likely to increase due to projected sea level rise, though the degree of projected increase has not been adequately studied.

There are likely to be more frequent deep low-pressure systems (strong storms) outside the tropics, with stronger winds and more extreme wave heights.

wrf recording  Recorded presentation

Session 11A, Climate Change Science Program Report 3.3
Wednesday, 14 January 2009, 4:00 PM-5:30 PM, Room 129A

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