finding 7.3 : key-finding-7-3

The frequency and intensity of heavy precipitation events are projected to continue to increase over the 21st century (high confidence). Mesoscale convective systems in the central United States are expected to continue to increase in number and intensity in the future (medium confidence). There are, however, important regional and seasonal differences in projected changes in total precipitation: the northern United States, including Alaska, is projected to receive more precipitation in the winter and spring, and parts of the southwestern United States are projected to receive less precipitation in the winter and spring (medium confidence).

This finding is from chapter 7 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: Based on numerous analyses of model simulations and our understanding of the climate system there is high confidence in the projected changes in precipitation extremes and medium confidence in projected changes in total precipitation over the United States.

Description of evidence base: Evidence for future changes in precipitation is based on climate model projections and our understanding of the climate system’s response to increasing greenhouse gases and of regional mechanisms behind the projected changes. In particular, Figure 7.7 documents projected changes in the 20-year return period amount using the LOCA data, and Figure 7.6 shows changes in 2 day totals for the 5-year return period using the CMIP5 suite of models. Each figure shows robust changes in extreme precipitation events as they are defined in the figure. However, Figure 7.5, which shows changes in seasonal and annual precipitation, indicates where confidence in the changes is higher based on consistency between the models and that there are large areas where the projected change is uncertain.

New information and remaining uncertainties: A key issue is how well climate models simulate precipitation, which is one of the more challenging aspects of weather and climate simulation. In particular, comparisons of model projections for total precipitation (from both CMIP3 and CMIP5, see Sun et al. 2015b63c9720-f770-4718-89cc-53b3616e2bec) by NCA3 region show a spread of responses in some regions (for example, the Southwest) such that they are opposite from the ensemble average response. The continental United States is positioned in the transition zone between expected drying in the subtropics and wetting in the mid- and higher-latitudes. There are some differences in the location of this transition between CMIP3 and CMIP5 models and thus there remains uncertainty in the exact location of the transition zone.

Assessment of confidence based on evidence: Based on evidence from climate model simulations and our fundamental understanding of the relationship of water vapor to temperature, confidence is high that extreme precipitation will increase in all regions of the United States. However, based on the evidence and understanding of the issues leading to uncertainties, confidence is medium that that more total precipitation is projected for the northern U.S. and less for the Southwest.

This finding was derived from figure -.2: Confidence / Likelihood

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