Today, scientists released a new report documenting the growing risks that climate change poses for communities across the United States.
The new report is part of the fourth edition of the National Climate Assessment (NCA4), developed by the U.S. Global Change Research Program (USGCRP). The new report reaffirms that evidence of human-caused climate change is robust, is extensive, and continues to strengthen. It also shows that risks to humans, ecosystems, and the economy are intensifying across the country [USGCRP, 2018].
Building on an evaluation of observed and projected changes in the physical climate system in the United States, released in 2017 (USGCRP ; see a plain language summary in Eos by Wuebbles et al. ), volume 2 of NCA4 specifically focuses on climate-related risks to humans and systems that support our well-being and economy. The new report is the most comprehensive and authoritative assessment to date of the state of knowledge on current and future impacts of climate change on society in the United States.
The new report notes a number of key findings.
First, if greenhouse gas emissions continue to rise throughout the 21st century, following historic trends—tracking the Intergovernmental Panel on Climate Change (IPCC) Representative Concentration Pathway 8.5 (RCP8.5) scenario—climate change is expected to cause substantial damage in different sectors of the U.S. economy by 2100, especially without increased adaptation.
Under the RCP8.5 scenario, annual losses in some sectors, such as labor productivity and coastal property, could reach hundreds of billions of dollars by the end of the century. Without adaptation, coastal property damage across the contiguous United States could reach several trillion in the same time frame. By then, annual damages associated with an increasing number of extreme temperature-related deaths are projected to be well more than $100 billion.
Second, economic damages and other impacts of climate change in the United States can be substantially reduced over this century through global-scale emissions cuts and regional adaptation measures.
For example, annual damages associated with an increasing number of deaths related to extreme temperatures in 2090 could be reduced by well more than 50% under a scenario with significantly lower emissions. In this scenario—the IPCC’s RCP4.5—emissions are substantially cut by 2050 with greater reductions thereafter that result in about 85% lower emissions than in RCP8.5 by the end of the century. Well-timed adaptation measures can also reduce costs significantly, in some sectors by half.
Third, reducing emissions is expected to significantly lower health risks from climate change faced by those living in the United States. For example, annual health impacts and health-related costs are projected to be approximately 50% lower under RCP4.5 versus under RCP8.5.
The impacts of climate change, both current and expected under future scenarios, are outlined in the new report’s 10 regional and 18 national topic chapters. They confirm that not only are effects already being felt in communities across the United States but that climate-related risks to resources and systems that sustain society are expected to grow in the coming decades.
What’s more, without further actions to reduce global greenhouse gas emissions and increased regional adaptation to changes that are underway, these risks and costs will be much higher.
What’s New in This Report?
NCA4’s regional and national topic chapters detail current and expected future impacts of and risks from climate change on interconnected human and natural systems across the country.
To a much greater degree than previous assessments, this volume of NCA4 includes broader and more systematic quantification of climate change impacts in the United States in economic terms under different future greenhouse gas emissions scenarios. Although this is an emerging body of literature that does not yet characterize differential economic impacts for each of the identified regions, it represents a valuable advance in understanding how physical climate changes translate into monetary damages and benefits. It also provides an indication of the potential for reducing risks through mitigation actions.
Risks and Costs to the Economy and Infrastructure
The report is clear: Many extreme weather and climate-related events are expected to become more frequent and more intense as temperatures continue to rise, creating greater risks of infrastructure disruption and failure that can cascade across economic sectors.
Coastal Risks. Along the U.S. coastline, public infrastructure and $1 trillion in national wealth held in coastal real estate are threatened by rising sea levels, higher storm surges, and the ongoing increase in high-tide flooding. The Atlantic and Gulf coasts face greater than average risk compared with other regions of the country.
In 2017, for example, Hurricane Harvey produced an unprecedented amount of rainfall over the greater Houston area, some of which has been attributed to human-caused climate change. Resulting power outages and floods had cascading impacts on businesses, critical infrastructure, and services, including a reduction in oil production and refining capacity in the Gulf of Mexico that caused spikes in gasoline prices regionally as well as nationally. Preliminary damage estimates place Harvey as one of the two costliest U.S. natural disasters in inflation-adjusted dollars, rivaling only Hurricane Katrina.
Without adaptation, cumulative damages to coastal property across the contiguous United States could reach $3.6 trillion through 2100 under RCP8.5. More than half of damages to coastal property from sea level rise and flooding are expected to be avoidable by proactive adaptation measures such as shoreline protection and beach replenishment.
Wildfire Risks. In the western United States, increasing wildfire activity is damaging ranches and rangelands as well as property in cities near the wildland-urban interface. For example, wildfires around Los Angeles from 1990 to 2009 caused $3.1 billion in damages (unadjusted for inflation). The report notes that the area burned by wildfire across the western United States from 1984 to 2015 is estimated to be twice what would have burned had climate change not occurred.
Drier conditions are expected to increase the risk of wildfire and damage to property and infrastructure in the coming decades as climate continues to change. Land use and forest management decisions can reduce—or exacerbate—risks to people and property from wildfires.
Risks to Agriculture. Yields of major U.S. crops such as corn, soybeans, and cotton are expected to decline, on average, across the country over this century as a result of climate change. Increases in temperatures during the growing season in the Midwest are projected to be the largest contributing factor to declines in the productivity of U.S. agriculture.
By midcentury, under the RCP8.5 emissions scenario, 5-day maximum temperatures are projected to have moved further above optimum conditions for many crops and closer to the reproductive failure temperature, especially for corn in the southern half of the Midwest.
Risks to the Workforce. Extreme heat poses a significant risk to human health and labor productivity in sectors of the economy reliant on outdoor workers, including agriculture, construction, and others (Figure 1).
Under the RCP8.5 emissions scenario, almost 2 billion labor hours are projected to be lost annually by 2090 from the impacts of temperature extremes, costing an estimated $160 billion in lost wages. However, following the RCP4.5 emissions scenario, the annual loss of hundreds of millions of labor hours from extreme temperatures nationwide could be avoided.
Risks and Costs to Natural Environment and Ecosystem Services
Climate change has already had observable impacts on biodiversity and ecosystems throughout the United States that are expected to continue. Without substantial and sustained reductions in global greenhouse gas emissions, transformative impacts on some ecosystems will occur. Although some new opportunities may emerge from ecosystem changes, economic and recreational opportunities and cultural heritage based around historical use of species or natural resources in many areas are at risk.
Losses to a few ecosystem services are outlined below.
Risks to Coral Reefs and the Economies They Support. Some coral reef ecosystems are already experiencing transformational changes. Warming has led to mass bleaching and/or outbreaks of coral diseases off the coastlines of Puerto Rico, the U.S. Virgin Islands, Florida, Hawaii, and the U.S.-affiliated Pacific islands, for example, that threaten reef ecosystems and the people who depend on them. In 2005, a mass bleaching event driven by 12 weeks of temperatures above the normal local seasonal maximum affected the entire Caribbean region, causing the loss of 40%–80% of the coral cover in the region.
In the Hawaiian archipelago, under the RCP8.5 emissions scenario, coral reef cover is projected to decline from the present level of 38% to 11% in 2050 and to less than 1% by the end of the century. In Hawaii alone, this lost habitat is projected to result in a total economic loss of $1.3 billion per year in 2050 and $1.9 billion per year in 2090 from reef-based recreation. Mitigation efforts consistent with the RCP4.5 emissions scenario would avoid 16% of coral cover loss and $470 million in damages per year by the end of the century.
Risks to Winter Recreation. Continued declines in snowpack in the western United States and shifts to more precipitation falling as rain than snow in many parts of the central and eastern United States will have significant effects on water resources. These declines and shifts also are expected to adversely impact the winter recreation industry. In northwestern Wyoming and western Montana, for example, the length of the cross-country ski season is projected to decline on average by 19% under the RCP4.5 emissions scenario and more than 50% under RCP8.5 by 2090.
The economic impacts of low-snowfall years can already be seen today in some regions. Comparing high-snowfall to low-snowfall years in the Northwest between 1999 and 2009, each low-snowfall year resulted in more than 2,100 fewer employees and a $173 million reduction in ski resort revenues ($189 million in 2015 dollars). These economic benefits are particularly important in rural and tribal communities in the region whose income base is largely dependent on natural resource economies and supporting industries.
Risks and Costs to Human Health and Well-Being
Higher temperatures, increasing air quality risks, more frequent and intense extreme weather and climate-related events, increases in coastal flooding, disruption of ecosystem services, and other changes increasingly threaten the health and well-being of Americans. Risks will be greater for populations that are already vulnerable, such as the elderly and lower-income communities.
Under RCP8.5, annual damages associated with an increasing number of extreme-temperature-related deaths in 2090 are projected to be $140 billion (in 2015 dollars). But annual damages associated with an increasing number of deaths related to extreme temperatures in 2090 could be reduced to a projected $60 billion nationwide under RCP4.5.
Overall, as noted earlier, annual health impacts (including from temperature extremes, poor air quality, and vector-borne diseases) and health-related costs are projected to be approximately 50% lower under a lower-emissions scenario (RCP4.5) versus under RCP8.5.
A few specific health effects are outlined below.
Risks from Exposure. With continued warming, cold-related deaths are expected to decrease, and heat-related deaths are projected to increase. In most regions, the increases in heat-related deaths are expected to outpace reductions in cold-related deaths.
Extreme temperature-related deaths are expected to increase by more than 9,000 annually in 49 major urban areas under a higher-emissions scenario (RCP8.5) by the end of the century, although this number would be lower if considering acclimatization or other adaptations (for example, increased use of air conditioning). This increase could be reduced by 60% under a lower-emissions scenario (RCP4.5).
Risks from Disease. Rising air and water temperatures and more intense extreme events are expected to increase exposure to waterborne and foodborne diseases, affecting food and water safety. Climate change is also projected to alter the geographic range and distribution of disease-carrying insects and pests, exposing more people to ticks that carry Lyme disease and mosquitoes that transmit viruses such as Zika, West Nile, and dengue, with varying impacts across regions.
Outbreaks occurring in other countries can impact U.S. populations and military personnel living abroad and can sometimes affect the United States directly. For example, the 2015–2016 El Niño, one of the strongest on record, may have contributed to the 2014–2016 Zika epidemic in the Americas. Warmer conditions may have facilitated expansion of the geographic range of mosquito populations and increased their capacity to transmit the Zika virus.
Risks to Mental Health. Extreme weather and climate-related events, some of which are projected to intensify as warming continues, can have lasting mental health consequences in affected communities, particularly if they result in degradation of livelihoods or community relocation. Coastal city flooding as a result of sea level rise and hurricanes, for example, can result in forced evacuation, with adverse effects on family and community stability as well as mental and physical health. This is an emerging area of research, and mental health impacts and risks are currently difficult to quantify.
Data for State and Local Decision-Makers
The newly released volume, drawing on foundational climate science covered in volume 1 [USGCRP, 2017] and nearly 6,000 unique references, puts the impacts of climate change in terms of issues that Americans care about. For example, how might climate change increase risks to important fisheries in the Northwest, impact water resources in the Southwest, or affect coastal infrastructure in the Southeast? In addition to observed and projected impacts and the implications of different greenhouse gas emissions scenarios, the report presents examples of steps being taken across the country to address risks through adaptation and mitigation actions.
In response to the growing demand for more localized information on the impacts of climate change, the report has an increased focus on regional-scale information (in addition to coverage of national-level topics required by law), drawing upon state-level climate summaries developed by the National Oceanic and Atmospheric Administration (NOAA) and downscaled projections of dozens of climate parameters. NCA4 highlights regional differences in sea level rise projections (see Figure 2), providing decision-makers with more localized data to inform long-term planning. This restructuring also includes expansion of the report’s regional coverage: The U.S. Caribbean is now featured as a separate region, and the Great Plains have been split into northern and southern regions along the Kansas–Nebraska border, covered in separate chapters.
Given scientific advances since the previous assessment, some emerging topics were given greater visibility in the new report, including a new chapter on the effects of climate change on air quality. Another new chapter focuses on the effects of climate change on U.S. international interests, including U.S. trade and businesses, national security, and U.S. humanitarian assistance and disaster relief. And another evaluates advances in analyzing how complex, interconnected human and natural systems will interact with and respond to a changing climate.
Current Levels of Mitigation and Adaptation Not Enough to Avoid Significant Damages
Since the Third National Climate Assessment [Melillo et al., 2014], the integration of climate risk into decision-making and the implementation of actions to adapt to changes that will occur have significantly increased, including in areas of financial risk reporting, capital investment planning, development of engineering standards, military planning, and disaster risk management. Transformations in the energy sector—including technological innovation that has contributed to the displacement of coal by natural gas and increased deployment of renewable energy—along with policy actions at the national, regional, state, and local levels are reducing greenhouse gas emissions in the United States (see Figure 3).
However, although response efforts have expanded substantially in the last 4 years, the assessment finds that neither global efforts to mitigate the causes of climate change nor regional efforts to adapt to the impacts currently approach the scales needed to avoid substantial damages to the U.S. economy, environment, and human health and well-being over the coming decades. Instead, more immediate and substantial global greenhouse gas emissions reductions and more regional adaptation efforts, would be needed to avoid the most severe consequences in the long term..
An Authoritative Voice on What Climate Changes Mean for American Communities
USGCRP, which coordinates federal research on global environmental change across its 13 member agencies, is required by Congress to evaluate and integrate findings on the current and future impacts of climate change on the United States every 4 years. Both volumes of NCA4 were conducted in fulfilment of this mandate.
NCA4 volume 2 was authored by more than 300 federal and nonfederal experts, including individuals from federal, state, and local governments; tribes and indigenous communities; national laboratories; universities; and the private sector. The entire process was informed by engagement with hundreds of external stakeholders, including a series of regional workshops that reached more than 1,000 individuals in more than 40 cities. Listening sessions, webinars, and public comment periods also provided valuable input to the authors.
The assessment aims to inform natural resource and utility managers, public health officials, emergency planners, financial risk managers, and other stakeholders as they consider climate-related risks in their decisions. By distilling a large body of research into relevant, accessible language framed around risks to people and important resources, the assessment provides a means for connecting the scientific and management communities. It also serves as a more general educational resource about what’s at stake for society as a result of climate change.
The new report underwent eight drafts and multiple rounds of review, including one by the public, as well as expert reviews by the 13 USGCRP federal member agencies and the National Academies of Sciences, Engineering, and Medicine. NOAA served as the lead administrative agency for the report, with editorial oversight by NOAA’s Technical Support Unit at the National Centers for Environmental Information. Development of the report was overseen by a federal steering committee.
Planning for Future USGCRP Assessment Efforts
USGCRP approaches assessment as a sustained process that enables integration of new information as it emerges. Sustained assessment products developed by USGCRP member agencies improve the thoroughness of the quadrennial report while serving particular stakeholder groups such as public health officials and food security decision-makers.
As part of the sustained assessment process, federal interagency groups are planning for future USGCRP assessment activities and have developed a suite of scenario products for use by the broader scientific community. These products include (1) downscaled sea level rise projections, (2) integrated climate and land use scenarios, and (3) demographic and population scenarios through at least 2100. USGCRP encourages researchers and decision-makers to use these products in their work, which can inform future sustained assessment products. More information can be found at the USGCRP’s webpage on climate scenarios.
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