Weather and Climate Extremes in a Changing Climate

Synopsis

• 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