Water and Climate Change: Snapshots from the New National Climate Assessment
After three years of intensive effort, research, writing, and review by hundreds of climate scientists, the latest update of the U.S. National Climate Assessment was released today. It includes many long, carefully prepared sectoral and regional studies, and covers the massive range of effects of climate change on the nation, including both changes already observed and expected in the future.
There are hundreds of pages of information, observations, projections, and conclusions to absorb -- almost all of it bad news. Here, in short form and in the actual wording from the NCA (with page numbers from the "Highlights" summary report), are some of the most important conclusions related to U.S. water resources:
Climate change is already affecting societies and the natural world (p. 32)
Climate change affects more than just temperature. The location, timing, and amounts of precipitation will also change as temperatures rise. (p. 29, Figure)
"There are significant trends in the magnitude of river flooding in many parts of the United States. River flood magnitudes have decreased in the Southwest and increased in the eastern Great Plains, parts of the Midwest, and from the northern Appalachians into New England." (p. 26)
Heavy downpours are increasing nationally, especially over the last three to five decades. The heaviest rainfall events have become heavier and more frequent, and the amount of rain falling on the heaviest rain days has also increased. Since 1991, the amount of rain falling in very heavy precipitation events has been significantly above average... The mechanism driving these changes is well understood. (p. 25).
- Agriculture, water, energy, transportation, and more, are all affected by climate change." (p.33)
"Risks of waterborne illness, and beach closures resulting from heavy rain and rising water temperatures are expected to increase in the Great Lakes region due to projected climate change." (p. 36)
Figure 2.18 from the National Assessment: The map shows percent increases in the amount of precipitation falling in very heavy events (defined as the heaviest 1 percent of all daily events) from 1958 to 2012 for each region of the continental United States.
Flooding along rivers, lakes, and in cities following heavy downpours, prolonged rains, and rapid melting of snowpack is exceeding the limits of flood protection infrastructure designed for historical conditions. (p. 38)
"Water quality and water supply reliability are jeopardized by climate change in a variety of ways that affect ecosystems and livelihoods." (p. 42)
"Annual precipitation and river-flow increases are observed now in the Midwest and the Northeast regions. Very heavy precipitation events have increased nationally and are projected to increase in all regions. The length of dry spells is projected to increase in most areas, especially the southern and northwestern portions of the contiguous United States." (p. 42)
The Northeast has experienced a greater recent increase in extreme precipitation than any other region in the U.S.; between 1958 and 2010, the Northeast saw more than a 70 percent increase in the mount of precipitation falling in very heavy events. (p. 70)
"Short-term (seasonal or shorter) droughts are expected to intensify in most U.S. regions. Longer-term droughts are expected to intensify in large areas of the Southwest, southern Great Plains, and Southeast." (p. 42)
"Climate change is expected to affect water demand, groundwater withdrawals, and aquifer recharge, reducing groundwater availability in some areas." (p. 42)
"Changes in precipitation and runoff, combined with changes in consumption and withdrawal, have reduced surface and groundwater supplies in many areas. These trends are expected to continue, increasing the likelihood of water shortages for many uses." (p. 42)
"In most U.S. regions, water resources managers and planners will encounter new risks, vulnerabilities and opportunities that may not be properly managed within existing practices." (p. 42)
"The effects of climate change, primarily associated with increasing temperatures and potential evapotranspiration, are projected to significantly increase water demand across most of the United States." (p. 43, Figure)
"Compared to 10 percent of counties today, by 2050, 32 percent of counties will be at high or extreme risk of water shortages." (p. 44, Figure)
The annual maximum number of consecutive dry days (less than 0.01 inches of rain) is projected to increase, especially in the western and southern part of the nation, negatively affecting crop and animal production. (p. 47, Figure).
Indigenous communities in various parts of the U.S. have observed climatic changes that result in impacts such as the loss of traditional foods, medicines, and water supplies. (p. 49)
Climate change impacts on ecosystems reduce their ability to improve water quality and regulate water flows. (p. 50)
In the Southern Plains, projected declines in precipitation in the south and greater evaporation everywhere due to higher temperatures will increase irrigation demand and exacerbate current stresses on agricultural productivity. (p. 77)
"Snowpack and streamflow amounts are projected to decline in parts of the Southwest, decreasing surface water supply reliability for cities, agriculture, and ecosystems." (p. 78)
"Increased warming, drought, and insect outbreaks, all caused by or linked to climate change, have increased wildfires and impacts to people and ecosystems in the Southwest." (p. 78)
"Tourism and recreation also face climate change challenges, including reduced streamflow and a shorter snow season, influencing everything from the ski industry to lake and river recreation." (p. 78)
"Changes in the timing of streamflow related to changing snowmelt are already observed and will continue, reducing the supply of water for many competing demands and causing far-reaching ecological and socioeconomic consequences." (p. 80)
"Observed regional warming has been linked to changes in the timing and amount of water availability in basins with significant snowmelt contributions to streamflow. By 2050, snowmelt is projected to shift three to four weeks earlier than the last century's average and summer flows are projected to be substantially lower. (p. 80, for the Northwest)"
"On most [U.S. Pacific] islands, increased temperatures coupled with decreased rainfall and increased drought will reduce the amount of freshwater available for drinking and crop irrigation. Climate change impacts on freshwater resources will vary with differing island size and topography, affecting water storage capability and susceptibility to coastal flooding. Low-lying islands will be particularly vulnerable due to their small land mass, geographic isolation, limited potable water sources, and limited agricultural resources." (p. 84)
"Snow accumulation in the West has decreased, and is expected to continue to decrease, as a result of observed and projected warming." (p. 87)
Although some additional climate change and related impacts are now unavoidable, the amount of future climate change and its consequences will still largely be determined by our choices, now and in the near future. There is still time to act to limit the amount of climate change and the extent of damaging impacts we will face. (p. 96)
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