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

Published date June 1, 2008

Weather and Climate Extremes in a Changing Climate


  • 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