This chapter summarizes how climate is changing, why it is changing, and what is projected for the future. While the focus is on changes in the United States, the need to provide context sometimes requires a broader geographical perspective. Additional geographic detail is presented in the regional chapters of this report. Further details on the topics covered by this chapter are provided in the Climate Science Supplement and Frequently Asked Questions Appendices.

The chapter presents 12 key messages about our changing climate, together with supporting evidence for those messages. The discussion of each key message begins with a summary of recent variations or trends, followed by projections of the corresponding changes for the future.

Key Message 6: Heavy Downpours Increasing

Heavy downpours are increasing nationally, especially over the last three to five decades. Largest increases are in the Midwest and Northeast. Increases in the frequency and intensity of extreme precipitation events are projected for all U.S. regions.

Supporting Evidence

Process for Developing Key Messages

Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment.

Key message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.² The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.¹ The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.⁸ The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.⁹ The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.²

The Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection.

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input.

Evidence that extreme precipitation is increasing is based primarily on analysis¹^,²^,³ of hourly and daily precipitation observations from the U.S. Cooperative Observer Network, and is supported by observed increases in atmospheric water vapor.⁴ Recent publications have projected an increase in extreme precipitation events,¹^,⁵ with some areas getting larger increases⁶ and some getting decreases.⁷^,²

Nearly all studies to date published in the peer-reviewed literature agree that extreme precipitation event number and intensity have risen, when averaged over the United States. The pattern of change for the wettest day of the year is projected to roughly follow that of the average precipitation, with both increases and decreases across the U.S. Extreme hydrologic events are projected to increase over most of the U.S.

New information and remaining uncertainties

A key issue (uncertainty) is the ability of climate models to simulate precipitation. This is one of the more challenging aspects of modeling of the climate system because precipitation involves not only large-scale processes that are well-resolved by models but also small-scale process, such as convection, that must be parameterized in the current generation of global and regional climate models.

Viable avenues to improving the information base are to perform some long, very high-resolution simulations of this century’s climate under different emissions scenarios.

Assessment of confidence based on evidence

Given the evidence base and uncertainties, confidence is high that heavy downpours are increasing in most regions of the U.S., with especially large increases in the Midwest and Northeast.

Confidence is high that further increases in the frequency and intensity of extreme precipitation events are projected for most U.S. areas, given the evidence base and uncertainties.

Confidence Level

Very High

Strong evidence (established theory, multiple sources, consistent results, well documented and accepted methods, etc.), high consensus

High

Moderate evidence (several sources, some consistency, methods vary and/or documentation limited, etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models incomplete, methods emerging, etc.), competing schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.), disagreement or lack of opinions among experts

Heavy Downpours Increasing

Figure 2.16: Observed U.S. Trend in Heavy Precipitation

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Across most of the United States, the heaviest rainfall events have become heavier and more frequent. The amount of rain falling on the heaviest rain days has also increased over the past few decades. Since 1991, the amount of rain falling in very heavy precipitation events has been significantly above average. This increase has been greatest in the Northeast, Midwest, and upper Great Plains – more than 30% above the 1901-1960 average (see Figure 2.18). There has also been an increase in flooding events in the Midwest and Northeast where the largest increases in heavy rain amounts have occurred.

Figure 2.17: Observed Change in Very Heavy Precipitation

Observed Change in Very Heavy Precipitation

U.S. AverageAlaskaNorthwestSouthwestHawai'iGreat Plains NorthGreat Plains SouthMidwestNortheastSoutheast

U.S. Average

Alaska

Northwest

Southwest

Hawai'i

Great Plains North

Great Plains South

Midwest

Northeast

Southeast

Figure 2.17: Percent changes in the annual amount of precipitation falling in very heavy events, defined as the heaviest 1% of all daily events from 1901 to 2012 for each region. The far right bar is for 2001-2012. In recent decades there have been increases nationally, with the largest increases in the Northeast, Great Plains, Midwest, and Southeast. Changes are compared to the 1901-1960 average for all regions except Alaska and Hawai‘i, which are relative to the 1951-1980 average. (Figure source: NOAA NCDC / CICS-NC).

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Warmer air can contain more water vapor than cooler air. Global analyses show that the amount of water vapor in the atmosphere has in fact increased over both land and oceans.¹⁰^,¹¹^,¹²^,¹³ Climate change also alters dynamical characteristics of the atmosphere that in turn affect weather patterns and storms. In the mid-latitudes, where most of the continental U.S. is located, there is an upward trend in extreme precipitation in the vicinity of fronts associated with mid-latitude storms.¹ Locally, natural variations can also be important.¹⁴

Figure 2.18: Observed Change in Very Heavy Precipitation

Figure 2.18: The map shows percent increases in the amount of precipitation falling in very heavy events (defined as the heaviest 1% of all daily events) from 1958 to 2012 for each region of the continental United States. These trends are larger than natural variations for the Northeast, Midwest, Puerto Rico, Southeast, Great Plains, and Alaska. The trends are not larger than natural variations for the Southwest, Hawai‘i, and the Northwest. The changes shown in this figure are calculated from the beginning and end points of the trends for 1958 to 2012. (Figure source: updated from Karl et al. 2009⁶).

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Projections of future climate over the U.S. suggest that the recent trend towards increased heavy precipitation events will continue. This is projected to occur even in regions where total precipitation is projected to decrease, such as the Southwest.¹^,⁷^,²

Figure 2.19: Projected Change in Heavy Precipitation Events

Projected Change in Heavy Precipitation Events

Rapid Emissions Reductions (RCP 2.6)Continued Emissions Increases (RCP 8.5)

Figure 2.19: Maps show the increase in frequency of extreme daily precipitation events (a daily amount that now occurs once in 20 years) by the later part of this century (2081-2100) compared to the later part of last century (1981-2000). Such extreme events are projected to occur more frequently everywhere in the United States. Under the rapid emissions reduction scenario (RCP 2.6), these events would occur nearly twice as often. For the scenario assuming continued increases in emissions (RCP 8.5), these events would occur up to five times as often. (Figure source: NOAA NCDC / CICS-NC).

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References

Balling, Jr., R. C., and G. B. Goodrich, 2011: Spatial analysis of variations in precipitation intensity in the USA. Theoretical and Applied Climatology, 104, 415-421, doi:10.1007/s00704-010-0353-0. | Detail ↩
Dai, A., 2006: Recent climatology, variability, and trends in global surface humidity. Journal of Climate, 19, 3589-3606, doi:10.1175/JCLI3816.1. URL | Detail ↩
Dai, A., 2012: Increasing drought under global warming in observations and models. Nature Climate Change, 3, 52-58, doi:10.1038/nclimate1633. URL | Detail ↩
Karl, T. R., J. T. Melillo, and T. C. Peterson, 2009: Global Climate Change Impacts in the United States. T.R. Karl, Melillo, J.T., and Peterson, T.C., Eds. Cambridge University Press, 189 pp. URL | Detail ↩
Kunkel, K. E., L. E. Stevens, S. E. Stevens, L. Sun, E. Janssen, D. Wuebbles, and J. G. Dobson, 2013: Regional Climate Trends and Scenarios for the U.S. National Climate Assessment: Part 9. Climate of the Contiguous United States. NOAA Technical Report NESDIS 142-9. 85 pp., National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service, Washington, D.C. URL | Detail ↩
Kunkel, K. E. et al., 2013: Monitoring and understanding trends in extreme storms: State of knowledge. Bulletin of the American Meteorological Society, 94, doi:10.1175/BAMS-D-11-00262.1. URL | Detail ↩
Peterson, T. C. et al., 2013: Monitoring and understanding changes in heat waves, cold waves, floods and droughts in the United States: State of knowledge. Bulletin of the American Meteorological Society, 94, 821-834, doi:10.1175/BAMS-D-12-00066.1. URL | Detail ↩
Santer, B. D., C. Mears, F. J. Wentz, K. E. Taylor, P. J. Gleckler, T. M. L. Wigley, T. P. Barnett, J. S. Boyle, W. Brüggemann, N. P. Gillett, S. A. Klein, G. A. Meehl, T. Nozawa, D. W. Pierce, P. A. Stott, W. M. Washington, and M. F. Wehner, 2007: Identification of human-induced changes in atmospheric moisture content. Proceedings of the National Academy of Sciences, 104, 15248-15253, doi:10.1073/pnas.0702872104. URL | Detail ↩
Simmons, A. J., K. M. Willett, P. D. Jones, P. W. Thorne, and D. P. Dee, 2010: Low-frequency variations in surface atmospheric humidity, temperature, and precipitation: Inferences from reanalyses and monthly gridded observational data sets. Journal of Geophysical Research, 115, 1-21, doi:10.1029/2009JD012442. | Detail ↩
Vose, R. S. et al., 2013: Monitoring and understanding changes in extremes: Extratropical storms, winds, and waves. Bulletin of the American Meteorological Society, in press, doi:10.1175/BAMS-D-12-00162.1. URL | Detail ↩
Wang, M., and J. E. Overland, 2009: A sea ice free summer Arctic within 30 years? Geophysical Research Letters, 36, L07502, doi:10.1029/2009GL037820. URL | Detail ↩
Wehner, M. F., 2013: Very extreme seasonal precipitation in the NARCCAP ensemble: Model performance and projections. Climate Dynamics, 40, 59-80, doi:10.1007/s00382-012-1393-1. | Detail ↩
Willett, K. M., P. D. Jones, N. P. Gillett, and P. W. Thorne, 2008: Recent changes in surface humidity: Development of the HadCRUH dataset. Journal of Climate, 21, 5364-5383, doi:10.1175/2008JCLI2274.1. | Detail ↩
Wuebbles, D. J., G. Meehl, K. Hayhoe, T. R. Karl, K. Kunkel, B. Santer, M. Wehner, B. Colle, E. M. Fischer, R. Fu, A. Goodman, E. Janssen, H. Lee, W. Li, L. N. Long, S. Olsen, A. J. Sheffield, and L. Sun, 2013: CMIP5 climate model analyses: Climate extremes in the United States. Bulletin of the American Meteorological Society, in press, doi:10.1175/BAMS-D-12-00172.1. URL | Detail ↩

Welcome to the National Climate Assessment

Heavy Downpours Increasing

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Heavy Downpours Increasing

Convening Lead Authors

Lead Authors

Contributing Authors

Introduction

Key Message 6: Heavy Downpours Increasing

Supporting Evidence

Process for Developing Key Messages

Description of evidence base

New information and remaining uncertainties

Assessment of confidence based on evidence

Confidence Level

Very High

High

Medium

Low

Heavy Downpours Increasing

Figure 2.17: Observed Change in Very Heavy Precipitation

Observed Change in Very Heavy Precipitation

U.S. Average

Alaska

Northwest

Southwest

Hawai'i

Great Plains North

Great Plains South

Midwest

Northeast

Southeast

Figure 2.18: Observed Change in Very Heavy Precipitation

Figure 2.19: Projected Change in Heavy Precipitation Events

Projected Change in Heavy Precipitation Events

References

National Climate Assessment

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