Table 1

Summary of analyses of different types of climate extremes, including extremes based on climate statistics and event-driven extremes (see text for explanation). The assessment of extremes here relies on very large scale changes that are physically plausible or representative of changes over many areas. In some regions the changes of certain extremes may not agree with the larger scale changes. Therefore, the assessment here is a general one where observed and model changes appear to be representative and physically consistent with a majority of changes globally. Additionally, certain changes in observed extremes may not have been specifically itemized from model simulations, but are physically consistent with changes of related extremes in the future climate experiments and are denoted as such. The definitions of the uncertainty estimates for the possibility of changes in extremes differ between observations and models. For observations they are based on the following probability ranges: Virtually certain, >99%; Very likely, 90 to 99%; Likely, 67 to 90%; Possible, 33 to 66%; Unlikely, 10 to 33%; Very unlikely, 1 to 10%; Improbable, <1%. For models they are based on the following degree of model agreement or physically plausibility: Virtually certain, many models have been analyzed for this change and all show it; Very likely, a number of models that have been analyzed have shown such a change, or that change is physically plausible and can readily be shown for a larger group of models; Likely, some models that have been analyzed have shown such a change, or the change is physically plausible and could be shown for a larger group of models; Possible, only a few models have shown such a change, it is not physically obvious that such a change should occur, or the results from analyses from various models are mixed; Unlikely, some models that have been analyzed have shown that such a change specifically did not occur, or it is physically implausible and could be shown for a larger group of models; Very unlikely, a number of models that have been analyzed have not shown such a change, or that change is physically implausible and could readily be shown for a larger group of models; Improbable, many models have been analyzed for this change and none show it. Note that changes in observations have already occurred, and the changes from models are projected to occur mainly as a result of increases in GHGs. Thus, where the observed changes agree with the models, they are qualitatively consistent with climate changes expected from increasing GHGs.

Observed (20th century)Modeling (end of 21st century)
Simple extremes based on climate statistics
Higher maximum temperaturesVery likelyVery likely
More hot summer daysLikelyVery likely
Increase in heat IndexLikelyVery likely
Higher minimum temperaturesVirtually certainVery likely
Fewer frost days (higher minimum temperatures)Virtually certainLikely*
More heavy 1-day precipitationLikelyVery likely
Events (increased intensity of precipitation events)
More heavy multiday precipitation events (increased intensity of precipitation events)LikelyVery likely
Complex event-driven climate extremes
More heat waves PossibleVery likely* (higher maximum temperatures)
Fewer cold waves Very likelyVery likely* (higher minimum temperatures)
More droughtUnlikelyVery likely (reduced mid-latitude summer soil moisture)
More wet spells LikelyLikely (increased precipitation at mid- and high latitudes in winter)
More tropical stormsUnlikelyPossible
More intense tropical stormsUnlikelyPossible
More intense mid-latitude stormsPossiblePossible
More intense El Niño eventsPossiblePossible
More common El Niño–like conditionsLikelyLikely*
  • * No direct model analyses, but these changes are physically plausible on the basis of other simulated model changes; comparable changes simulated by the models are noted in parentheses.