Effects of Human-Caused Greenhouse Gas Emissions on U.S. Climate, Health, and Welfare (2025)
Chapter: 6 Impacts on Public Welfare
6
Impacts on Public Welfare
6.1 KEY MESSAGES
Climate-driven changes in temperature and precipitation extremes and variability are leading to negative impacts on agricultural crops and livestock, even as technological and other changes have increased agricultural production. There is increasing evidence of effects of excess heat and precipitation extremes on crop yields in the southeast United States, of increasing drought conditions in western U.S. agriculture, and negative impacts on agricultural crops in the Midwest. Impacts of heat stress on livestock include increased susceptibility to disease and mortality, and reduced milk production and reproduction rates.
Climate change, including increases in climate variability and wildfires, is changing the composition of forests and affecting grassland ecosystems and the services they provide. Changes in temperature and precipitation also alter phenology, tree migration (range), and interactions with pests and pathogens. Drought and other climate conditions are increasing the risk of fire, and the increase in ozone production as temperatures warm, reduces crop yield and may reduce the chance of survival for some tree species.
Climate-related changes in water availability and quality vary across regions in the United States with some regions showing a decline. Drought affects the production of food which leads to supply shortages and increased prices. Reduced water quality in lakes and coastal waters has been linked to increased temperature, oxygen depletion in deeper waters, harmful algal blooms, and effects on freshwater and marine fisheries.
U.S. energy systems, infrastructure, and many communities are experiencing increasing stress and costs owing to the effects of climate change. Increased temperatures have reduced efficiency in energy generation and transmission, while U.S. energy demand continues to increase. Transport systems have multiple stresses from climate change. Communities in Arctic regions are facing multiple threats from permafrost thaw, sea level rise, and declines in sea ice extent. Sea level rise and extreme weather pose increasing threats to the nearly 40% of the U.S. population who live in coastal counties.
6.2 CONSIDERING THE EVIDENCE
Climate change influences public welfare in a multitude of ways that affect the places where we live, work, and recreate; the food we consume; the water we drink and rely on to support agriculture and energy production; and the air we breathe. This chapter focuses on climate change effects only on major environmental systems that affect public welfare and are addressed in EPA (2009a), including agriculture, forests, grasslands, freshwaters,
coastal oceans, and the built environment (energy, infrastructure, and settlements). Aspects of climate change, notably temperature and precipitation, will manifest differently in different regions of the United States. Thus, an evaluation of the evidence of the changing climate’s impacts on public welfare is most appropriate when it considers the regional, if not subregional, level.
This chapter does not cover all ecosystems or public welfare impacts. Choices on what to highlight were guided by the committee’s overall focus on impacts with more direct attribution to climate conditions and more direct impacts on human health and well-being, as well as responses to a public Request for Information. Nonetheless, the committee recognizes that growing bodies of literature address many other areas relevant to well-being, such as recreation, sport, hunting/fishing, and cultural heritage. Likewise, there is growing literature related to climate change and economics (see Box 6.1). Some examples of economic impacts of climate change are included, although an exhaustive review of economic impacts was beyond the scope of this report.
Like the approach used in EPA (2009a), this chapter includes discussion of direct impacts of climate change but also considers some other key indirect effects on public welfare, to put climate change impacts in the broader context of observed changes in environmental systems. The most significant and well-documented climate change effects on public welfare and the changes in the evidence that have occurred since 2009 are discussed here, including a better understanding of regional variability. In addition to addressing public welfare topics broadly, a few cross-cutting topics where linkages to public health (discussed in Chapter 5) are strong are highlighted, including
BOX 6.1
Economic Impacts
Many of the effects on human health and welfare discussed in this report have associated economic impacts. A growing number of studies since 2009 have estimated the economic effects associated with climate impacts on a range of economic sectors. For example, a study that calculated the costs associated with lost agricultural productivity is described in this chapter. For individuals who have directly experienced a climate impact, these economic impacts may directly affect their earning potential, the value of property they own, or other factors that contribute to their financial stability. Significant progress has been made since 2009 in analyzing sector-specific and economy-wide impacts of climate change, though challenges remain in considering climate impacts in the context of other economic drivers, such as changes in demographics, technology, and policy (NASEM, 2024c).
Empirical evidence of economic impacts has been used to establish exposure-response functions, which relate sector-specific economic impacts to climate indicators. These exposure-response functions can then be used to estimate potential climate-related damages associated with different future climate scenarios. Studies in the literature using empirical data to establish exposure-response relationships have expanded greatly since 2009 (e.g., Carleton and Greenstone, 2022; Clarke et al., 2018; Cromar et al., 2022; Depsky et al., 2023; Diaz, 2016; Moore et al., 2017; Rode et al., 2021; Shindell et al., 2020). It is now possible to estimate future economy-wide damages by aggregating these empirical exposure-response relationships for individual sectors (EPA, 2023). This is a significant advance since 2009, when most economy-wide analyses used relationships between large-scale economic indicators (e.g., Gross Domestic Product) and average changes in climate indicators (NASEM, 2017a). An alternative approach that is also substantially advanced since 2009 uses improved empirical data to evaluate economy-wide damages associated with each unit of change in the mean or variability of surface temperature or precipitation (e.g., Kalkuhl and Wenz, 2020; Kotz et al., 2021, 2022; Waidelich et al., 2024). Both the aggregated and economy-wide approaches estimate significant costs associated with future climate change and, although they have large uncertainties, their ranges overlap, providing increased confidence in their results.
wildfire (discussed in Chapters 3 and 5), the nutritional status of food, harmful algal blooms, and toxic-laden sediments from exposed lake beds carried as dust.
6.3 DRIVERS OF ECOSYSTEM CHANGE
Ecosystems are complex, encompassing interactions among many biological communities with important linkages with the physical environment and public welfare (IPBES, 2019). Climate is a key controller of the structure and function of ecosystems. Climate change-driven shifts, particularly in temperature and precipitation, are affecting the range of services that ecosystems and the built environment provide. Linkages among temperature, precipitation, and other climate factors are explored throughout this chapter.
Generally, across the public welfare areas discussed in EPA (2009a), recent evidence has strengthened the 2009 conclusions. New evidence has also led to improved understanding of the complex interactions among climate and non-climate drivers that influence observed changes in ecosystems and the built environment, and public welfare they support. In particular, the understanding of the regional variability of impacts and the complexity of other factors (e.g., land use, air quality, pests, and pathogens) that interact with climate impacts has grown. Discussion of these interactions and variability are addressed in this chapter.
Elevated Carbon Dioxide Effects on Plant Growth
Increases in atmospheric carbon dioxide (CO2) concentrations and a climate-driven lengthening of the growing season offer positive “carbon fertilization” effects on plant growth (Norby et al., 2005; Song et al., 2019). However, these beneficial impacts will not likely fully mitigate losses associated with climate factors including heat stress, increased water demand, decreased water or nitrogen availability, or enhanced transfer of carbon below ground as plants respond to the need for additional nitrogen (Long, 1991; Mason et al., 2022; Possinger et al., 2025; Wolfe et al., 1998). Carbon fertilization benefits have been difficult to detect in forests (Girardin et al., 2016; Possinger et al., 2025). Moreover, rapid growth does not necessarily translate to higher crop yields because faster development results in smaller plants, a shortened reproductive period, and reduced yield (Hatfield and Prueger, 2015; Hatfield et al., 2011; Zhu et al., 2021).
Other Effects of Greenhouse Gases on Plants
Methane (CH4) emissions lead to increased ground-level ozone (see Chapter 3), which damages many crops and trees. For the United States, analysis by the United Nations Environment Programme provides estimates of crop yield losses driven by CH4 emissions (via induced climate, ozone, and CO2 changes), finding yield losses of roughly 1,750,000 metric tons of maize (corn), 340,000 metric tons of wheat, 60,000 metric tons of rice, and 790,000 metric tons of soybeans for every 100 million metric tons emitted (UNEP and CCAC, 2021).1 For context, 100 million metric tons represents about 25% of current anthropogenic CH4 emissions, and these losses represent 0.6–1.3% of global yields of these crops with values up to 3–4% for individual countries (UNEP and CCAC, 2021). Non-CO2 greenhouse gases (GHGs), such as nitrous oxide (N2O) or fluorinated gases, affect plants only via climate change.
Non-Climate Drivers
Important drivers of environmental system changes beyond those linked to climate can amplify or mitigate public welfare effects. These non-climate drivers include land use and land cover change, pests and pathogens, nitrogen deposition, and changes in air quality driven by air pollution emissions not mediated through climate change. These non-climate drivers occur coincidently with climate change but are highly variable in space and time. At the same time, some of these non-climate drivers are affected by climate change (e.g., ozone and nitro-
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1 These values were obtained from Tables 3.6a-d in UNEP and CCAC (2021) and scaled to report values per 100 million metric tons.
gen deposition, noted in Chapter 3) and have also been shown to either amplify or mitigate ecosystem response to a changing climate (e.g., Baron et al., 2013; Bytnerowicz et al., 2007). Non-climate drivers contributing to the impacts discussed are noted throughout relevant sections in this chapter.
6.4 FOOD PRODUCTION AND AGRICULTURE
Agriculture production and climate are intrinsically linked, and evidence collected since 2009 strengthens messages conveyed in EPA (2009a). In this section, the committee details major effects of climate and interacting non-climate drivers on crop production, agricultural pests and weeds, and livestock.
Climate Effects on Crop Production
Increases in temperatures and variability in precipitation amount and intensity have negatively affected agricultural production in the United States (Eck et al., 2020; Hatfield et al., 2011; Lesk et al., 2022), although the extent of this impact varies by region. For the period of 1991–2017, temperature-related crop losses have resulted in $27 billion in crop insurance claims (Diffenbaugh et al., 2021). Increased temperature has lengthened the frost-free days by 2 weeks since 1970;2 however, a longer growing season increases the need for water and nutrients (fertilizer) to take advantage of the additional crop growth potential.
Winters are also warming, as documented by the U.S. Department of Agriculture Plant Hardiness Zone Map,3 which provides guidance on where specific plants are most likely to thrive. The hardiness zones have been adjusted northward, most recently in both 2012 and 2023. These maps are developed using 30-year averages of the lowest annual winter temperature at given locations, reflecting a trend in temperatures with direct implications for plant growth.
Extreme heat, drought, and moisture excess are increasingly co-occurring within a single growing season since 2000, resulting in up to 30% yield losses globally, with the United States noted as a region of greatest losses (Lesk et al., 2022). Similarly, extreme heat events and warm nights have decreased yields, and episodic temperature increases that exceed plant physiologic thresholds reduce yield and cause plant stress throughout the life of the crop, especially during flowering (Hatfield et al., 2011; Schlenker and Roberts, 2009).
Variability in temperature and precipitation effects has been observed across U.S. regions. For example, excess heat and precipitation extremes within the growing season have negatively affected crop yield in the southeast United States (Eck et al., 2020). The western United States has become hotter and drier in recent years (1976–2019) with associated negative impacts on agriculture production and other ecosystems (Su et al., 2021; Zhang et al., 2021).
Increasing temperatures across the United States have increased evaporation and plant transpiration (water evaporation from the plant surface). This change leads to an increase in water deficits and crop economic losses (Hatfield et al., 2011). In some parts of the United States, water will become less available from both reductions in rainfall and increasing drawdown of water for irrigation (e.g., the Ogallala region in the Great Plains). Hot-dry-windy events have significantly increased in the U.S. Great Plains from 1982 to 2020. These events have resulted in a 4% yield reduction per 10 hours of hot-dry-windy conditions during the reproductive stage of wheat (Zhao et al., 2022).
In addition to direct effects of temperature and precipitation, changes in the nutritional value of crops have been observed when grown under elevated CO2 conditions (see also Chapter 5 discussion). Non-legume crop species often have lower protein content when grown under elevated CO2 (Kimball, 2010). C3 (cool season) grains and legumes have lower concentrations of zinc and iron when grown under elevated CO2, while C4 (warm season) crops are less affected (Dietterich et al., 2015; Myers et al., 2014). These nutritional changes affect dietary needs for both human food crops and livestock forage.
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2 See https://www.epa.gov/climate-indicators/climate-change-indicators-length-growing-season (accessed September 2, 2025).
3 See https://planthardiness.ars.usda.gov (accessed September 8, 2025).
While many climate impacts have been observed, negative impacts might be more severe in the absence of efforts to adapt or improve agricultural practices in response to observed change. These adaptive measures include actions such as plant breeding, crop switching, soil management, and improved technologies (e.g., irrigation, water conservation, and precision agriculture).
Crop Pests and Weeds
Temperature is the single most important factor affecting insect ecology, epidemiology, the number of generations per growing season, and insect distribution (Skendžić et al., 2021). Warmer winters affect crops and weeds and also expand the potential habitable range of some insect and disease pests. Plant pathogens are highly responsive to humidity and rainfall, as well as temperature (Lahlali et al., 2024). Since 1960, data have documented a northward shift in pests (Bebber et al., 2013), and pests are expected to reduce crop yield as a result of warming (Deutsch et al., 2018). Increased pests may also result in more chemical applications at a cost to the farmer and the environment.
Many C3 weed species show substantial growth increases and resistance to herbicides when grown at elevated CO2 levels (Ziska, 2003; Ziska et al., 1999). As a result, rising atmospheric CO2 could lead to yield reductions when weed control is insufficient, potentially increasing the need for chemical applications, increasing costs to farmers and chemicals in the environment.
Climate Effects on Livestock
The EPA (2009a) discussion of livestock production is supported with new evidence and strengthened by recent research findings. A growing body of evidence indicates that summer heat stress has negative impacts on animal behavior. Livestock performance depends on their environment. The direct effects of temperature, variable precipitation, and extreme events impact thermoregulation, metabolism, and immune system function (Cheng et al., 2022). Heat stress increases susceptibility of livestock to diseases and death and decreases weight gain, milk production, and reproduction rate. For instance, from 2012 to 2016, milk yield was shown to be reduced by 1% due to heat stress resulting in $253 million in lost revenue across 19 states (Hutchins et al., 2025). Another study found that in 2010, lower milk production due to heat stress resulted in up to $1.2 billion in losses to the dairy sector (Key et al., 2014).
In addition to hotter summer temperatures generally, animals are experiencing more extreme heat events and temperature swings, which also impact animal behavior and stress. This heat exposure is more acute for cattle on grazing lands because fewer options exist for mitigating heat effects. In addition, having cattle on grazing lands that are susceptible to fires brings the animals into close proximity to both fire and smoke.
Indirect effects of climate change on livestock relate to feed production (declines in and reduced nutritional value), changes in water availability linked to shifting precipitation patterns, and increased exposure to pests and parasites. In grazing systems, livestock production is reduced by lower forage quality due to higher temperature, elevated CO2, and drought stress (Polley et al., 2013).
Climate Impacts on Commercial Fisheries
Climate change resulting from GHG emissions has impacted commercial marine fisheries in every coastal region of the United States. Impacts on fisheries include losses in the abundance and quality of harvested species and fisheries-related revenue and job loss (Fisher et al., 2021; Free et al., 2019; Pershing et al., 2018). Changes in climate are not the only drivers affecting fish populations but are additional stressors that can exacerbate other negative impacts and overwhelm and outpace even gold-standard fisheries’ management regimes, such as the North Pacific Fishery Management Council and the National Oceanic and Atmospheric Administration Fisheries in the Gulf of Alaska. Although some marine species have benefitted from ocean warming, for example the northern stock of American lobster (Le Bris et al., 2018), a variety of climate conditions related to warming have produced
declines in other species. For example, low sea ice conditions and a long period of warming temperatures in the Bering Sea led to declines in stocks of Pacific cod and snow crab (Fedewa et al., 2020; Spies et al., 2020).
Negative responses have been observed in marine fisheries’ populations near the warmer edge of their range, though a history of overfishing and other ocean changes are also contributing factors (Free et al., 2019). Populations showing a positive response were those on the cold edge of the species range. USGCRP (2023) noted that the incidence of disaster declarations for commercial fisheries rose from 1994 to 2019, and the majority of those disasters (more than 84%) were linked to extreme environmental events.
6.5 FORESTS
Forests are a critical resource for the United States, covering approximately a third of the nation’s land area. The forest products industry represents about 4.7% of total U.S. manufacturing gross domestic product and serves as an important manufacturing sector in the United States (AF&PA, 2022). Forest cover has decreased slightly in the contiguous United States over the last 20 years largely due to expansion of croplands and urbanization, which includes increases in development at the wildland–urban interface (USGCRP, 2023). Forests are dominated by trees and woody vegetation and are commonly situated in the headwaters of freshwater ecosystems (i.e., wetlands, rivers, streams, lakes). As a result, forest and freshwater ecosystems are often intimately connected, with the structure and function of each dependent on the processes and resources the other supplies.
Forests provide a suite of services including marketable forest products, cleansing the atmosphere of pollutants, retaining nutrients, influencing water supply, and flood and erosion control (USGCRP, 2023). Spending time in forests has also been shown to have positive health effects for people (Jimenez et al., 2021). Additionally, forests also serve to regulate climate by the net removal of CO2 from the atmosphere through photosynthesis and storage in tree biomass and soils. Forests also provide rich biodiversity, aesthetics, recreation, and cultural experiences (USGCRP, 2023).
Climate Effects on Forests
Climate change, including increases in climate variability, is changing the community composition and function of forest ecosystems and the services they provide (Campbell et al., 2022). Often these changes are subtle, manifested over decades and difficult to detect without careful long-term observation (Jones and Driscoll, 2022). In contrast, extreme events such as intense storms, extreme heat or prolonged wet or dry conditions, increases in pests and pathogens, and fire can have more marked effects on forests than those observed under gradual change (Andrus et al., 2025; Smith, 2011; Ummenhofer and Meehl, 2017).
There are several mechanisms by which climate change affects forest ecosystems. Increases in temperature may either increase or decrease tree growth due to changes in soil nutrient availability, hydraulic conductivity, and vapor pressure deficit (Grossiord et al., 2020; McDowell et al., 2020). Increases in precipitation and humidity are expected to increase tree growth through decreases in water deficit stress and increases in weathering and nutrient availability. At the same time, decreases in precipitation correspondingly decrease tree growth, but it has been shown that extreme dry conditions decrease growth more than wet extremes increase growth (Dannenberg et al., 2019). Both changes in temperature and precipitation also alter phenology and tree migration (range).
Forest ecosystems also sequester and store carbon, thereby providing some mitigation of human-caused CO2 emissions (Pan et al., 2011). Understanding of this benefit has been greatly strengthened since EPA (2009a). In the United States, forests are a large net carbon sink, meaning they remove much more carbon from the atmosphere than they release (USGCRP, 2023). However, the strength of the U.S. forest carbon sink has declined over recent decades, due in part to climate-related disturbances largely associated with wildfires and insect outbreaks, and in part to forest management and land use change associated with increases in urbanization and agriculture (USGCRP, 2023). Hogan et al. (2024) evaluated trends in forest productivity in the United States, finding generally positive trends in productivity in the eastern United States under mild warming and increases in precipitation. In contrast, forest productivity was found to decline in much of the West where warming was more pronounced and precipitation decreased.
The U.S. Department of Agriculture Forest Inventory and Analysis database is a critical tool for quantifying forest resources within the United States and understanding forest change. Recent analysis of changes in the growth and survival of tree species across the contiguous United States demonstrate mixed responses to climate drivers (Clark et al., 2024) with potential implications for the services forest provide. Growth of 44 of 153 tree species studied decreased with increases in mean annual temperature, whereas fewer than 20 species in either the eastern or western United States exhibited negative growth associations with trends in mean annual precipitation. Average annual growth and decadal survival generally decreased with wetter conditions in the East and drier conditions in the West. Only eight species considered were tolerant of increases in temperature. In the East, 24 species were found to be tolerant of increases in precipitation and only seven in the West were tolerant of decreases in precipitation. There were at least a few species that had a similar response (either positive or negative) across the contiguous United States for growth and survival metrics.
The Forest Inventory and Analysis database has also recently been used to evaluate ozone impacts on forest growth and survival of 88 tree species in the contiguous United States (Pavlovic et al., 2025). As a whole, ozone exposure was generally below critical levels to impair tree growth, but exceeded levels needed to protect survival for some species.
As discussed in Chapter 3 of this report, wildland fires are a growing climate concern for forested ecosystems. At present the public health risks associated with these fires (discussed in Chapter 5 of this report) are a dominant impact on public welfare, though other effects such as increased homeowners’ insurance premiums in fire-prone areas are growing.
Forest Pests and Pathogens
As with crops, pests and pathogens are an important driver of tree growth and mortality and fungal composition and function, whose impacts have been markedly altered and intensified by a changing climate (Simler-Williamson et al., 2019). These impacts occur through tree physiology, mortality, and morbidity (Andrus et al., 2025; Cobb and Metz, 2017; Preston et al., 2016). Increases in pathogens and insect pests cause changes in forest composition (Metz et al., 2012), disrupt food webs (Ellison et al., 2005), and alter biogeochemical processes (Preston et al., 2016). Climate change alters the survival rates of pests and pathogens (Simler-Williamson et al., 2019). Often rates of over-winter survival limit outbreaks, but increasing winter temperatures have been linked to increasing pest occurrence and impacts (McAvoy et al., 2017). Moreover, changes in temperature, humidity, and precipitation affect the reproduction and growth rates of pests and pathogens. In addition to effects on pests and pathogens, climate change can also impact the susceptibility of a tree host to infection, invasion, or damage resulting in changes in physiology, morphology, and population or community structure.
6.6 GRASSLANDS
Grasslands account for approximately 29% of U.S. land area and serve as an important ecosystem for livestock grazing and supporting wildlife. Grasslands are an important ecosystem for carbon storage; globally, more than 30% of terrestrial carbon occurs in grasslands soils (Bai and Cotrufo, 2022). Grasslands in the Great Plains were estimated to contain 34.9% of the total carbon stocks in the region from 2001 to 2005 (Pendall et al., 2018). It has been estimated that U.S. grazing lands contribute 14.7% of the U.S. soil carbon sequestration potential (Lal et al., 2003). Thus, grasslands are important for the economy and ecosystem services.
Increasing temperature can reduce grass productivity in tallgrass prairie (Koerner et al., 2023). Since 1984, the amount of plant biomass (known as annual net primary productivity) has increased in Great Plains grasslands due to increased growing season precipitation (Reeves et al., 2021). However, there are regional differences. The northern Great Plains may benefit from a longer growing season while the southwestern Great Plains will likely show a decline due to increased drought, higher temperature, and greater variability (McCollum et al., 2017). Since 2000, below average precipitation and above average temperature have been observed in the southwestern United States, indicative of a changing climate (Williams et al., 2023). This has reduced grassland productivity, with implications for grazing livestock production. Rangeland grazing capacity in New Mexico has declined by
43% over a 52-year period (1967–2018) due to higher growing season temperatures and increased frequency of drought (McIntosh et al., 2019).
Fire is a part of grassland ecology and historically has helped to suppress woody plants. However, a combination of elevated CO2, increasing temperatures, and land and fire management practices is contributing to the expansion of trees into grasslands (Morford et al., 2022) resulting in a loss of livestock productivity.
6.7 COASTAL OCEAN ECOSYSTEMS
Coastal ocean ecosystems are experiencing warmer waters, sea level rise, increasing pressures from human development, and other stressors (May et al., 2023). These ecosystems play an important role in protecting coastlines. Tidal wetlands, which include mangroves and salt marshes, provide crucial habitats for fish and wildlife, and their dense vegetation helps to slow and absorb floodwaters, protecting inland areas from storm surges and high tides. Wetlands serve as important nursery grounds and feeding areas for many commercial fish species.
Sea level rise and increasing coastal hazards associated with climate change can drive tidal wetland loss (Weis et al., 2021). Tidal wetlands can move landward to escape rising sea levels, a process called inland migration. This can occur if there is space and time for the wetland to move inland before it erodes or is submerged. The evidence of potential loss has grown since EPA (2009a). A net loss of tidal wetlands is expected throughout the United States, but the rate and extent of loss will vary significantly from place to place depending on local conditions and inland migration. For example, loss and migration of tidal wetlands linked to sea level rise has been observed in the Chesapeake Bay (Schieder et al., 2018), Florida (Raabe and Stumpf, 2016), and New Jersey (Weis et al., 2021). Along the Pacific Coast, tidal wetlands cannot migrate inland due to coastal development and steep topography, increasing the chances of net tidal wetland loss due to sea level rise.
6.8 WATER RESOURCES
The amount and quality of water available for use by humans has direct impacts on welfare in a variety of ways. Drought affects the production of food, which leads to supply shortages and increased prices. Increases in land area inundated by flood waters imply increases in losses of life and property. Deteriorating water quality limits human use of water for multiple purposes, including drinking water and recreation.
Impacts of climate change on water resources, including water quality and water availability, droughts, and floods, are affected by regional hydroclimatology. For example, despite increasing air temperatures everywhere in the United States, some regions, such as much of the East, are experiencing significant increases in total precipitation, while in other regions, including parts of the West, precipitation is decreasing. This section provides technical information about impacts and recent trends and discusses the regional variability observed. Additional discussion of climate impacts on precipitation and drought is provided in Chapter 3 of this report.
Water Quality
The quality of many streams and rivers across the contiguous United States is declining in response to climate change, with implications for drinking water and municipal use, energy, fisheries, and other uses of freshwater. Water quality is affected by land cover and land use, and by many direct (e.g., sewage disposal) and indirect (e.g., use of fertilizers) human influences, creating complexity in understanding climate and non-climate drivers of water quality change. A recent review of 965 case studies indicated that 56% of observed water quality issues were related to climate change due to increasing water temperatures and changes in low flow periods (van Vliet et al., 2023). Some substances, such as nutrients and pharmaceuticals, show mostly increasing trends in concentrations, whereas others, such as sediment, biochemical oxygen demand, and metals show a mixture of increasing and decreasing trends in concentrations (van Vliet et al., 2023). Lakes have also been impacted by increasing temperatures in multiple ways, including a strengthening and lengthening period of thermal stratification, enhanced depletion of dissolved oxygen in deeper waters, loss of habitat for cold-water fisheries, and other threats such as spread of invasive species and loss of biodiversity (Jane et al., 2021, 2024; Woolway et al., 2022).
Harmful Algal Blooms
A consequence of warming waters and enhanced stratification of lakes and coastal waters is an increase in harmful algal blooms (Lefebvre et al., 2025; Townhill et al., 2018; Trainer et al., 2020). Additional changes in environmental conditions, such as increases in nutrients, can stimulate growth and blooms of cyanobacteria, called harmful algal blooms (Chapra et al., 2017). Understanding and documentation of climate-driven impacts of harmful algal blooms have increased greatly since 2009. In freshwaters, cyanobacteria (also known as blue-green algae) occur naturally and are able to outcompete other types of algae under warm water conditions (Cottingham et al., 2021). Some species of cyanobacteria can release toxins when environmental conditions are favorable, and the cyanobacteria present can express genes that produce the toxins. These toxins can harm people and animals drinking or recreating in contaminated waters or inhaling air near affected water sources (Plaas and Paerl, 2021). Under extreme conditions, when drinking water sources are affected, closures or additional treatment of the water supply may be needed.
In coastal waters, dinoflagellates or diatoms are the most common algae causing harmful algal blooms (Anderson et al., 2021). In coastal waters, fish kills have been reported when water temperatures are much higher than normal and associated with harmful algal bloom events. Shellfish contamination has been documented, with toxins making shellfish unsafe to consume (e.g., Lefebvre et al., 2025; McCabe et al., 2016).
For recreational fresh and coastal waters, many states post advisories in response to the occurrence of harmful algal blooms, warning people against contact during recreational activities and the potential for respiratory distress. Advisories have been listed for fresh and marine waters across the United States, from Florida to Alaska. There has been a marked increase in the number of advisories during the warm water months and in the annual total number of advisories since compilation was initiated in 2015.4 These increases likely reflect increases in harmful algal bloom events, increased awareness of the problem, and improvements in monitoring efforts. Economic losses linked to harmful algal bloom impacts on coastal fisheries and aquaculture in the United States have been estimated to be tens of millions of dollars (Anderson et al., 2021; Jin et al., 2020).
Water Supply and Availability
Impacts of climate change on water supply and availability are complicated by the impacts of non-climate factors, such as changes in water consumption. Overall water withdrawals for all uses in the contiguous United States reported for 2015 were the lowest since 1970 (Warziniack et al., 2022). The primary consumptive uses of water are for irrigation and thermoelectricity generation. Water withdrawals for irrigation have remained relatively constant for decades despite increases in acreage under irrigation, presumably because of increases in efficiency (Warziniack et al., 2022). Withdrawals for thermoelectric power plant cooling have declined, again because of technological improvements (Warziniack et al., 2022).
Hydroclimatic changes driven by atmospheric warming include changes in precipitation and evapotranspiration demand. These changes interact with the land surface through exchanges of energy and water. The consequences of these changes are reflected in spatial and temporal patterns of precipitation, evapotranspiration, and soil moisture (Herrera et al., 2023). Changes of hydroclimate across the contiguous United States have changed the size and timing of rainfall over the past several decades (Marvel et al., 2021).
Baseflow Drought
A meteorological drought is a period of low precipitation that has cascading effects. The lack of precipitation delivered to the soil surface is accompanied by increased evaporation demand, which leads to soil moisture drought. Additionally, reduced recharge to shallow groundwater leads to baseflow (i.e., streamflow and groundwater flow) drought. The discussion in this section refers to baseflow drought (see Chapter 3 for discussion of meteorological drought). EPA (2009a) did not report significant evidence of increases in drought due to climate change in the decades leading up to 2009. There is now some evidence for increasing drought severity in some areas of the United States.
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4 See the EPA Tracking CyanoHAB website at https://storymaps.arcgis.com/stories/d4a87e6cdfd44d6ea7b97477969cb1dd (accessed September 3, 2025).
Historical data show that drought magnitude is increasing in some regions and decreasing in others. For the period 1981–2020, drought duration and deficit were studied using reference (i.e., largely unimpacted by significant land-use changes) watersheds and were found to have decreased in the north and east and increased in the Southwest and south-central United States (Hammond et al., 2022). Alonso-Vicario et al. (2025) included catchments that were impacted by agriculture and urbanization, but the broad pattern for droughts across the United States was similar.
The impact of climate change in many regions may be confounded by the impact of other human activities (Vicente-Serrano et al., 2022). Attribution of increasing drought trends to climate change is difficult because the areas where these trends are observed overlap with areas of increasing water demand and land cover change. In the U.S. Southwest, decreasing flows in the Colorado River have been attributed to increased evapotranspiration driven by climate warming (Milly and Dunne, 2020). The observed 9.3% flow decrease per degree Celsius of warming is likely to continue in the future although it may be partially offset by increases in precipitation (Milly and Dunne, 2020).
Lag times between precipitation decreases and baseflow decreases are typically months and the duration of baseflow droughts from months to years. Baseflow droughts in watersheds unimpacted by human water use increased over the period from 1982 to 2012 in the mild temperate zone (Lee and Ajami, 2023). This zone consists generally of the Southeast, the eastern portion of the southern Great Plains, and the West Coast. In watersheds impacted by groundwater withdrawals, links to climate change are indirect (e.g., through the increased use of groundwater for irrigation). Meteorological droughts are linked to groundwater level declines in broad areas of the United States (Singh et al., 2025).
Lake levels have mostly increased across Alaska and the northern tier of the contiguous United States and have decreased in the intermountain West and the Southeast over the past two decades (Feng et al., 2022). Decreased water levels in terminal lakes in the Great Basin have been partially due to climate change (Hall et al., 2023). Declines in water levels in these terminal lakes have direct implications for human welfare as portions of exposed lake beds result in wind-blown sediments that contain toxic metals (see Chapter 5 for discussion of health effects of these toxins) (NASEM, 2020).
Floods
Across North America, the magnitude of extreme precipitation at the continental scale and at broad regional scales has increased (see also Chapter 3 of this report) (Kirchmeier-Young and Zhang, 2020). Like other aspects of water resources, flooding is influenced by many factors beyond climate. River floods are affected by characteristics of the land and by both the amount and timing of precipitation. Because of this combination of factors, regional-scale flood-hazard changes are not necessarily directly linked to precipitation changes (Blöschl, 2022), and some regions report no clear signal of increases in riverine floods despite the increases in extreme precipitation (Kundzewicz and Pińskwar, 2022). A more nuanced analysis that separates areas of rain-induced flooding and areas of snowmelt-induced flooding indicates that the annual maximum flood is increasing across the former and decreasing across the latter (Zhang et al., 2022).
In the western United States, data from reference watersheds indicate a decrease in the magnitude and frequency of rain-on-snow flooding, an increase for convective storm flooding, and little change in floods caused by other mechanisms (Huang et al., 2022). Patterns of flood magnitude trends studied across the contiguous United States show land use and hydroclimate change to be equally important in determining the trends (Kemter et al., 2023). Seasonal changes in flood frequency are also quite heterogeneous spatially, but in general show more declining trends in spring and summer and more increasing trends in autumn and winter (Gu et al., 2025). Trends in baseflow in rivers have been linked to trends in annual floods in North America (Berghuijs and Slater, 2023). The complex interactions among climate and land variables that result in river flows indicate that attribution of observed changes in flood magnitudes and frequencies will be uncertain (Scussolini et al., 2024).
Annual maximum snowpack decreased significantly in the contiguous United States from 1982 to 2016, and the snow season shortened by about a month (Zeng et al., 2018). These changes in snowpack have important implications for soil moisture limitations during summer, wildfires (see Chapter 3), and water supplies.
USGCRP (2023) highlights that flood hazards have disproportionate impacts on communities across the country. Coastal communities, communities situated on rivers, and agricultural and fishing communities experience more flood hazards (Edmonds et al., 2020; Thiault et al., 2019).
6.9 ENERGY, INFRASTRUCTURE, AND SETTLEMENTS
This section provides examples of climate change impacts on the built environment and describes their links to public welfare. For energy, this includes discussion of energy production as well as changes in demand associated with climate change. Similar to other welfare impacts discussed, impacts can be wide-ranging and have considerable geographic variability. Where people live (e.g., coastal regions or in or near forested areas) affects their vulnerability to some climate change impacts, such as coastal erosion or wildfires, with attendant economic loss (Deilami et al., 2018).
Heating and Cooling Requirements
Climate warming is increasing the number of cooling degree days and reducing heating degree days5 in the United States (EPA, 2024b). The result is significantly increased demand for air conditioning. Air conditioning can help people remain in a temperature range that is comfortable and safe (see Box 5.1). Heat waves can result in both increased deaths and illness, with elderly and low-income populations particularly vulnerable to increased heat in urban areas6 (Qian and Liu, 2025). Urban areas can be especially vulnerable to heat waves. Where the albedo (proportion of incident light that is reflected versus absorbed by a surface) is low, incoming heat is absorbed by the built environment, creating Urban Heat Islands, but attribution to increased GHG is difficult to establish (Martilli et al., 2020). Chapter 5 of this report provides a more detailed discussion of temperature effects on public health and associated impacts.
Energy Production
Demand for electricity is continuing to increase as a result of population growth, higher incomes, electrification of transportation, and internet services and data centers. Cooling needs due to warmer temperatures also has a significant impact on electricity demand. The peak demand for U.S. electricity was set in late afternoon in July 2025 at 759,180 megawatts, representing a 2% increase from the previous summer peak in 2024 (EIA, 2025c). Increasing peaks require investment in new generating, transmission, and distribution capacity.
Climate change is also increasing the costs of generating power (Bartos and Chester, 2015). As noted in EPA (2009a), warmer waters make the cooling of power plants less efficient, thus more costly, and warmer temperatures make energy transmission less efficient. Extreme weather events coupled with sea level rise in coastal areas create energy supply disruptions that require resiliency investments. Hydroelectric power generation is susceptible to decreases due to droughts and decreased snowpacks.
Infrastructure and Settlements
Climate change is affecting numerous settlements, especially in coastal regions.7 For example, in Alaska, observed increases in coastal and riverbank erosion are causing damage and growing risks to settlements in numerous communities (Huntington et al., 2023). Rapidly warming temperatures in the Arctic region are leading to increased erosion through interactions with permafrost thaw, sea level rise, declines in sea ice extent, the lengthening of open water period, and increased impacts of storms along coastlines (Gibbs et al., 2021; Irrgang et al., 2022; Jones et al., 2020). In extreme cases, abandonment or relocation of affected settlements is needed. Inland areas are also subject to permafrost thaw, which can cause ground subsidence and landslides, affecting
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5 Degree days are calculated using the assumption that when it is 65°F, neither heating or cooling is needed, then calculating the difference between the daily temperature mean and 65°F. If the temperature mean is above 65°F, then 65 is subtracted from the mean to calculate Cooling Degree Days. If the temperature mean is below 65°F, the mean is subtracted from 65 to calculate the Heating Degree Days. See https://www.weather.gov/key/climate_heat_cool (accessed September 3, 2025).
6 See https://www.cdc.gov/heat-health/risk-factors/heat-and-older-adults-aged-65.html (accessed September 3, 2025).
7 See https://oceanservice.noaa.gov/facts/population.html (accessed September 3, 2025).
settlements and infrastructure (Makopoulou et al., 2025). The Gulf Coast is also experiencing similar settlement stresses due to sea level rise, hurricanes, and erosion that can cause relocations of communities (NASEM, 2024b).
Nearly 40% of the U.S. population currently lives in coastal counties. Sea level rise (discussed in Chapter 3) coupled with extreme weather has direct impacts on coastal infrastructure and settlements, including flooding, erosion, wind damage, and saltwater incursion to water supplies and cropland. For example, saltwater incursion and the pressure of the water even when it is not moving may damage structural foundations (Abdelhafez et al., 2022).
Increased wildfire severity (discussed in Chapter 3) is impacting infrastructure and settlements. In the western United States, structural losses due to wildfires increased over 200% between the decades 1999–2009 and 2010–2020 (Higuera et al., 2023). Wildland–urban interfaces are growing as settlements expand spatially into fire-prone areas, posing significant risks to structures and human health (see Chapter 5) (NASEM, 2022). The cost of insurance is also rising while the ability to retain and obtain insurance is declining in areas with wildfire risks (Auer, 2024).
Climate change also affects U.S. highway infrastructure, where impacts are driven by temperature, precipitation, sea level rise, and hurricanes (see discussion of hurricanes in Chapter 3). Table 6.1 details highway impacts (TRB, 2014). Increased costs are already being observed from effects such as softening asphalt due to high temperatures (Sias et al., 2025). Thermal expansion on roadways and bridges is also a significant issue (Zhiyuan et al., 2025). Other infrastructure systems experience similar impacts, including energy, public transportation, railroads, ports, military facilities, and water infrastructure.
TABLE 6.1 Summary of Climate Change Impacts on the Highway System
| Climatic/Weather Change | Impact to Infrastructure | Impact to Operations/Maintenance |
|---|---|---|
| Temperature | ||
| Change in extreme maximum temperature |
|
|
| Change in range of maximum and minimum temperature |
|
|
| Climatic/Weather Change | Impact to Infrastructure | Impact to Operations/Maintenance |
|---|---|---|
| Precipitation | ||
| Greater changes in precipitation levels |
|
|
| Increased intense precipitation, other change in storm intensity (except hurricanes) |
|
|
| Sea Level | ||
| Sea level rise |
|
|
| Climatic/Weather Change | Impact to Infrastructure | Impact to Operations/Maintenance |
|---|---|---|
| Hurricanes | ||
| Increased hurricane intensity |
|
|
SOURCE: TRB, 2014. Table formatting has been modified.
