IPCC Report.pdf - Adam Curry

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentExecutive SummaryThis chapter addresses changes in weather and climate events relevant to extreme impacts and disasters.An extreme (weather or climate) event is generally defined as the occurrence of a value of a weather or climatevariable above (or below) a threshold value near the upper (or lower) ends (‘tails’) of the range of observed values ofthe variable. Some climate extremes (e.g., droughts, floods) may be the result of an accumulation of weather or climateevents that are, individually, not extreme themselves (though their accumulation is extreme). As well, weather orclimate events, even if not extreme in a statistical sense, can still lead to extreme conditions or impacts, either bycrossing a critical threshold in a social, ecological, or physical system, or by occurring simultaneously with otherevents. A weather system such as a tropical cyclone can have an extreme impact, depending on where and when itapproaches landfall, even if the specific cyclone is not extreme relative to other tropical cyclones. Conversely, not allextremes necessarily lead to serious impacts. [3.1]Many weather and climate extremes are the result of natural climate variability (including phenomenasuch as El Niño), and natural decadal or multi-decadal variations in the climate provide the backdrop foranthropogenic climate changes. Even if there were no anthropogenic changes in climate, a wide variety of naturalweather and climate extremes would still occur. [3.1]A changing climate leads to changes in the frequency, intensity, spatial extent, duration, and timing ofweather and climate extremes, and can result in unprecedented extremes. Changes in extremes can also bedirectly related to changes in mean climate, because mean future conditions in some variables are projected to liewithin the tails of present-day conditions. Nevertheless, changes in extremes of a climate or weather variable are notalways related in a simple way to changes in the mean of the same variable, and in some cases can be of oppositesign to a change in the mean of the variable. Changes in phenomena such as the El Niño-Southern Oscillation ormonsoons could affect the frequency and intensity of extremes in several regions simultaneously. [3.1]Many factors affect confidence in observed and projected changes in extremes. Our confidence in observedchanges in extremes depends on the quality and quantity of available data and the availability of studies analyzingthese data. It consequently varies between regions and for different extremes. Similarly, our confidence in projectingchanges (including the direction and magnitude of changes in extremes) varies with the type of extreme, as well asthe considered region and season, depending on the amount and quality of relevant observational data and modelprojections, the level of understanding of the underlying processes, and the reliability of their simulation in models(assessed from expert judgment, model validation, and model agreement). Global-scale trends in a specific extrememay be either more reliable (e.g., for temperature extremes) or less reliable (e.g., for droughts) than some regionalscaletrends, depending on the geographical uniformity of the trends in the specific extreme. ‘Low confidence’ inobserved or projected changes in a specific extreme neither implies nor excludes the possibility of changes in thisextreme. [3.1.5, 3.1.6, 3.2.3; Box 3-2; Figures 3-3, 3-4, 3-5, 3-6, 3-7, 3-8, 3-10]There is evidence from observations gathered since 1950 of change in some extremes. It is very likely thatthere has been an overall decrease in the number of cold days and nights, and an overall increase in the number ofwarm days and nights, at the global scale, that is, for most land areas with sufficient data. It is likely that these changeshave also occurred at the continental scale in North America, Europe, and Australia. There is medium confidence of awarming trend in daily temperature extremes in much of Asia. Confidence in observed trends in daily temperatureextremes in Africa and South America generally varies from low to medium depending on the region. Globally, in many(but not all) regions with sufficient data there is medium confidence that the length or number of warm spells or heatwaves has increased since the middle of the 20th century. It is likely that there have been statistically significantincreases in the number of heavy precipitation events (e.g., 95th percentile) in more regions than there have beenstatistically significant decreases, but there are strong regional and subregional variations in the trends. There islow confidence that any observed long-term (i.e., 40 years or more) increases in tropical cyclone activity are robust,after accounting for past changes in observing capabilities. It is likely that there has been a poleward shift in themain Northern and Southern Hemisphere extratropical storm tracks. There is low confidence in observed trends in111