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mate model simulation and found that the most extreme precipitation rates
increased at a rate nearly double that inferred from the roughly 7 percent
per degree C Clausius-Clapyron rate of increase in atmospheric moisture
content for constant relative humidity. The regional climate model gener-
ally produced changes in precipitation extremes that roughly matched those
inferred from observations, i.e., approximately 14% increase per degree C
warming.
Kharin et al. (2007) compare 20-year return period precipitation amounts
simulated by IPCC AR4 model runs globally for mid- and late 21st century
relative to model simulations for 1981-2000. They find that the models' abil-
ity to reproduce current climate precipitation extremes in the extratropics is
plausible, but that there are large differences in the simulations in the trop-
ics, which are compounded by observational issues. On a global basis, the
average rate of increase in 20-year return period precipitation was slightly
less than the Clausius-Clapyron rate of increase in atmospheric water hold-
ing capacity (see Figure 4.10).
Considering the combined literature on the physical basis for changes
in extreme precipitation in a warming climate, along with model results and
observational studies, there is a strong basis for concluding that precipitation
extremes should increase with temperature in most parts of the globe. Model
results are roughly consistent with simple physical arguments, although there
are major issues with model parameterizations that complicate model-based
interpretations in the tropics. Attempts to estimate changes in precipitation
extremes from observations are complicated by large natural variability and
spatial differences, but nonetheless observed changes over the past century
are mostly consistent in sign with the expectation of increases in a warming
climate (see also USCCSP, 2008c; and Section 4.2). We conclude the follow-
ing:
Extreme precipitation is likely to increase as the atmospheric moisture
content increases in a warming climate, with changes likely to be greater
in the tropics than in the extratropics.
Typical magnitudes are 3-10% per
degree C warming, with potentially larger values in the tropics, and in the
most extreme events globally. However, despite general agreement in the
likely direction of future changes, our current understanding of precipitation
extremes is not sufficient to infer the likely magnitudes of future changes
for the large return periods used for infrastructure design. Although these
changes in precipitation extremes could lead to changes in flood frequency,
the linkage between precipitation changes and flooding will be modulated
by interactions between precipitation characteristics and river basin hydrol-
ogy, the nature of which are not yet well known.
