Abstract
Human-caused climate change can affect weather and climate extremes, as well as mean climate properties. Analysis of observations and climate model results shows that previously rare (5th percentile) summertime average temperatures are presently occurring with greatly increased frequency in some regions of the 48 contiguous United States. Broad agreement between observations and a mean of results based upon 16 global climate models suggests that this result is more consistent with the consequences of increasing greenhouse gas concentrations than with the effects of natural climate variability. This conclusion is further supported by a statistical analysis based on resampling of observations and model output. The same climate models project that the prevalence of previously extreme summer temperatures will continue to increase, occurring in well over 50% of summers by mid-century.
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Acknowledgements
Claudia Tebaldi acknowledges support from the US Department of Energy, Office of Biological and Environmental Research, grant DE-SC0004956 and thanks the Climate and Global Dynamics division of the National Center for Atmospheric Research, Climate Change Research section, for hosting her.
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Figure S1
Like Fig. 1 but here we set to 5% all values that are not determined significantly different from 5% on the basis of the bootstrap procedure described in detail in the Materials and methods section. (PDF 816 kb)
Figure S2
Ratio of the value of a linear trend fitted to the 50 year period 1950–1999 (in degrees Celsius per year) divided by the inter-annual standard deviation computed on the same 50 year period, for the two observational datasets top row and the ensemble of GCM simulations (presented here as the ensemble average ratio). These results confirm the different magnitude of this ratio in the different regions that the analysis of exceedance has highlighted. Note that observations in some regions show negative trends, while the ensemble average of the GCMs do not, consistent with the idea that negative trends are caused by internal variability that is not replicated consistently and simultaneously across the ensemble of models. (PDF 267 kb)
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Duffy, P.B., Tebaldi, C. Increasing prevalence of extreme summer temperatures in the U.S.. Climatic Change 111, 487–495 (2012). https://doi.org/10.1007/s10584-012-0396-6
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DOI: https://doi.org/10.1007/s10584-012-0396-6