A. Ecosystems
Ecosystems and nature—along with the services and benefits they provide to people—are under substantial threat due to ongoing climate change.
Climate change affects land use, land cover, and ecosystems both directly and indirectly. It changes the suitability of land for specific uses and habitats (e.g., types of crops or ecosystems), alters disturbances that occur (e.g., wildfire, pests, and disease), and shifts distributions of where species live and occur.
By altering the productivity of ecosystems, species interactions, and the spread of invasive species, climate change is reconfiguring ecosystems in unprecedented ways.
The impacts of climate change on ecosystems, in turn, threaten agricultural and fisheries production, water supply and quality, the buffering of extreme weather and climate events, and resources of cultural value.
Species on land, in rivers and lakes, and in the oceans are changing in their geographic ranges, their seasonal behaviors and migrations, and their interactions within ecosystems.
About one-half of all species have shifted polewards or to higher elevations. These shifts are leading to widespread deterioration of ecosystem structure, function, resilience, and adaptive capacity.
Local losses of species are occurring, and increasing fractions of species on land and in rivers and lakes face elevated extinction risks under increasing temperatures and other climate changes, in interaction with other stressors to ecosystems (e.g., pollution, habitat fragmentation). For instance, at 1.5°C warming above pre-industrial levels, 3-14% of terrestrial species assessed to date will face very high risk of extinction.
Many species will be unable to relocate to suitable climates under even moderate rates of climate change. Adaptation actions, such as assisting species to migrate to more suitable locations, can reduce impacts to ecosystems but not eliminate them.
As a specific example, the structure and function of forests are being and will continue to be affected by climate-related hazards and extremes.
The impacts of climate change include severe ecological disturbances, impacts on forest productivity and health, and changing distribution and abundance of species. Warming in high-latitude regions will worsen the effects of wildfire, drought, and pests on high-latitude forests. Under scenarios of continued high emissions, abrupt and irreversible regional-scale changes in ecosystems could occur, such as tree death and forest dieback. In turn, this would release additional carbon and greenhouse gases into the atmosphere, exacerbating warming and affecting water quality, ecosystem services, and economies.
In high-mountain and polar regions, reductions in snow cover, thawing of permafrost, and disappearance of ice cover have already had profound effects on terrestrial and freshwater species and ecosystems. Impacts include shifts in seasonal activities and species abundance, disturbance of ecosystems (e.g., through wildfire or abrupt permafrost thaw), and alteration of ecosystem functioning. Under additional climate change, changes in ice and snow cover will continue to alter ecosystem structure and functioning and eventually lead to loss of biodiversity. Further, wildfire is anticipated to increase across the tundra and high-latitude regions in the rest of the 21st century. Beyond impacts on ecosystems themselves, these changes have cultural and economic consequences.
In the oceans, the geographical ranges and seasonal activities of many marine species have shifted in complex ways due to warming of the oceans, changes in sea ice, and oxygen loss in the ocean environment. From the equator to the poles, the species composition and the amount of biological matter produced by marine ecosystems have changed. Warming of the oceans in the 20th and 21st centuries has decreased the amount of fish available for fisheries’ catches (called the maximum catch potential), adding to impacts from overfishing of some fish stocks. In the United States, ocean ecosystems have been disrupted by loss of iconic and valued habitats and shifts in species composition and interactions among predators and prey.
Ocean acidification, especially with continued high emissions of greenhouse gases, poses risks to coral reefs, as well as to polar ecosystems and other marine habitats.
Decreases in the global biomass of marine ecosystems and in fisheries’ catch potential, in addition to regional changes in species composition, are projected under all emissions scenarios for the remainder of the 21st century.
The most rapid and largest declines are projected in the tropics, as marine species shift globally in response to warming. These shifts will have differential impacts for fisheries productivity—for example, increasing high-latitude invasions while exacerbating extinction in the tropics and semi-enclosed seas. The effects of warming will be exacerbated in some cases by ocean acidification, loss of oxygen from ocean waters, and reduced sea ice extent, with compounding hazards projected to become more common and severe in the future. Impacts on fish species, in turn, will affect the livelihoods, incomes, and food security of communities dependent upon marine resources. Detecting, forecasting, and mitigating adverse conditions will become increasingly important.
Coastal ecosystems, such as mangroves, coral reefs, and estuaries, also have been impacted by multiple climate change drivers, including the warming of the oceans, increasing marine heat waves, acidification, oxygen loss in coastal waters, intrusion of saline waters inland, and sea-level rise. These and other impacts are affecting biodiversity, the extent of different habitats, and the functioning and services of ecosystems, such as protecting coastlines from storms. Climate change impacts on coastal ecosystems have in turn had cascading consequences for fisheries, tourism, human health, and public safety.
Increases in harmful algal blooms have resulted from both climate drivers (e.g., ocean warming, marine heat waves, oxygen loss) and non-climatic drivers (e.g., nutrients runoff), which has negatively impacted food security, human health, tourism, and local economies. Especially for high magnitudes of climate change over the 21st century, risks of severe impacts are projected, including potential losses of species habitat and diversity and degradation of ecosystem functions.
Because ecosystems are directly exposed to the changing climate, the prospects for reducing risks through ecological adjustments and adaptation are fundamentally linked to how much climate change occurs and therefore to progress with reductions of greenhouse gas emissions. Species and ecosystems can adapt—for example, through changes in their geographic ranges or the timing of activities such as reproduction—but the degree to which ecosystems can adapt to current and future climate change is limited, especially given the rate of projected climate change compared with historical shifts. Under scenarios of ambitious emission reductions, the capacity of species to adjust is greater because the rate and amount of climate change are lower, although some ecosystems, such as warm-water corals, will be at very high risk even with global warming limited to 1.5°C increase above pre-industrial levels.
Many human-assisted adaptations can reduce the impacts of climate change on ecosystems. For example, sustainable land and forest management, such as thinning of forests to manage fuels, can prevent and reduce degradation and some adverse climate impacts. Monitoring and early warning systems have helped reduce the impacts of harmful algal blooms. Fisheries, agriculture, and other livelihoods and activities dependent upon ecosystems are shifting in locations and timing. Cost-effective and beneficial measures are available to communities in supporting jobs and livelihoods.
B. Human Health and Well-Being
Climate change is already adversely affecting human health and well-being, including in the United States.
Heat-related mortality has increased due to global warming, and cold-related mortality has decreased. Urbanization combined with global warming amplifies heat in cities and their surroundings, and this heat island effect increases night-time as well as daytime temperatures. Higher night-time temperatures can affect vulnerable populations such as the elderly or outdoor workers, as less physiological recovery occurs overnight when night-time temperatures are high. Further, climate change is altering exposures to climate extremes and posing stresses to mental health.
The health impacts of climate change are differentially experienced by people with varying levels of exposure and sensitivity to hazards.
Health risks are amplified among the young and the elderly, low-income households, and some communities of color. In addition, the impacts of climate change impacts often result from multiple climate hazards and different forms of exposure and sensitivity to climate-related events and trends.
Under continuing climate change, human health impacts will include increases in morbidity and mortality from heat waves and fires, reduced labor productivity, and increases in food and waterborne diseases as well as vector-borne diseases.
Risks from vector-borne diseases such as malaria and dengue fever also result from the ways in which climate change shifts their geographic ranges. Under high magnitudes of climate change, human activities including agriculture and outdoor work will be compromised in some regions for some parts of the year due to temperature and humidity in combination.
Climate change is also exacerbating air pollution through increased wildfire smoke, air-pollution co-emitted with heat-trapping greenhouse gases, and increased formation of ozone.
In the United States, over 100 million people already experience air pollution exceeding health-based air quality standards, with impacts on respiratory and cardiovascular health. Wildfire smoke poses health risks, and intensification of wildfires from climate change diminishes air quality, with impacts for respiratory illnesses, visibility, and outdoor activities. Allergic illnesses, including asthma and hay fever, will likely increase due to climate changes increasing plant growth and pollen production, such as earlier onset of spring, warmer temperatures, and higher carbon dioxide levels in the atmosphere. Other environmental consequences of air pollution include reductions in visibility and damages to agricultural crops and forests.
In the Arctic and high mountain regions, the rapid warming of what scientists call the “cryosphere”—snow cover, glaciers and ice, permafrost, and seasonally frozen ground—has negatively impacted multiple aspects of human health and well-being since the middle of the 20th century.
Categories of impacts include increased food insecurity and adverse consequences for water resources, infrastructure, tourism, and Indigenous cultures. For example, changes in snow cover, ice, and permafrost in the Arctic have disrupted access of Indigenous communities to areas in which fishing, herding, hunting, and gathering have traditionally occurred, as well as the availability of food in these areas. Under additional climate change in the region, impacts on water resources will affect uses such as hydropower and irrigation of agriculture, as well as livelihoods. Changes in floods, avalanches, and the stability of frozen ground will impact infrastructure, cultures, tourism, and recreation.
For climate change impacts on human health and well-being, effective adaptation options include basic public health measures, essential health care, disaster preparedness and response, infrastructure planning, urban design, livelihood diversification, and poverty reduction.
In many communities around the United States, increased preparedness involves consideration of climate in domains already relevant to human health and well-being. For example, physicians are increasingly considering the ways that extreme heat may affect young children, the elderly, or pregnant women, with impacts exacerbated where air conditioning is not available or not affordable for households. Air pollution worsened by wildfire or ozone formation on hot days is particularly impactful for outdoor workers, people with asthma, or individuals facing prolonged exposure
Public health, support of livelihoods, and attention to the most vulnerable communities are all important considerations in adapting and preparing for the impacts of climate change on human health and well-being. Adaptation actions span social scales, from the individual and household level through to national approaches. Adaptation actions also range from activities done in advance of an acute event, such as a heat wave or a wildfire, to warning and emergency management once intense exposures or disasters are underway.
C. Food Production and Security
Climate change has already impacted the production of food and food security as a result of warming, shifts in rainfall patterns, and increases in some types of extreme events.
To date, the impacts of climate change on agricultural crop yields have been both positive and negative. While crops such as maize and wheat in lower latitude regions have been negatively impacted by climate changes, some crop yields in higher latitude regions have benefited. On balance, however, climate change impacts on crop yields have more often been negative than positive, especially for wheat and maize, and food security is being adversely impacted.
In tropical and temperate growing areas, major crops, including wheat, maize, and rice, will be negatively impacted overall under increasing levels of warming, although some locations may benefit. In drylands, climate change combined with desertification will negatively affect both crop and livestock productivity. Climate change has been altering patterns of agricultural pest and disease infestations. Increases in carbon dioxide in the atmosphere also can decrease the nutritional quality of crops.
In some pastoral systems, such as in Africa, animal growth and productivity have been adversely impacted. Increasing temperature extremes negatively affect the health of livestock, with economic implications for producers and also markets and supply chains.
Increasing temperatures pose risks for agricultural workers, including heat exhaustion, heat stroke, and heart attacks. Both heat and air pollution, sometimes in combination, raise issues regarding the time of work, the use of protective equipment, and provision of adequate breaks.
Multiple aspects of food security are affected by climate change, including access to food and the stability of food prices. Projected increases in prices of cereal crops disproportionately increase risks of food insecurity and hunger for low-income, marginalized people. Increases in extreme weather events are also projected to increase food insecurity through disruptions of food chains. Risk to food security increase with continued warming, and food security risks are greater in possible future scenarios with lower income, increases in food demand, more land competition, and limited trade.
In the United States, climate change impacts on agricultural productivity will result from increasing drought and rangeland wildfires, decreases in water supplies for irrigation, and expanding distributions of pests and disease for crops and livestock.
Many of these impacts will have long-term consequences. For example, increasing extreme precipitation events degrade soils and water resources through excessive runoff, leaching, and flooding.
Throughout the food system, adaptation options extend from production to consumption, including food loss and waste. They include the use of stress-tolerant crops, heat-tolerant livestock, improved animal housing, and health services in rural areas. Commonly occurring adaptations involve the timing of planting, considerations of water availability and irrigation, and the varieties planted in different locations.
D. Water Resources and Security
Climate change is already negatively impacting water security.
Increasing magnitudes of climate change will increase the numbers of people globally who will experience water scarcity. For example, with 2°C global warming above pre-industrial levels, in some snowmelt dependent river basins, water available for irrigation will decline by up to 20%.
The impacts of climate change on water resources relate to both the quantity and quality of water available for different uses. Current, much less future, impacts require consideration of water infrastructure and management, especially where current systems are already struggling to meet demands for water resources in both rural and urban areas. Integrated water management addresses the interacting risks related to water for both freshwater resources and flooding.
Especially in dry subtropical regions, climate change over the century will decrease the availability of renewable surface waters and groundwater resources. Drought is expected to increase in frequency in already dry regions, whereas water resources will increase at higher latitudes. Risks to water quality result from interactions of increased temperatures, increased sediment and pollution from heavy rainfall, concentration of pollutants during droughts, mobilization of pollutants and sediments by high-intensity rainfall events, and disruption of water treatment during floods.
In the United States, changes in the quantity and quality of water resources are already evident and are expected to worsen.
Variability in rainfall and increasing temperatures are intensifying droughts, increasing heavy downpours, and reducing snowpack. Shifts toward rainfall instead of snow are also affecting the timing of water supply.
The deterioration of water infrastructure within the United States compounds climate risks, yet this infrastructure will need to manage increases in extreme precipitation and drought events in the future. Much of current infrastructure was built before there was awareness of climate change risks. Even today, the design and operation of infrastructure does not usually consider the changing climate, the impacts of co-occurring climate-related hazards, or the potential for cascading failures, despite growing evidence and awareness of climate change risks. Given the uncertainties in hydrological changes, adaptation includes scenario planning, strategies grounded in learning, and flexible solutions.
E. Infrastructure, Economies, and Finance
Climate change poses economic and financial risks, which emerge from impacts on infrastructure and the built environment and consequences for markets and other instruments that manage and are affected by such risks. Economic and financial impacts and damages from climate change are already occurring, and the potential costs and losses in the future are substantial, increasing non-linearly with the amount of global warming that occurs.
The impacts of climate change on infrastructure result from weather and climate events and trends that exceed the tolerances to which infrastructure was initially built, and infrastructure that is inadequately maintained and deteriorating is especially at risk. For example, the U.S. energy system, which is essential for most economic sectors in the United States, has already been impacted by extreme weather and climate events. Under increasing climate change, risks include more frequent or longer power outages.
Climate change impacts also pose risks for the reliability of the U.S. transportation system due to such hazards as heavy precipitation, coastal flooding, increasing temperatures, and wildfire.
For example, recurrent and disaster floods alike disrupt transportation, and exposure to temperatures outside of design standards threaten the integrity of roadways, bridges, and electricity transmission facilities. Disruption of transportation networks has both economic and social consequences.
The impacts of climate change and climate-related extremes affect trade and the economy—through, for example, internationally interconnected import and export prices or supply chains.
Where climate change slows or reverses socioeconomic development globally, the effectiveness of international aid and investments will be decreased, requiring increases in disaster relief and humanitarian assistance.
Coastal economies and properties are at risk.
The United States has a trillion-dollar coastal property market. Both properties and public infrastructure will be increasingly impacted by high-tide flooding, storm surge, and heavy rainfall, all of which are exacerbated by sea-level rise. Coastal flooding and erosion are also exacerbating social inequities within coastal communities, raising difficult questions about financing adaptation and hazard mitigation and the potential needs for relocation in some places. Under high magnitudes of climate change, coastal communities will be fundamentally altered by the end of the century. Even with ambitious emissions reductions, financial impacts will increase. Coastal communities will likely be among the first in the United States to test legal frameworks relevant to climate change, because the effects are evident, costly, and directly attributable to human activity.
Adaptation relevant to infrastructure often involves substantial investments with long infrastructural lifetimes. Considering climate change from the start can substantially reduce the costs of adaptive adjustments. Additional considerations are the aging of infrastructure and the complex interconnections across the transportation system and sectors dependent upon it.
Adjustments that increase the climate resilience of infrastructure can have widespread benefits. For example, plans and investments that reduce the frequency, scope, and severity of coastal flooding reduce both the direct impacts of coastal flooding and cascading consequences across economies.
F. Compounding Risks Across Regions
Climate change science to date has necessarily prioritized analysis of climate change impacts within sectors or within regions. However, as climate change increases, it is becoming clear that impacts can cascade and compound both within and across sectors and regions. The sectors and systems exposed to climate change interact with one another and depend upon each other.
Increasing climate change increases cascading risks resulting from interactions across categories of risk.
Complex dynamics and outcomes can result, and they are generally difficult to fully predict in advance. Examples of interacting risks include increases in dryland water scarcity, erosion of soils, wildfire damages, loss of vegetation, thawing of permafrost, degradation of coastal regions, and declines in tropical crop yields.
As a particular example, coastal communities are affected by multiple interacting climate-related hazards, including tropical cyclones, coastal flooding, marine heat waves, loss of sea ice, and thawing of permafrost. Coastal systems and low-lying areas will increasingly be inundated by coastal waters and experience coastal flooding and erosion due to continuing sea-level rise. Low-lying developing countries and small islands already face substantial impacts, and even with ambitious reductions in greenhouse gas emissions, low-lying coastal settlements in atoll islands or Arctic communities face future risks so severe that sufficient adaptation will be extremely difficult. Under continued high emissions, delta regions and high-income coastal cities are expected to face substantial risks after 2050. Without large increases in adaptation and context-specific, integrated responses to reduce risks, infrastructure, homes, communities, financial viability, and livelihoods will all be affected.
Climate change can also exacerbate multiple existing challenges in urban areas, including deteriorating infrastructure, degraded ecosystems, and social inequities. The interdependence of infrastructural, social, and ecological systems in urban areas can transmit and amplify climate change risks—for example when communications or transportation networks fail.
In rural areas, climate change impacts have consequences for water availability, food security, and agricultural livelihoods. Further, the livelihoods, health, and economies of Indigenous peoples are threatened by climate change.
Climate change affects agriculture, hunting and gathering, fishing, forestry, energy, recreation, and tourism. Impacts can simultaneously degrade the foundations of economies and practices, locations, and relationships that have cultural importance.
Different groups will be differentially affected by risks. Within populations, some individuals will be more at risk than others, including the poor, the young and the elderly, and women. Over this century, climate change impacts will make poverty reduction more difficult, especially due to multiple compounding climate-related risks.
Finally, with regard to conflict and security, climate change over the 21st century will increase the displacement of people and amplify well-documented drivers of violent conflict. Changes in climate will cause environmentally induced migration internally within countries and also across national borders. In addition, shared resources along borders are impacted by climate change, necessitating transboundary processes for their management. The impacts of climate change already have consequences for U.S. military infrastructure—for example, through damages to roads, runways, and other infrastructure in low-lying coastal areas. Violent conflict also substantially increases vulnerability to the impacts of climate change through its adverse consequences for government institutions, infrastructure, and livelihoods.
Cascading impacts are already putting risk management to the test and revealing limits to capacities to cope and adapt. Jointly managing interacting systems can increase their resilience, although such coordination can be challenged by the many different agencies and levels of government, along with the private sector and communities, that are involved.
For example, adaptation responses in almost any local community in the United States often benefit from coordination of local, state, and federal efforts, and actions at one level of government can enable or constrain actions by other levels of government. In addition, most responses to date have been reactive rather than proactive. For instance, adaptive actions implemented have generally occurred after extreme events and disasters rather than before.
In many cases, adaptation relevant to cascading risks has favored “hard” infrastructure, such as sea walls, levees, and storm barriers. Approaches combining ecosystems and infrastructure, such as the buffering of waves that occurs through wetlands or mangroves combined with “armoring” infrastructure near human settlements, are starting to become more common. In addition, adaptation actions are starting to address future sea-level rise or changes in extremes. There is increasing recognition of the profound importance of combining infrastructural approaches with preparedness measures relevant to the response capacity of government agencies, communities, and the private sector.
VI. Conclusion
Climate change is substantially impacting people and ecosystems globally. Risks will increase into the future, depending on both climate-related hazards and the exposure and vulnerability of societies and ecosystems to these hazards. Ultimately, the level of heat-trapping greenhouse gases in the atmosphere, arising mostly from fossil fuels and industry, will be the most important determinant of climate risks and impacts that occur, especially into the second half of the century and beyond.
Through mitigation and adaptation at different scales, from the local to the national to the global, these risks can be reduced. But current and future generations will nevertheless experience losses and damages, with risks growing enormously if mitigation remains adequate. These negative impacts will disproportionately be borne by marginalized and underserved communities in the United States and other countries, although all regions, societies, and ecosystems will be affected by severe, pervasive, and in some cases irreversible impacts and damages.
