Extreme Precipitation

Summary

  • The frequency and intensity of severe storms has increased. This trend will likely continue as the effects of climate change become more pronounced.
  • The amount of precipitation falling in the heaviest 1% of storms increased by 42% in the Midwest and 55% in the Northeast from 1958 through 2016.
  • Heavier storms are projected to increase in frequency at a faster rate than storms that are less intense.
  • The amount of precipitation falling during intense multi-day events has increased dramatically.

Observed Extreme Precipitation

The increase in extreme precipitation is one of the clearest changes in climate observed in the Great Lakes region. In the Midwest and Northeast United States, as defined by the USGCRP, the most intense precipitation events have become stronger and more frequent. 1 The amount of precipitation falling in the most intense 1% of precipitation events increased by 42% in the Midwest and 55% in the Northeast from 1958 through 2016. 2 In the extreme western extent of the Great Lakes region, as much as 50% of annual total precipitation falls during only 10 days of the year. Accumulated precipitation during these 10 days has increased dramatically over that same region of the country, with increases of 20-30% observed from 1971 to 2000 in many locations.3

The observed changes in heavy precipitation from 1901 through 2016. Image provided by the United States Global Change Research Program (USGCRP) Climate Science Special Report (CSSR, 2017).

More Total Precipitation from More Heavy Precipitation

Heavier, more frequent storms have been responsible for most of the observed increase in total precipitation during the last 60 years. In the comparison of NOAA’s Climate Normal periods of 1951-1980 and 1981-1990, the average of 230 observational stations in the Great Lakes region recorded that the heaviest 1% of daily precipitation events have become 5.1% more intense. Similarly, the frequency of these same heavy storms increased by 23.6% and the amount of precipitation falling in those storms increased by 20.2%.

In many areas, heavy precipitation has increased while changes in the frequency and intensity of moderate precipitation events have been less significant. Annual average precipitation over the United States saw an increase of about 4% from 1901 to 2015. 2015 was the third wettest year on record in the U.S., just behind 1973 and 1983 (all three years were marked by El Niño events). 4 5

Regional analysis of extreme precipitation. Values are based on observations from 230 U.S. and Canadian GHCN stations across the Great Lakes region.

Change in intensity of the heaviest 1% of daily precipitation events for GLISA quality-controlled GHCN-Daily stations in the Great Lakes region.

More Multi-Day Heavy Precipitation

Precipitation falling during heavy, multi-day wet periods has also increased dramatically. From 1931 through 1996, the amount of precipitation falling during week-long, once a year precipitation events increased by 25% to 100% in a broad region from the central Great Plains through the southern Great Lakes basin. 6 Two-day precipitation events that typically occurred once in 5, 10, and 20 years have all become substantially more frequent since the early 1960s. 7

The index of the number of 2-day precipitation events exceeding the 5-year recurrence interval of GHCN-Daily Stations in CONUS. Image provided by the United States Global Change Research Program (USGCRP) Climate Science Special Report (CSSR, 2017).

Projected Extreme Precipitation

Model projections of future precipitation vary greatly, but overall, the current observed trends of increasing frequency and intensity of extreme precipitation events is anticipated to continue. Climate models project the Great Lakes region to experience a greater increase in total precipitation than most other regions of North America. The amount of precipitation falling in the most intense 1% of precipitation events has increased significantly in the Midwest (42%) and Northeast (55%) from 1958 to 2016. These numbers are projected to increase by another 40% or more by late century (2070-2099), relative to 1986-2015 amounts. 8 The frequency of severe thunderstorm environments is also projected to rise across the United States by mid to late century. The greatest projected increases occur during spring over the Midwest and northern Great Plains regions, with increases of up to 2.4 days of such environments per season. 9

The number of days per year exceeding one inch of precipitation is projected to increase by 15 to 23% on average in the Midwest and Northeast regions of the United States through mid-century. The number of days exceeding 2 inches of precipitation is projected to increase at a faster rate, by 37 to 46%, on average. 10

The projected change in the number of days with over 1 inch precipitation in the Great Lakes region. Data provided by the University of Wisconsin-Madison Nelson Institute Center for Climatic Research (UW-RegCM4).

The projected change in the number of days with over 2 inches precipitation in the Great Lakes region. Data provided by the University of Wisconsin-Madison Nelson Institute Center for Climatic Research (UW-RegCM4).

Climate Change, Extreme Precipitation, and Drought

Though it seems counterintuitive, as temperatures warm, the potential for both wetter and drier conditions can increase. While annual precipitation totals have generally increased, the seasonal and regional distribution of precipitation can also change. In the Great Lakes region, precipitation totals during the fall, winter, and spring have increased in most locations, while summer precipitation has remained relatively stable or even declined. On top of that, the distribution of the intensity of precipitation events has also changed so that more precipitation is falling during heavier storms. These combined effects, the clustering of precipitation into heavier storms and the polarization of wet and dry seasons, may increase the chance of both extreme precipitation events and prolonged dry periods by extending the time between rainfalls.

Impacts of Extreme Precipitation

In general, climate change will likely amplify existing stressors across a range of sectors in the Great Lakes region. 11 As disruptive, potentially damaging storms become more frequent and more intense, many agricultural, economic, and environmental sectors will be impacted. Most directly, changing extreme precipitation events will affect stormwater management, water quality, public health, and transportation.

Stormwater Management

The observed increase in heavy rain events across the region is already contributing to more stormwater management problems. 12 13 14 Intense precipitation overloads sewage systems and water treatment facilities, increasing the risk of waterborne disease outbreaks when untreated sewage is discharged into surface water. 15 An increase in the frequency and volume of combined sewer overflows is projected in the Great Lakes under high emissions scenarios. [enf_note]Tavakol-Davani, H., E. Goharian, C. H. Hansen, D. Apul, and S. J. Burian, 2016: How does climate change affect combined sewer overflow in a system benefiting from rainwater harvesting systems? Sustainable Cities and Society, 27, 430–438. doi:10.1016/j.scs.2016.07.003.[/efn_note] Increased urbanization further amplifies flooding risks, because more impervious surfaces channel storm water into fast-moving flows that are more difficult to control. 16 17 In areas with poor stormwater management, more frequent heavy rains, impervious surfaces, and degrading infrastructure will likely result in even greater damage.

Water Quality and Public Health

Heavy rainfall increases contaminated runoff, and warmer lake waters mobilize pollutants stored in sediment, contaminating fish. 18 19 20 Floods could increase the likelihood of waterborne diseases and lake contamination. 21 22 23 Warmer temperatures may conspire with stronger storms and greater nutrient loading to increase the occurrence of toxic algal blooms and oxygen depleted dead zones. 24

Transportation

Increased extreme precipitation will likely impact transportation infrastructure, such as roads and bridges. The projected increase in the number of extreme precipitation events has been linked to an increased risk of traffic accidents.  25 Extreme precipitation can also cause scour, which is the erosion of the base of bridges due to faster streamflow. According to an estimate by the EPA, when taking into account the projected increase in scour damage, the annual cost of maintaining Midwestern bridges may reach $400 million by the year 2050. 26

Potential Causes of Increasing Extreme Precipitation

There are several possible causes for the observed increase in extreme precipitation events over the last century. Perhaps the most prominent reasoning is that rising global surface temperatures have increased evaporation rates and added water vapor to the atmosphere. With more heat and moisture, two of the key ingredients for storm development are now more abundant, leading to an increase in extreme precipitation. 27 28 29 30 31

References

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