TABLE SA-1Observed Changes in North American Extreme Events, Assessment of Human Influence for the Observed Changes, and Likelihood That the Changes Will Continue Through the Twenty-first Centurya

Phenomenon and direction of changeWhere and when these changes occurred in past 50 yearsLinkage of human activity to observed changesLikelihood of continued future changes in this century
Warmer and fewer cold days and nightsOver most land areas, the last 10 years had lower numbers of severe cold snaps than any other 10-year periodLikely warmer extreme cold days and nights and fewer frostsbVery likelyd
Hotter and more frequent hot days and nightsOver most of North AmericaLikely for warmer nightsbVery likelyd
More frequent heat waves and warm spellsOver most land areas, most pronounced over northwestern two-thirds of North AmericaLikely for certain aspects, e.g., night-time temperatures; and linkage to record high annual temperaturebVery likelyd
More frequent and intense heavy downpours and higher proportion of total rainfall in heavy precipitation eventsOver many areasLinked indirectly through increased water vapor, a critical factor for heavy precipitation eventscVery likelyd
Increases in area affected by droughtNo overall average change for North America, but regional changes are evidentLikely, southwest USA.c Evidence that 1930s and 1950s droughts were linked to natural patterns of sea surface temperature variabilityLikely in Southwest USA, parts of Mexico, and Carribeand
More intense hurricanesSubstantial increase in Atlantic since 1970; likely increase in Atlantic since 1950s; increasing tendency in W. Pacific and decreasing tendency in E. Pacific (Mexico West Coast) since 1980eLinked indirectly through increasing sea surface temperature, a critical factor for intense hurricanes;e a confident assessment requires further studycLikelyd

Based on frequently used family of IPCC emission scenarios.


Based on formal attribution studies and expert judgment.


Based on expert judgment.


Based on model projections and expert judgment.


As measured by the Power Dissipation Index (which combines storm intensity, duration, and frequency).

From: Summary and Assessment

Cover of Global Climate Change and Extreme Weather Events
Global Climate Change and Extreme Weather Events: Understanding the Contributions to Infectious Disease Emergence: Workshop Summary.
Institute of Medicine (US) Forum on Microbial Threats.
Washington (DC): National Academies Press (US); 2008.
Copyright © 2008, National Academy of Sciences.

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