Water
climate highly influential scientific assessment government
Ariane O. Pinson Elizabeth A. Payton Tirusew Asefa Laura E. Condon Benjamin L. Harding Julie Kiang Deborah H. Lee Stephanie A. McAfee Justin Pflug Imtiaz Rangwala
Chapter 4 of the Fifth National Climate Assessment
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Fifth National Climate Assessment
KEY MESSAGE 4.1

Climate Change Will Continue to Cause Profound Changes in the Water Cycle

Changes to the water cycle pose risks to people and nature. Alaska and northern and eastern regions of the US are seeing and expect to see more precipitation on average, while the Caribbean, Hawaiâi, and southwestern regions of the US are seeing and expect to see less precipitation . Heavier rainfall events are expected to increase across the Nation , and warming will increase evaporation and plant water use where moisture is not a limiting factor . Groundwater supplies are also threatened by warming temperatures that are expected to increase demand . Snow cover will decrease and melt earlier . Increasing aridity, declining groundwater levels, declining snow cover, and drought threaten freshwater supplies .

Read about Confidence and Likelihood

Description of Evidence Base

The hydrologic component maps shown in Figures 4.3, 4.4, 4.5, 4.6, 4.7, and 4.9 constitute part of the evidence base. They show mid-21st-century projections of water cycle components based on an intermediate scenario (RCP4.5). Projections of water cycle components are available for both RCP4.5 and RCP8.5 scenarios, but both scenarios show similar hydrologic responses at midcentury, neither are available as 100-year projections, and space in this chapter is limited; as a result, only RCP4.5 projections are presented here. The central map of the contiguous US (CONUS) in each of these figures represents the average of all 32 Coupled Model Intercomparison Project Phase 5 (CMIP5) projections chosen for this discussion.197 The Alaska and Hawaiâi maps represent the average of 10 CMIP5 projections. The wettest and driest 20% of projections show the range of outcomes from the 32-projection set for CONUS, illustrating the uncertainty surrounding water cycle responses to climate change. Outside CONUS, downscaled climate projections are limited, especially those needed to map projected changes in hydrologic components for the US Caribbean and US-Affiliated Pacific Islands. The absence of projections for actual evapotranspiration, soil moisture, and runoff contribute to uncertainty when assessing future water security challenges for these regions. Further information about the data used to generate the maps can be found in the figure metadata.

Because the focus of this chapter is terrestrial fresh water, the authors relied heavily on Chapter 2 (Climate Trends) and Chapter 3 (Earth Systems Processes) for their assessments of precipitation trends and projections, particularly extreme precipitation trends and projections.

Regarding evapotranspiration, there is general consensus that warming temperatures will enhance evaporative demand (potential evapotranspiration, PET) across the Nation (Ch. 3);43,44,75 however, uncertainties in vegetation response to warming reduce confidence in evapotranspiration (ET) projections.75 In many parts of the country, projected changes in annual evapotranspiration by the end of this century are not robust, and there is disagreement among models across the southern states and parts of the central US.43 The degree and sometimes direction of observed changes in PET and ET are also less certain, particularly east of the Rocky Mountains, due to differences in the trends of the variables that force PET.12 Nor are these trends well supported by direct observation. There is a lack of information on more recent trends in pan evaporation across the US. Pan evaporation is a useful concept to estimate atmospheric evaporative demand but it is strongly affected by local environmental conditions, which can drive contradictory trends in pan evaporation across a broader region,198 as is observed across the US.199 For example, increases in local humidity (e.g., from irrigation) or land-use changes (e.g., changes in tree density near the pans) could affect evaporation from the pans. Therefore, pan evaporation may not provide a reliable indication of regional-scale trends in evaporative demand. The disagreement among observational data and reanalyses limits our confidence in past ET and PET trends. Complexities related to vegetation, as well as the competing effects of multiple evaporation drivers, make assigning nationally consistent likelihood and confidence challenging. However, the balance of evidence suggests with medium confidence that evaporation is expected to increase in places where moisture is not a limiting factor to atmospheric demand.

There is widespread consensus that increases in temperature will decrease the proportion of US precipitation that falls as snow,14,15,24,43 decrease snow extents,24,25 advance the timing of snowmelt rates and pulses,16,27 increase the prevalence of rain-on-snow events,70,71 and influence how snow water resources are partitioned to runoff.19,20

Since parts of Alaska and the highest elevations in the contiguous US may be cold enough to sustain snowfall in future climates, some studies have projected increases in snow volume in these locations with future increases in precipitation. However, those increases in snow are expected to be vastly outweighed by the future decreases in snow elsewhere, particularly across the western US and by the late 21st century for all intermediate (RCP4.5 and SSP2-4.5) and higher scenarios.

It is well established that groundwater and surface water are connected resources and that groundwater can help stabilize surface water supplies.47,48 Similarly, there is agreement that loss of shallow groundwater can exacerbate droughts and decrease streamflow. There is also agreement that warmer temperatures will increase water demand and that this could increase groundwater pumping.52,53,54

Major Uncertainties and Research Gaps

Uncertainties stem from future projections of climate. This may be particularly true for late-21st-century projections that are dependent on the degree to which societies will respond to climate change. The literature employs different projections and emissions scenarios, as well as metrics and measurements that vary in their degree of climate sensitivity, resulting in studies that are not always directly comparable.

Understanding recent and potential future flood responses to climate change is difficult for several reasons. Floods are the product of complex subseasonal to interannual interactions between rainfall, soil moisture, evapotranspiration, snowpack/melt, and other processes. Isolating climate change impacts on inland flooding is further complicated by the hydrologic âreplumbingâ wrought by urbanization and dams. For these reasons, the translation of rainfall trends into flood changes is complex and poorly understood. National65,200 and global201 examination of historical flood records has concluded that climate influences have been relatively limited, contradicting an earlier study that argued that the largest floods have increased in severity.202 This latter argument is further contradicted by evidence that floods in the central US have become more common but not more intense.203,204

However, major floods are by definition rare, making detection and attribution of changes difficult. Thus, a lack of statistically significant trends in observed floods does not necessarily indicate that such events are not changing. Indeed, a relatively limited number of geographically focused case studies have painted complex pictures of climate-related flood changes that are lacking in broader regional and national analyses. Additional place-based case studiesâas opposed to regional- or national-scale analysesâcould help unravel the complex interactions between climate and non-climate flood drivers.

Given the first-order influence of temperature and precipitation change on snowfall, there is high certainty that future US snow cover, snow volume, and snow persistence will change.24 However, there is some disagreement in the literature about the extent and direction (positive or negative) of change in surface water availability with future changes in climate. Existing studies indicate both increases and decreases in future runoff for different US hydroclimatic regimes.

In particular, there is uncertainty in the degree to which temperature may impact flow in some major river systems in the West.205,206 Significant disagreements in the direction of observed soil moisture trends remain,38,39,40 largely because it can be challenging to estimate with remote sensing or models, and the existing in situ soil moistureâmonitoring network is insufficient.37 Uncertainties can also be introduced because not all products are directly comparable, capturing trends over slightly different depths, although modest differences are probably not a major source of error. There is also uncertainty in soil moisture projections related to model, season, and soil depth.38,42,43

Similarly, there is uncertainty in both the magnitude and direction of groundwater storage changes, primarily due to uncertainty in future groundwater management policy and uncertainty in future recharge. This is due to uncertainty in both the human response to changing climate conditions and research gaps in quantifying natural groundwater recharge. Groundwater pumping is controlled by a myriad of human factors such as population, water policy, crop choices, and irrigation technology. While it is well established that warmer temperatures can increase water demand,52,53,54 and historical trends demonstrate unsustainable groundwater usage in the past (as discussed in NCA4), future groundwater pumping increases will depend on water management practices and policy. Groundwater recharge is similarly uncertain.50,51 Projected increases in large precipitation and flooding events are expected to increase recharge (known as episodic recharge events). However, the quantity of this recharge is less certain and highly dependent on the nature and timing of the storms that occur. Also, while increases in recharge may be counteracted by changes in plant water usage and snowpack that can decrease natural recharge, the magnitude of these recharge changes has not been well quantified. Separating the impacts of groundwater pumping from climate trends is particularly challenging due to a lack of long-term groundwater monitoring wells, especially outside of the most heavily groundwater-developed areas.

Description of Confidence and Likelihood

The author team determined that the evidence points to medium confidence that there will continue to be increases in precipitation in Alaska and in the northern and eastern regions of the US and decreases in precipitation in the Caribbean and the Southwest. Despite lingering uncertainties around average precipitation, there is very high confidence from both observations and projections that extreme precipitation events are becoming more frequentÂ nationwide, and that it is very likely this trend will continue in the future. The disagreement among observational data and reanalyses limits our confidence in past ET and PET trends. Complexities related to vegetation, as well as the competing effects of multiple evaporation drivers, make assigning nationally consistent likelihood and confidence challenging. However, the balance of evidence suggests with medium confidence that evaporation will increase in places where moisture is not a limiting factor to atmospheric demand. Based on current trends and climate model projections, there is high confidence and it is very likely that warming temperatures will increase the demand for surface and groundwater for crops and human use. Given the direct influence of rising temperatures on snow, there is high confidence and it is very likely that the extent, volume, and duration of snow cover and melt upon which human and natural systems rely is and will continue to be reduced by warming.

KEY MESSAGE 4.2

Water Cycle Changes Will Affect All Communities, with Disproportionate Impacts for Some

Natural and human systems have evolved under the water cycleâs historical patterns, making rapid adaptation challenging. Heavier rainfall, combined with changes in land use and other factors such as soil moisture and snow, is leading to increasing flood damage . Drought impacts are also increasing , as are flood- and drought-related water quality impacts . All communities will be affected, but in particular those on the frontline of climate changeâincluding many Black, Hispanic, Tribal, Indigenous, and socioeconomically disadvantaged communitiesâface growing risks from changes to water quantity and quality due to the proximity of their homes and workplaces to hazards and limited access to resources and infrastructure .

Read about Confidence and Likelihood

Description of Evidence Base

Observational records now span time periods long enough to evaluate changes in the volume, variability, and timing of water availability.11 The magnitude of these changes, and their agreement with model projections, vary with hydroclimate regimes across the US.

While it has been difficult to establish clear linkages between increases in extreme precipitation and trends in âtraditionalâ measures of flood activity such as peak streamflow rate, attribution studies have apportioned some of the historical increases in flood damage to precipitation change.68,69 It is probable that many of these increases have been concentrated in urbanized watersheds, which are more sensitive to rainfall than rural and natural settings.74 Flood vulnerability, including in urbanized areas, tends to be concentrated in historically marginalized and socioeconomically disadvantaged neighborhoods.130 There is increasing consensus that systematically disadvantaged communities have been and will continue to be most impacted by these hazards, due to factors such as inadequate climate/hydrological monitoring, deferred infrastructure maintenance, and insufficient access to recovery resources.207

There is ample literature describing the impacts of floods, fires, and drought events on a wide variety of water quality hazards.6 These studies provide insights into impacts to water quality hazards from intensified events due to climate change, and studies specific to climate change impacts on water quality are becoming more prevalent. There are some reports of specific benefits to contaminant concentrations from increased or decreased precipitation, but there is no consensus that water quality will improve with climate change.

There is widespread consensus that increases in air temperature will impact water quality by increasing water temperatures, resulting in less oxygen-rich water, exacerbating harmful algal blooms, increasing pathogens, and creating problems with drinking water taste and odor.6,7

Similarly, there is consensus that increased precipitation and intensity will degrade water quality due to urban storm water and combined sewer overflows, increased agricultural runoff, and riverine flooding. There is less certainty in regions of the country where precipitation is not increasing or decreasing. Compounding factors of increasing temperatures and aging stormwater and sewer systems and water reservoirs can exacerbate problems due to too much or too little water.

The literature is rife with observations of segments of the population being negatively affected by climate change, especially water-related hazards. There is consensus that these negative impacts of water-related climate change will be felt disproportionately among marginalized and low-income people.122

Major Uncertainties and Research Gaps

There is moderate uncertainty about the degree to which land-surface changes will drive nonstationary changes to the volume and timing of water resources. There is a lack of research on the linkages between climate change and flooding.68 There is uncertainty about the extent to which traditional design stormsâthat is, storms of particular intensity and duration, used in floodplain and built environment planningâand flooding assumptions based on older observations reflect current and future flood conditions.171 Additional research into the effects of climate change on water quality would improve our understanding of impacts, particularly in the face of compounding factors such as aging infrastructure, wildfires, and increased agricultural runoff.

Description of Confidence and Likelihood

There is strong evidence that climate change imparts a number of important shifts in local and regional hydrologic cycles, and that when combined with land-use changes and other human factors, increases are likely in the frequency, severity, duration, and damages from floods (high confidence) and drought impacts are increasing (medium confidence). There is a more limited body of work on the effects of climate change on water quality; thus there is medium confidence that climate change is degrading water quality. However, there is still uncertainty about how climate drivers may shape harmful algal blooms, a significant factor in water quality. Based on the vast literature documenting current, disparate impacts to frontline communities from floods, droughts, and the exposures they bring, there is high confidence and it is very likely that frontline communities will be at disproportionate risk from water-related hazards exacerbated by climate change.