Working Group I: The Scientific Basis

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F.4 Projections of Future Changes in Precipitation

Figure 23: Analysis of inter-model consistency in regional precipitation change. Regions are classified as showing either agreement on increase with an average change of greater than 20% (‘Large increase’), agreement on increase with an average change between 5 and 20% (‘Small increase’), agreement on a change between –5 and +5% or agreement with an average change between –5 and 5% (‘No change’), agreement on decrease with an average change between –5 and -20% (‘Small decrease’), agreement on decrease with an average change of less than -20% (‘Large decrease’), or disagreement (‘Inconsistent sign’). A consistent result from at least seven of the nine models is deemed necessary for agreement. [Based on Chapter 10, Box 1, Figure 2]
Globally averaged water vapour, evaporation and precipitation are projected to increase. At the regional scale both increases and decreases in precipitation are seen. Results (see Figure 23) from recent AOGCM simulations forced with SRES A2 and B2 emissions scenarios indicate that it is likely for precipitation to increase in both summer and winter over high-latitude regions. In winter, increases are also seen over northern mid-latitudes, tropical Africa and Antarctica, and in summer in southern and eastern Asia. Australia, central America, and southern Africa show consistent decreases in winter rainfall.

Based on patterns emerging from a limited number of studies with current AOGCMs, older GCMs, and regionalisation studies, there is a strong correlation between precipitation interannual variability and mean precipitation. Future increases in mean precipitation will likely lead to increases in variability. Conversely, precipitation variability will likely decrease only in areas of reduced mean precipitation.

F.5 Projections of Future Changes in Extreme Events It is only recently that changes in extremes of weather and climate observed to date have been compared to changes projected by models (Table 4). More hot days and heat waves are very likely over nearly all land areas. These increases are projected to be largest mainly in areas where soil moisture decreases occur. Increases in daily minimum temperature are projected to occur over nearly all land areas and are generally larger where snow and ice retreat. Frost days and cold waves are very likely to become fewer. The changes in surface air temperature and surface absolute humidity are projected to result in increases in the heat index (which is a measure of the combined effects of temperature and moisture). The increases in surface air temperature are also projected to result in an increase in the “cooling degree days” (which is a measure of the amount of cooling required on a given day once the temperature exceeds a given threshold) and a decrease in “heating degree days”. Precipitation extremes are projected to increase more than the mean and the intensity of precipitation events are projected to increase. The frequency of extreme precipitation events is projected to increase almost everywhere. There is projected to be a general drying of the mid-continental areas during summer. This is ascribed to a combination of increased temperature and potential evaporation that is not balanced by increases of precipitation. There is little agreement yet among models concerning future changes in mid-latitude storm intensity, frequency, and variability. There is little consistent evidence that shows changes in the projected frequency of tropical cyclones and areas of formation. However, some measures of intensities show projected increases, and some theoretical and modelling studies suggest that the upper limit of these intensities could increase. Mean and peak precipitation intensities from tropical cyclones are likely to increase appreciably.

For some other extreme phenomena, many of which may have important impacts on the environment and society, there is currently insufficient information to assess recent trends, and confidence in models and understanding is inadequate to make firm projections. In particular, very small-scale phenomena such as thunderstorms, tornadoes, hail, and lightning are not simulated in global models. Insufficient analysis has occurred of how extra-tropical cyclones may change.

Table 4: Estimates of confidence in observed and projected changes in extreme weather and climate events. The table depicts an assessment of confidence in observed changes in extremes of weather and climate during the latter half of the 20th century (left column) and in projected changes during the 21st century (right column)a. This assessment relies on observational and modelling studies, as well as physical plausibility of future projections across all commonly used scenarios and is based on expert judgement (see Footnote 4). [Based upon Table 9.6]
Confidence in observed changes (latter half of the 20th century) Changes in Phenomenon Confidence in projected changes (during the 21st century)
Likely Higher maximum temperatures and more hot days over nearly all land areas Very likely
Very likely Higher minimum temperatures, fewer cold days and frost days over nearly all land areas Very likely
Very likely Reduced diurnal temperature range over most land areas Very likely
Likely, over many areas Increase of heat index8 over land areas Very likely, over most areas
Likely, over many Northern Hemisphere mid- to high latitude land areas More intense precipitation eventsb Very likely, over many areas
Likely, in a few areas Increased summer continental drying and associated risk of drought Likely, over most mid-latitude continental interiors (Lack of consistent projections in other areas)
Not observed in the few analyses available Increase in tropical cyclone peak wind intensitiesc Likely, over some areas
Insufficient data for assessment Increase in tropical cyclone mean and peak precipitation intensitiesc Likely, over some areas
a For more details see Chapter 2 (observations) and Chapters 9, 10 (projections).
b For other areas there are either insufficient data or conflicting analyses.
c Past and future changes in tropical cyclone location and frequency are uncertain.

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