Annual precipitation since the beginning of the last century has increased across most of the northern and eastern United States and decreased across much of the southern and western United States. Over the coming century, significant increases are projected in winter and spring over the Northern Great Plains, the Upper Midwest, and the Northeast (medium confidence). Observed increases in the frequency and intensity of heavy precipitation events in most parts of the United States are projected to continue (high confidence). Surface soil moisture over most of the United States is likely to decrease (medium confidence), accompanied by large declines in snowpack in the western United States (high confidence) and shifts to more winter precipitation falling as rain rather than snow (medium confidence).

This finding is from chapter 2 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

This chapter is based on the collective effort of 32 authors, 3 review editors, and 18 contributing authors comprising the writing team for the Climate Science Special Report (CSSR),75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 a featured U.S. Global Change Research Project (USGCRP) deliverable and Volume I of the Fourth National Climate Assessment (NCA4). An open call for technical contributors took place in March 2016, and a federal science steering committee appointed the CSSR team. CSSR underwent three rounds of technical federal review, external peer review by the National Academies of Sciences, Engineering, and Medicine, and a review that was open to public comment. Three in-person Lead Authors Meetings were conducted at various stages of the development cycle to evaluate comments received, assign drafting responsibilities, and ensure cross-chapter coordination and consistency in capturing the state of climate science in the United States. In October 2016, an 11-member core writing team was tasked with capturing the most important CSSR key findings and generating an Executive Summary. The final draft of this summary and the underlying chapters was compiled in June 2017.

The NCA4 Chapter 2 author team was pulled exclusively from CSSR experts tasked with leading chapters and/or serving on the Executive Summary core writing team, thus representing a comprehensive cross-section of climate science disciplines and supplying the breadth necessary to synthesize CSSR content. NCA4 Chapter 2 authors are leading experts in climate science trends and projections, detection and attribution, temperature and precipitation change, severe weather and extreme events, sea level rise and ocean processes, mitigation, and risk analysis. The chapter was developed through technical discussions first promulgated by the literature assessments, prior efforts of USGCRP,75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 e-mail exchanges, and phone consultations conducted to craft this chapter and subsequent deliberations via phone and e-mail exchanges to hone content for the current application. The team placed particular emphasis on the state of science, what was covered in USGCRP,75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 and what is new since the release of the Third NCA in 2014.dd5b893d-4462-4bb3-9205-67b532919566

Description of evidence base:

The Key Message and supporting text summarize extensive evidence documented in the climate science peer-reviewed literature and previous National Climate Assessments (e.g., Karl et al. 2009, Walsh et al. 2014a6a312ba-6fd1-4006-9a60-45112db52190^,e251f590-177e-4ba6-8ed1-6f68b5e54c8a). Evidence of long-term changes in precipitation is based on analysis of daily precipitation observations from the U.S. Cooperative Observer Network (http://www.nws.noaa.gov/om/coop/) and shown in Easterling et al. (2017),e8089a19-413e-4bc5-8c4a-7610399e268c Figure 7.1. Published work, such as the Third National Climate Assessment and Figure 7.1e8089a19-413e-4bc5-8c4a-7610399e268c, show important regional and seasonal differences in U.S. precipitation change since 1901.

Numerous papers have been written documenting observed changes in heavy precipitation events in the United States (e.g., Kunkel et al. 2003, Groisman et al. 2004642a65e4-fe5d-4655-97e1-49a8a9bfc297^,dd26dcd2-9abb-4533-9763-d56b2e1f0a1a), which were cited in the Third National Climate Assessment, as well as those cited in this assessment. Although station-based analyses (e.g., Westra et al. 2013e941a5b9-10b7-462b-9042-5760a82fc415) do not show large numbers of statistically significant station-based trends, area averaging reduces the noise inherent in station-based data and produces robust increasing signals (see Easterling et al. 2017,e8089a19-413e-4bc5-8c4a-7610399e268c Figures 7.2 and 7.3). Evidence of long-term changes in precipitation is based on analysis of daily precipitation observations from the U.S. Cooperative Observer Network (http://www.nws.noaa.gov/om/coop/) and shown in Easterling et al. (2017),e8089a19-413e-4bc5-8c4a-7610399e268c Figures 7.2, 7.3, and 7.4.

Evidence of historical changes in snow cover extent and reduction in extreme snowfall years is consistent with our understanding of the climate system’s response to increasing greenhouse gases. Furthermore, climate models continue to consistently show future declines in snowpack in the western United States. Recent model projections for the eastern United States also confirm a future shift from snowfall to rainfall during the cold season in colder portions of the central and eastern United States. Each of these changes is documented in the peer-reviewed literature and cited in the main text of this chapter.

Evidence of future change in precipitation is based on climate model projections and our understanding of the climate system’s response to increasing greenhouse gases, and on regional mechanisms behind the projected changes. In particular, Figure 7.7 in Easterling et al. (2017)e8089a19-413e-4bc5-8c4a-7610399e268c documents projected changes in the 20-year return period amount using the LOCA data, and Figure 7.6e8089a19-413e-4bc5-8c4a-7610399e268c 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 7.5e8089a19-413e-4bc5-8c4a-7610399e268c shows changes in seasonal and annual precipitation and shows where confidence in the changes is higher based on consistency between the models, and there are large areas where the projected change is uncertain.

New information and remaining uncertainties:

The main issue that relates to uncertainty in historical trends is the sensitivity of observed precipitation trends to the spatial distribution of observing stations and to historical changes in station location, rain gauges, the local landscape, and observing practices. These issues are mitigated somewhat by new methods to produce spatial grids596a7f1e-6ce5-4bdf-b144-d0715a7567bd through time.

This includes the sensitivity of observed snow changes to the spatial distribution of observing stations and to historical changes in station location, rain gauges, and observing practices, particularly for snow. Future changes in the frequency and intensity of meteorological systems causing heavy snow are less certain than temperature changes.

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 (e.g., 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 projected wetting in the mid- and higherlatitudes. 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:

Confidence is medium that precipitation has increased and high that heavy precipitation events have increased in the United States. Furthermore, confidence is also high that the important regional and seasonal differences in changes documented here are robust.

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 more total precipitation is projected for the northern United States and less for the Southwest.

Based on the evidence and understanding of the issues leading to uncertainties, confidence is medium that average annual precipitation has increased in the United States. Furthermore, confidence is also medium that the important regional and seasonal differences in changes documented in the text and in Figure 7.1 in Easterling et al. (2017)e8089a19-413e-4bc5-8c4a-7610399e268c are robust.

Given the evidence base and uncertainties, confidence is medium that snow cover extent has declined in the United States and medium that extreme snowfall years have declined in recent years. Confidence is high that western U.S. snowpack will decline in the future, and confidence is medium that a shift from snow domination to rain domination will occur in the parts of the central and eastern United States cited in the text, as well as that soil moisture in the surface (top 10cm) will decrease.

Provenance

This finding was derived from scenario rcp_8_5