finding 3.1 : key-message-3-1

Significant changes in water quantity and quality are evident across the country. These changes, which are expected to persist, present an ongoing risk to coupled human and natural systems and related ecosystem services (high confidence). Variable precipitation and rising temperature are intensifying droughts (high confidence), increasing heavy downpours (high confidence), and reducing snowpack (medium confidence). Reduced snow-to-rain ratios are leading to significant differences between the timing of water supply and demand (medium confidence). Groundwater depletion is exacerbating drought risk (high confidence). Surface water quality is declining as water temperature increases (high confidence) and more frequent high-intensity rainfall events mobilize pollutants such as sediments and nutrients (medium confidence).

This finding is from chapter 3 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

Chapter authors were selected based on criteria, agreed on by the chapter lead and coordinating lead authors, that included a primary expertise in water sciences and management, knowledge of climate science and assessment of climate change impacts on water resources, and knowledge of climate change adaptation theory and practice in the water sector.

The chapter was developed through technical discussions and expert deliberation among chapter authors, federal coordinating lead authors, and staff from the U.S. Global Change Research Program (USGCRP). Future climate change impacts on hydrology, floods, and drought for the United States have been discussed in the Third National Climate Assessment3ff0e30a-c5ee-4ed9-8034-288be428125b and in the USGCRP’s Climate Science Special Report.e8089a19-413e-4bc5-8c4a-7610399e268c,a29b612b-8c28-4c93-9c18-19314babce89 Accordingly, emphasis here is on vulnerability and the risk to water infrastructure and management presented by climate variability and change, including interactions with existing patterns of water use and development and other factors affecting climate risk. The scope of the chapter is limited to inland freshwater systems; ocean and coastal systems are discussed in their respective chapters in this report.

Description of evidence base:

Increasing air temperatures have substantially reduced the fraction of winter precipitation occurring as snow, particularly over the western United States,d9661451-b35d-4e0c-9551-cbc60c45c5ef,d1069afd-d9c4-4cc1-bd29-c50f637502bd,87575740-3c5e-4669-8558-55621962abb8,73760c11-7b97-4876-a24f-8fb54b01bca9,ea2c3f43-b493-4001-8489-959c0f55080a,17850be8-349d-4e41-8f34-2510ce678c7b,0615b4ff-d185-4e14-9d4d-5bea1ce6ca51 and warming has resulted in a shift in the timing of snowmelt runoff to earlier in the year.87575740-3c5e-4669-8558-55621962abb8,c88eee01-cbe5-46d7-983f-20d26c04758c,52c72f28-b78c-4a82-9ca6-df2bbb15ef58,ce5dabfd-3d19-4eee-948e-a14ed158b1f7,67d16340-8e47-4ddb-bb4a-2b1855636daf

As reported in the Climate Science Special Report and summarized in Chapter 2: Climate, average annual temperature over the contiguous United States has increased by 1.2°F (0.7°C) for the period 1986–2016 relative to 1901–1960, and by 1.8°F (1.0°C) based on a linear regression for the period 1895–2016. Surface and satellite data are consistent in their depiction of rapid warming since 1979. Paleo-temperature evidence shows that recent decades are the warmest of the past 1,500 years. Additionally, contiguous U.S. average annual temperature is projected to rise. Increases of about 2.5°F (1.4°C) are projected for the next few decades in all emission scenarios, implying that recent record-setting years may be common in the near future. Much larger rises are projected by late century: 2.8°–7.3°F (1.6°–4.1°C) in a lower scenario (RCP4.5) and 5.8°–11.9°F (3.2°–6.6°C) in a higher scenario (RCP8.5).

Annual precipitation has decreased in much of the West, Southwest, and Southeast and increased in most of the Northern and Southern Great Plains, Midwest, and Northeast. There are important regional differences in trends, with the largest increases occurring in the northeastern United States. In particular, mesoscale convective systems (organized clusters of thunderstorms)—the main mechanism for warm season precipitation in the central part of the United States—have increased in occurrence and precipitation amounts since 1979 (see Easterling et al. 2017, Key Finding 1e8089a19-413e-4bc5-8c4a-7610399e268c).

Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901 (see Easterling et al. 2017, Key Finding 2e8089a19-413e-4bc5-8c4a-7610399e268c) and are projected to continue to increase over this century. 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 (see Easterling et al. 2017, Key Finding 3e8089a19-413e-4bc5-8c4a-7610399e268c).

Projections indicate large declines in snowpack in the western United States and shifts to more precipitation falling as rain rather than snow in the cold season in many parts of the central and eastern United States (see Easterling et al. 2017, Key Finding 4e8089a19-413e-4bc5-8c4a-7610399e268c).

The human effect on recent major U.S. droughts is complicated. Little evidence is found for a human influence on observed precipitation deficits, but much evidence is found for a human influence on surface soil moisture deficits due to increased evapotranspiration caused by higher temperatures (see Wehner et al. 2017, Key Finding 2a29b612b-8c28-4c93-9c18-19314babce89).

Future decreases in surface (top 10 cm) soil moisture from anthropogenic forcing over most of the United States are likely as the climate warms under higher scenarios (see Wehner et al. 2017, Key Finding 3a29b612b-8c28-4c93-9c18-19314babce89). Substantial reductions in western U.S. winter and spring snowpack are projected as the climate warms. Earlier spring melt and reduced snow water equivalent have been formally attributed to human-induced warming and will very likely be exacerbated as the climate continues to warm. Under higher scenarios, and assuming no change to current water resources management, chronic, long-duration hydrological drought is increasingly possible by the end of this century (see Wehner et al. 2017, Key Finding 4a29b612b-8c28-4c93-9c18-19314babce89).

Even though national water withdrawal has remained steady irrespective of population growth,81bd7c9e-d465-4eb4-a0d9-5b3f244c839e there is a significant spatiotemporal variability in water withdrawal (for example, a higher rate over the South) and water-use efficiency across the United States.d68b9064-d5de-4eb8-8665-04427b77e030 Siebert et al. 2010c9641496-7c75-4570-adcb-ef3d77239fb6 reported that irrigation use of groundwater has increased substantially over the past century and that groundwater use for irrigation in some areas has exceeded natural aquifer recharge rates.

Changes in air temperature and precipitation affect water quality in predictable ways. Attribution of water quality changes to climate change, however, is complicated by the multiple cascading, cumulative effects of climate change, land use, and other anthropogenic stressors on water quality. There has been a widespread increase in water temperatures across the United States.af1c492e-fcd5-4491-bea3-45c80688e788,dd8ab4ea-4ced-4289-bfb3-b7022b8617e0 These trends are expected to continue in the future, with increased water temperatures likely across the country.0d1a4c40-01d6-41ec-8c2b-54ce0c3e174f Runoff from more frequent and intense precipitation events can increase the risk of pollutant loading as nutrients,c3662163-50aa-4c98-903f-eb8b9b477f51,0ea21dd7-301a-483d-8e31-d172fd18680d,715479a7-d3ea-40fe-99f4-f26381b2b554 sediment,06553223-ed2b-494a-956f-b2ba386f25c1,43e7bfdb-30c7-407d-89ae-e94f7bff36a1,738a6b0e-7c98-44e9-87e2-f08f9c3e73b4 and pathogensd4ed906f-cc7b-422c-aef1-96a1b1d5c80f,7571af21-b0b8-4602-9955-9509571e630b are transported from upland sources to water bodies. Pollutant loading is also strongly influenced by local watershed conditions (for example, land use, vegetative ground cover, pollutant sources). Increases in summer–fall water temperatures, excess nutrient loading events (driven by heavy precipitation events), and longer dry periods (associated with calm, quiescent water conditions) can expand the seasonal window for cyanobacteria and present an increased risk of bloom events.d4ed906f-cc7b-422c-aef1-96a1b1d5c80f,28077cd1-c29f-48ae-a068-2cdcef880807

Figure 3.2 shows net, average volumetric rates of groundwater depletion (km3/year) in 40 assessed aquifer systems or subareas in the contiguous 48 states.35520257-6694-45bb-a0bf-bd14ba88a77c Variation in rates of depletion in time and space within aquifers occurs but is not shown. For example, in the Nebraska part of the northern High Plains, small water-table rises occurred in parts of this area, and the net depletion was negligible. In contrast, in the Texas part of the southern High Plains, development of groundwater resources was more extensive, and the depletion rate averaged 1.6 km3/year.35520257-6694-45bb-a0bf-bd14ba88a77c

New information and remaining uncertainties:

There is high uncertainty associated with projected scenarios, as they include many future decisions and actions that remain unknown. There also is high uncertainty with estimates of precipitation; this uncertainty is reflected in the wide range of climate model estimates of future precipitation. In contrast, because climate model simulations generally agree on the direction and general magnitude of future changes in temperature (given specific emission scenarios), there is a medium level of uncertainty associated with temperature projections. Overall, changes in land use are associated with a medium level of uncertainty. Even though there is low uncertainty regarding the expansion of urban areas, there is greater uncertainty regarding changes in agricultural land use. A medium level of uncertainty for water supply reflects a combination of high uncertainty in streamflow and low uncertainty in water demand. Uncertainty in water demand is low because of adaptation and increased water-use efficiency and because of water storage in reservoirs. Water storage capacity also reduces uncertainty in future groundwater conditions. Water temperature changes are relatively well understood, but other changes in water quality, particularly pollutant loads (such as nutrients, sediment, and pathogens), are associated with high uncertainty due to a combination of uncertain land-use changes and high uncertainty in streamflow and hydrologic processes.

Assessment of confidence based on evidence:

Increasing temperature is highly likely to result in early snowmelt and increased consumptive use. Uncertainty in precipitation and emission scenarios leads to low confidence in predicting water availability and the associated quality arising from changes in land-use scenarios. However, surface water and groundwater storage ensures medium confidence in water quantity and quality reliability, but spatial disparity in water efficiency could be better addressed through increased investment in water infrastructure for system maintenance.

This finding was derived from scenario rcp_4_5
This finding was derived from scenario rcp_8_5

References :

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