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Historic Trends in U. S. Drought Forcing in a Warming Climate


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      The mean North American and world climates have warmed significantly since the beginning of climatologically significant anthropogenic emission of greenhouse gases in the 19th Century. It has been suggested that warming may increase the frequency or severity of droughts. We define and study the statistics of an aridity index that describes the precipitation forcing function of a drought, considering drought to be a season with low enough precipitation to be significant for agriculture. Our aridity index is a reciprocal function of the seasonal precipitation, which is more significant for agriculture than mean precipitation. Using NOAA data from sites in 13 diverse climate regimes in the 48 contiguous United States with time series running over the period 1940--1999 but including two data series from 1900 or 1910, and computing their decadal averages, we search for linear trends in their aridity indices. We find no linear trends significant at the \(2\sigma\) level. At five sites \(3\sigma\) upper bounds on any systematic trends are in the range 1.0--2.8%/decade, while at two sites \(3\sigma\) lower bounds are -0.5%/decade and -2.2%/decade; at other sites the bounds are less restrictive.

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      Constraints on future changes in climate and the hydrologic cycle.

      What can we say about changes in the hydrologic cycle on 50-year timescales when we cannot predict rainfall next week? Eventually, perhaps, a great deal: the overall climate response to increasing atmospheric concentrations of greenhouse gases may prove much simpler and more predictable than the chaos of short-term weather. Quantifying the diversity of possible responses is essential for any objective, probability-based climate forecast, and this task will require a new generation of climate modelling experiments, systematically exploring the range of model behaviour that is consistent with observations. It will be substantially harder to quantify the range of possible changes in the hydrologic cycle than in global-mean temperature, both because the observations are less complete and because the physical constraints are weaker.

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        28 January 2014

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        11 pages, 2 figures physics.soc-ph


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