Jun 1, 2008

Weather and Climate Extremes in a Changing Climate

Thomas R. Karl, Gerald A. Meehl, Christopher D. Miller, Susan J. Hassol, Anne M. Waple, and William L. Murray
The U.S. Climate Change Science Program Preface


  • Many extremes and their associated impacts are now changing
    • In recent decades most of North America has been experiencing more unusually hot days and nights, fewer unusually cold days and nights, and fewer frost days
    • Heavy downpours have become more frequent and intense
    • Droughts are becoming more severe in some regions, though there are no clear trends for North America as a whole
    • The power and frequency of Atlantic hurricanes have increased substantially in recent decades, though North American mainland land-falling hurricanes do not appear to have increased over the past century
    • Outside the tropics, storm tracks are shifting northward and the strongest storms are becoming even stronger.
  • It is well established through formal attribution studies that the global warming of the past 50 years is due primarily to human-induced increases in heat-trapping gases
  • The increase in heavy precipitation events is associated with an increase in water vapor, and the latter has been attributed to human-induced warming
  • No formal attribution studies for changes in drought severity in North America have been attempted
  • There is evidence suggesting a human contribution to recent changes in hurricane activity as well as in storms outside the tropics, though a confident assessment will require further study
  • In the future, with continued global warming, heat waves and heavy downpours are very likely to further increase in frequency and intensity
  • Substantial areas of North America are likely to have more frequent droughts of greater severity. Hurricane wind speeds, rainfall intensity, and storm surge levels are likely to increase
  • The strongest cold season storms are likely to become more frequent, with stronger winds and more extreme wave heights
  • Current and future impacts resulting from these changes depend not only on the changes in extremes, but also on responses by human and natural systems