Section 27 of the Fourth National Climates Assessment, Volume 2 by USG-CRP. This LibriVox recording is in the public domain. Recording by Warren Coddy, Gurnee, Illinois. CHAPTER 25 SOUTHWEST KEY MESSAGE 1. WATER RESOURCES Water for people and nature in the southwest has declined during droughts due in part to human-caused climate change. Intensifying droughts and occasional large floods combined with critical water demands from a growing population, deteriorating infrastructure, and groundwater depletion suggests the need for flexible water management techniques that address changing risks over time, balancing declining supplies with greater demands. KEY MESSAGE 2. ECOSYSTEMS AND ECOSYSTEM SERVICES The integrity of southwest forests and other ecosystems and their ability to provide natural habitat, clean water, and economic livelihoods have declined as a result of recent droughts and wildfire due in part to human-caused climate change. Greenhouse gas emissions reductions, fire management, and other actions can help reduce future vulnerabilities of ecosystems and human well-being. KEY MESSAGE 3. COAST Many coastal resources in the southwest have been affected by sea-level rise, ocean warming, and reduced ocean oxygen, all impacts of human-caused climate change, and ocean acidification resulting from human emissions of carbon dioxide, homes and other coastal infrastructure, marine flora and fauna, and people who depend on coastal resources face increased risks under continued climate change. KEY MESSAGE 4. INDIGENOUS PEOPLES Traditional foods, natural resource-based livelihoods, cultural resources, and spiritual well-being of indigenous peoples in the southwest are increasingly affected by drought, wildfire, and changing ocean conditions. Because future changes would further disrupt the ecosystems on which indigenous peoples depend, tribes are implementing adaptation measures and emissions reduction actions. KEY MESSAGE 5. ENERGY The ability of hydropower and fossil fuel electricity generation to meet growing energy use in the southwest is decreasing as a result of drought and rising temperatures. Many renewable energy sources offer increased electricity reliability, lower water intensity of energy generation, reduced greenhouse gas emissions, and new economic opportunities. KEY MESSAGE 6. FOOD Food production in the southwest is vulnerable to water shortages. Increased drought, heat waves, and reduction of winter chill hours can harm crops and livestock, exacerbate competition for water among agriculture, energy generation, and municipal uses, and increase future food insecurity. KEY MESSAGE 7. HEAT Heat associated deaths and illnesses, vulnerabilities to chronic disease, and other health risks to people in the southwest result from increases in extreme heat, poor air quality, and conditions that foster pathogen growth and spread. Improving public health systems, community infrastructure, and personal health can reduce serious health risks under future climate change. The southwest region encompasses diverse ecosystems, cultures, and economies, reflecting a broad range of climate conditions, including the hottest and driest climate in the United States. Water for people and nature in the southwest region has declined during droughts due in part to human-caused climate change. Higher temperatures intensified the recent severe drought in California and our amplifying drought in the Colorado River Basin. Since 2000, Lake Mead on the Colorado River has fallen 130 feet, 40 meters, and lost 60 percent of its volume, a result of the ongoing Colorado River Basin drought and continued water withdrawals by cities and agriculture. The reduction of water volume in both Lake Powell and Lake Mead increases the risk of water shortages across much of the southwest. Federal water utilities, the governments of seven U.S. states and the federal governments of the United States and Mexico have voluntarily developed and implemented solutions to minimize the possibility of water shortages for cities, farms, and ecosystems. In response to the recent California drought, the state implemented a water conservation plan in 2014 that set allocations for water utilities and major users and banned wasteful practices. As a result, the people of the state reduced water use 25 percent from 2014 to 2017. Exposure to hotter temperatures and heat waves already leads to heat-associated deaths in Arizona and California. Mortality risk during a heat wave is amplified on days with high levels of ground-level ozone or particulate air pollution. Given the proportion of the U.S. population in the southwest region, a disproportionate number of West Nile virus, plague, hantavirus pulmonary syndrome, and valley fever cases occur in the region. Analysis estimated that the area burned by wildfire across the western United States from 1984 to 2015 was twice what would have burned had climate change not occurred. Wildfires around Los Angeles from 1990 to 2009 caused $3.1 billion in damages, unadjusted for inflation. Tree death in mid-elevation conifer forests doubled from 1955 to 2007 due, in part, to climate change, allowing naturally ignited fires to burn in wilderness areas and preemptively setting low-severity prescribed burns in areas of unnatural fuel accumulations can reduce the risk of high-severity fires under climate change. Reducing greenhouse gas emissions globally can also reduce ecological vulnerabilities. At the Golden Gate Bridge in San Francisco, sea level rose 9 inches, 22 centimeters, between 1854 and 2016. Climate change caused most of this rise by melting of land ice and thermal expansion of ocean water. Local governments on the California coast are using projections of sea level rise to develop plans to reduce future risks. Ocean water acidity off the coast of California increased 25 percent to 40 percent decreases of 0.1 to 0.15 pH units from the pre-industrial era circa 1750 to 2014 due to increasing concentrations of atmospheric carbon dioxide from human activities. The marine heat wave along the Pacific coast from 2014 to 2016 occurred due to a combination of natural factors and climate change. The event led to the mass stranding of sick and starving birds and sea lions and shifts of red crabs and tuna into the region. The ecosystem disruptions contributed to closures of commercially important fisheries. Agricultural irrigation accounts for approximately three-quarters of water use in the southwest region, which grows half of the fruits, vegetables, and nuts and most of the wine grapes, strawberries, and lettuce for the United States. Increasing heat stress during specific phases of the plant's life cycle can increase crop failures. Drought and increasing heat intensify the arid conditions of reservations where the United States restricted some tribal nations in the southwest region to the driest portions of their traditional homelands. In response to climate change, indigenous peoples in the region are developing new adaptation and mitigation actions. The severe drought in California, intensified by climate change, reduced hydroelectric generation two-thirds from 2011 to 2015. The efficiency of all water-cooled electric power plants that burn fuel depends on the temperature of the external cooling water, so climate change could reduce energy efficiency up to 15% across the southwest by 2050. Solar, wind, and other renewable energy sources, except biofuels, emit less carbon and require less water than fossil fuel energy. Economic conditions and technological innovations have lowered renewable energy costs and increased renewable energy generation in the southwest. For full chapter, including references and traceable accounts, see HTTPS colon double backslash nca2018.globalchange.gov backslash chapter backslash southwest. End of Section 27. Section 28 of the Fourth National Climate Assessment, Volume 2 by USGCRP. This Slibervox recording is in the public domain. Recording by Warren Cotty, Gurney, Illinois. Chapter 26 Alaska. Key Message 1. Marine Ecosystems. Alaska's marine fish and wildlife habitats, species distributions, and food webs, all of which are important to Alaska's residents, are increasingly affected by retreating and thinning Arctic summer sea ice, increasing temperatures, and ocean acidification. Climate warming will accelerate related ecosystem alterations in ways that are difficult to predict, making adaptation more challenging. Key Message 2. Terrestrial Processes. Alaska residents, communities, and their infrastructure continue to be affected by permafrost thaw, coastal and river erosion, increasing wildfire, and glacier melt. These changes are expected to continue into the future with increasing temperatures, which would directly impact how and where many Alaskans will live. Key Message 3. Human Health. A warming climate brings a wide range of human health threats to Alaskans, including increased injuries, smoke inhalation, damage to vital water and sanitation systems, decreased food and water security, and new infectious diseases. The threats are greatest for rural residents, especially those who face increased risk of storm damage and flooding, loss of vital food sources, disrupted traditional practices, or relocation. Implementing adaptation strategies would reduce the physical, social, and psychological harm likely to occur under a warming climate. Key Message 4. Indigenous Peoples. The subsistence activities, culture, health, and infrastructure of Alaska's Indigenous peoples and communities are subject to a variety of impacts, many of which are expected to increase in the future. Flexible community-driven adaptation strategies would lessen these impacts by ensuring that climate risks are considered in the full context of the existing socio-cultural systems. Key Message 5. Economic Costs. Climate warming is causing damage to infrastructure that will be costly to repair or replace, especially in remote Alaska. It is also reducing heating costs throughout the state. These effects are very likely to grow with continued warming. Timely repair and maintenance of infrastructure can reduce the damages and avoid some of these added costs. Key Message 6. Adaptation. Proactive adaptation in Alaska would reduce both short and long-term costs associated with climate change, generate social and economic opportunity, and improve livelihood security. Direct engagement and partnership with communities is a vital element of adaptation in Alaska. Alaska is the largest state in the nation, almost one-fifth the size of the combined lower 48 United States, and is rich in natural capital resources. Alaska is often identified as being on the front lines of climate change since it is warming faster than any other state and faces a myriad of issues associated with a changing climate. The cost of infrastructure damage from a warming climate is projected to be very large, potentially ranging from $110 to $270 million per year, assuming timely repair and maintenance. Although climate change does and will continue to dramatically transform the climate and environment of the Arctic, proactive adaptation in Alaska has the potential to reduce costs associated with these impacts. This includes the dissemination of several tools, such as guidebooks to support adaptation planning, some of which focus on indigenous communities. While many opportunities exist with a changing climate, economic prospects are not well captured in the literature at this time. As the climate continues to warm, there is likely to be a nearly sea ice-free Arctic during the summer by mid-century. Ocean acidification is an emerging global problem that will intensify with continued carbon dioxide, CO2, emissions, and negatively affects organisms. Climate change will likely affect management actions and economic drivers, including fisheries in complex ways. The use of multiple alternative models to appropriately characterize uncertainty in future fisheries biomass trajectories and harvests could help manage these challenges. As temperature and precipitation increase across the Alaska landscape, physical and biological changes are also occurring throughout Alaska's terrestrial ecosystems. Degredation of permafrost is expected to continue with associated impacts to infrastructure, river and stream discharge, water quality, and fish and wildlife habitat. Longer sea ice-free seasons, higher ground temperatures, and relative sea level rise are expected to exacerbate flooding and accelerate erosion in many regions, leading to the loss of terrestrial habitat in the future and in some cases requiring entire communities or portions of communities to relocate to safer terrain. The influence of climate change on human health in Alaska can be traced to three sources. Direct exposures, indirect effects, and social or psychological disruption. Each of these will have different manifestations for Alaskans when compared to residents elsewhere in the United States. Climate change exerts indirect effects on human health in Alaska through changes to water, air, and soil, and through ecosystem changes affecting disease ecology and food security, especially in rural communities. Alaska's rural communities are predominantly inhabited by indigenous peoples who may be disproportionately vulnerable to socioeconomic and environmental change. However, they also have rich cultural traditions of resilience and adaptation. The impacts of climate change will likely affect all aspects of Alaska native societies from nutrition, infrastructure, economics, and health consequences to language, education, and the communities themselves. The profound and diverse climate-driven changes in Alaska's physical environment and ecosystems generate economic impacts through their effects on environmental services. These services include positive benefits directly from ecosystems, for example, food, water, and other resources, as well as services provided directly from the physical environment, for example, temperature moderation, stable ground for supporting infrastructure, and smooth surface for overland transportation. Some of these effects are relatively assured and, in some cases, are already occurring. Other impacts are highly uncertain due to their dependence on the structure of global and regional economies and future human alterations to the environment decades into the future, but they could be large. In Alaska, a range of adaptations to changing climates and related environmental conditions are underway, and others have been proposed as potential actions, including measures to reduce vulnerability and risk, as well as more systematic institutional transformation. For full chapter, including references and traceable accounts, see HTTPS colon double backslash nca2018.globalchange.gov backslash chapter backslash Alaska. End of Section 28. Section 29 of the Fourth National Climate Assessment, Volume 2 by USGCRP. This LibriVox recording is in the public domain. Recording by Warren Coddy, Gurnee, Illinois. Chapter 27 Hawaii and US-affiliated Pacific Islands. Key Message 1. Threats to Water Supplies Dependable and safe water supplies for Pacific Island communities and ecosystems are threatened by rising temperatures, changing rainfall patterns, sea level rise, and increased risk of extreme drought and flooding. Islands are already experiencing saltwater contamination due to sea level rise, which is expected to catastrophically impact food and water security, especially on low-lying atolls. Residents to future threats relies on active monitoring and management of watersheds and freshwater systems. Key Message 2. Terrestrial Ecosystems, Ecosystem Services, and Biodiversity. Pacific Island ecosystems are notable for the high percentage of species found only in the region, and their biodiversity is both an important cultural resource for island people and a source of economic revenue through tourism. Terrestrial habitats and the goods and services they provide are threatened by rising temperatures, changes in rainfall, increased storminess, and land use change. These changes promote the spread of invasive species and reduce the ability of habitats to support protected species and sustain human communities. Some species are expected to become extinct and others to decline to the point of requiring protection and costly management. Key Message 3. Coastal Communities and Systems. The majority of Pacific Island communities are confined to a narrow band of land within a few feet of sea level. Sea level rise is now beginning to threaten critical assets, such as ecosystems, cultural sites and practices, economies, housing and energy, transportation, and other forms of infrastructure. By 2100, increases of 1 to 4 feet in global sea level are very likely with even higher levels than the global average in the US-affiliated Pacific Islands. This would threaten the food and freshwater supply of Pacific Island populations and jeopardize their continued sustainability and resilience. As sea level rise is projected to accelerate strongly after mid-century, adaptation strategies that are implemented sooner can better prepare communities and infrastructure for the most severe impacts. Key Message 4. Oceans and Marine Resources. Fisheries, coral reefs, and the livelihoods they support are threatened by higher ocean temperatures and ocean acidification. Widespread coral reef bleaching and mortality have been occurring more frequently, and by mid-century, these events are projected to occur annually, especially if current trends and emissions continue. Bleaching and acidification will result in loss of reef structure leading to lower fisheries yields and loss of coastal protection and habitat. Declines in oceanic fishery productivity of up to 15% and 50% of current levels are projected by mid-century and 2100, respectively, under the higher scenario, RCP 8.5. Key Message 5. Indigenous Communities and Knowledge. Indigenous peoples of the Pacific are threatened by rising sea levels, diminishing freshwater availability, and shifting ecosystem services. These changes imperil communities' health, well-being, and modern livelihoods, as well as their familial relationships with lands, territories, and resources. Built on observations of climatic changes over time, the transmission and protection of traditional knowledge and practices, especially via the central role played by Indigenous women, are intergenerational, place-based, localized, and vital for ongoing adaptation and survival. Key Message 6. Cumulative Impacts and Adaptation. Climate change impacts in the Pacific Islands are expected to amplify existing risks and lead to compounding economic, environmental, social, and cultural costs. In some locations, climate change impacts on ecological and social systems are projected to result in severe disruptions to livelihoods that increase the risk of human conflict or compel the need for migration. Early interventions, already occurring in some places across the region, can prevent costly and lengthy rebuilding of communities and livelihoods, and minimize displacement and relocation. The US Pacific Islands are culturally and environmentally diverse, treasured by the 1.9 million people who call them home. Pacific Islands are particularly vulnerable to climate change impacts due to their exposure and isolation, small size, low elevation, in the case of atolls, and concentration of infrastructure and economy along the coasts. A prevalent cause of year-to-year changes in climate patterns around the globe and in the Pacific Islands region is the El Niño Southern Oscillation, ENSO. The El Niño and La Niña phases of ENSO can dramatically affect precipitation, air and ocean temperature, sea surface height, storminess, wave size, and trade winds. It is unknown exactly how the timing and intensity of ENSO will continue to change in the coming decades, but recent climate model results suggest a doubling in frequency of both El Niño and La Niña extremes in this century as compared to the 20th century under scenarios with more warming, including the higher scenario, RCP 8.5. In islands, all natural sources of fresh water come from rainfall received within their limited land areas. Severe droughts are common, making water shortage one of the most important climate-related risks in the region. As temperature continues to rise and cloud cover decreases in some areas, evaporation is expected to increase, causing both reduced water supply and higher water demand. Stream flow in Hawaii has declined over approximately the past 100 years, consistent with observed decreases in rainfall. The impacts of sea level rise in the Pacific include coastal erosion, episodic flooding, permanent inundation, heightened exposure to marine hazards, and saltwater intrusion to surface water and groundwater systems. Sea level rise will disproportionately affect the tropical Pacific and potentially exceed the global average. Invasive species, landscape change, habitat alteration, and reduced resilience have resulted in extinctions and diminished ecosystem function. Inundation of atolls in the coming decades is projected to impact existing on-island ecosystems. Wildlife that relies on coastal habitats will likely also be severely impacted. In Hawaii, coral reefs contribute an estimated $477 million to the local economy every year. Under projected warming of approximately 0.5 degrees Fahrenheit per decade, all near-shore coral reefs in the Hawaii and Pacific Islands region will experience annual bleaching before 2050. An ecosystem-based approach to international management of open ocean fisheries in the Pacific that incorporates climate-informed catch limits is expected to produce more realistic future harvest levels and enhance ecosystem resilience. Indigenous communities of the Pacific derive their sense of identity from the islands. Emerging issues for indigenous communities of the Pacific include the resilience of marine managed areas and climate-induced human migration from their traditional lands. The rich body of traditional knowledge is place-based and localized and is useful in adaptation planning because it builds on intergenerational sharing of observations. Documenting the kinds of governance structures or decision-making hierarchies created for management of these lands and waters is also important as a learning tool that can be shared with other island communities. Across the region groups are coming together to minimize damage and disruption from coastal flooding and inundation as well as other climate-related impacts. Social cohesion is already strong in many communities making it possible to work together to take action. Early intervention can lower economic, environmental, social, and cultural costs and reduce or prevent conflict and displacement from ancestral land and resources. For a full chapter including references and traceable accounts, see HTTPS colon double backslash nca2018.globalchange.gov backslash chapter backslash hawaii-pacific. End of section 29 section 30 of the fourth national climate assessment volume two by USG-CRP. This LibriVox recording is in the public domain. Recording by Warren Coddy, Gurney, Illinois. Chapter 28 Reducing Risks Through Adaptation Actions Key message one. Adaptation implementation is increasing. Adaptation planning and implementation activities are occurring across the United States in the public, private, and non-profit sectors. Since the third national climate assessment, implementation has increased, but is not yet commonplace. Key message two. Climate change outpaces adaptation planning. Successful adaptation has been hindered by the assumption that climate conditions are and will be similar to those in the past. Incorporating information on current and future climate conditions into design guidelines, standards, policies, and practices would reduce risk and adverse impacts. Key message three. Adaptation entails iterative risk management. Adaptation entails a continuing risk management process. It does not have an endpoint. With this approach, individuals and organizations of all types assess risks and vulnerabilities from climate and other drivers of change, such as economic, environmental, and societal, take actions to reduce those risks and learn over time. Key message four. Benefits of proactive adaptation exceed costs. Proactive adaptation initiatives, including changes to policies, business operations, capital investments, and other steps yield benefits in excess of their costs in the near term, as well as over the long term. Evaluating adaptation strategies involves consideration of equity, justice, cultural heritage, the environment, health, and national security. Key message five. New approaches can further reduce risk. Integrating climate considerations into existing organizational and sectoral policies and practices provides adaptation benefits. Further reduction of the risks from climate change can be achieved by new approaches that create conditions for altering regulatory and policy environments, cultural and community resources, economic and financial systems, technology applications, and ecosystems. Across the United States, many regions and sectors are already experiencing the direct effects of climate change. For these communities, climate impacts from extreme storms made worse by sea level rise to longer lasting and more extreme heat waves to increased numbers of wildfires and floods are an immediate threat, not a far off possibility. Because these impacts are expected to increase over time, communities throughout the United States face the challenge not only of reducing greenhouse gas emissions, but also of adapting to current and future climate change to help mitigate climate risks. Adaptation takes place at many levels, national and regional, but mainly local, as governments, businesses, communities, and individuals respond to today's altered climate conditions and prepare for future change based on the specific climate impacts relevant to their geography and vulnerability. Adaptation has five general stages, awareness, assessment, planning, implementation, and monitoring and evaluation. These phases naturally build on one another, though they are often not executed sequentially and the terminology may vary. The third national climate assessment, released in 2014, found the first three phases underway throughout the United States, but limited in terms of on the ground implementation. Since then, the scale and scope of adaptation implementation have increased, but in general, adaptation implementation is not yet commonplace. One important aspect of adaptation is the ability to anticipate future climate impacts and plan accordingly. Public and private sector decision makers have traditionally made plans assuming that the current and future climate in their location will resemble that of the recent past. This assumption is no longer reliably true. Increasingly, planners, builders, engineers, architects, contractors, developers, and other individuals are recognizing the need to take current and projected climate conditions into account in their decisions about the location and design of buildings and infrastructure, engineering standards, insurance rates, property values, land use plans, disaster response preparations, supply chains, and cropland and forest management. In anticipating and planning for climate change, decision makers practice a form of risk assessment known as iterative risk management. Iterative risk management emphasizes that the process of anticipating and responding to climate change does not constitute a single set of judgments at any point in time. Rather, it is an ongoing cycle of assessment, action, reassessment, learning, and response. In the adaptation context, public and private sector actors manage climate risk using three types of actions, reducing exposure, reducing sensitivity, and increasing adaptive capacity. Climate risk management includes some attributes and tactics that are familiar to most businesses and local governments, since these organizations already commonly manage or design for a variety of weather-related risks, including coastal and inland storms, heatways, water availability threats, droughts, and floods. However, successful adaptation also requires the often unfamiliar challenge of using information on current and future climate rather than past climate, which can prove difficult for those lacking experience with climate change data sets and concepts. In addition, many professional practices and guidelines, as well as legal requirements, still call for the use of data based on past climate. Finally, factors such as access to resources, culture, governance, and available information can affect not only the risk faced by different populations, but also the best ways to reduce their risks. Achieving the benefits of adaptation can require upfront investments to achieve longer-term savings, engaging with differing stakeholder interests and values, and planning in the face of uncertainty. But adaptation also presents challenges, including difficulties in obtaining the necessary funds, insufficient information and relevant expertise, and jurisdictional mismatches. In general, adaptation can generate significant benefits in excess of its costs. Benefit cost analysis can help guide organizations toward actions that most efficiently reduce risks, in particular those that, if not addressed, could prove extremely costly in the future. Beyond those attributes explicitly measured by benefit cost analysis, effective adaptation can also enhance social welfare in many ways that can be difficult to quantify and that people will value differently, including improving economic opportunity, health, equity, security, education, social connectivity, and sense of place, as well as safeguarding cultural resources and practices and environmental quality. A significant portion of climate risk can be addressed by mainstreaming, that is, integrating climate adaptation into existing organizational and sectoral investments, policies, and practices, such as planning, budgeting, policy development, and operations and maintenance. Mainstreaming of climate adaptation into existing decision processes has already begun in many areas, such as financial risk reporting, capital investment planning, engineering standards, military planning, and disaster risk management. Further reduction of the risks from climate change, in particular those that arise from futures with high levels of greenhouse gas emissions, calls for new approaches that create conditions for altering regulatory and policy environments, cultural and community resources, economic and financial systems, technology applications, and ecosystems. For full chapter, including references and traceable accounts, see HTTPS colon double backslash nca2018.globalchange.gov backslash chapter backslash adaptation. End of section 30. Section 31 of the Fourth National Climate Assessment, Volume 2 by USG CRP. This LibriVox recording is in the public domain. Recording by Warren Coddy, Gurnee, Illinois. Chapter 29. Reducing risks through emissions mitigation. Key Message 1. Mitigation-related activities within the United States. Mitigation-related activities are taking place across the United States, at the federal, state, and local levels, as well as in the private sector. Since the Third National Climate Assessment, a growing number of states, cities, and businesses have pursued or deepened initiatives aimed at reducing emissions. Key Message 2. The risks of inaction. In the absence of more significant global mitigation efforts, climate change is projected to impose substantial damages on the US economy, human health, and the environment. Under scenarios with high emissions and limited or no adaptation, annual losses in some sectors are estimated to grow to hundreds of billions of dollars by the end of the century. It is very likely that some physical and ecological impacts will be irreversible for thousands of years, while others will be permanent. Key Message 3. Avoided or reduced impacts due to mitigation. Many climate change impacts and associated economic damages in the United States can be substantially reduced over the course of the 21st century through global scale reductions in greenhouse gas emissions, though the magnitude and timing of avoided risks vary by sector and region. The effect of near-term emissions mitigation on reducing risks is expected to become apparent by mid-century and grow substantially thereafter. Key Message 4. Interactions between mitigation and adaptation. Interactions between mitigation and adaptation are complex and can lead to benefits, but they also have the potential for adverse consequences. Adaptation can complement mitigation to substantially reduce exposure and vulnerability to climate change in some sectors. This complementarity is especially important given that a certain degree of climate change due to past and present emissions is unavoidable. Current and future emissions of greenhouse gases and thus emission mitigation actions are crucial for determining future risks and impacts of climate change to society. The scale of risks that can be avoided through mitigation actions is influenced by the magnitude of emissions reductions, the timing of those reductions, and the relative mix of mitigation strategies for emissions of long-lived greenhouse gases, namely carbon dioxide, short-lived greenhouse gases such as methane, and land-based biologic carbon. Many actions at national, regional, and local scales are underway to reduce greenhouse gas emissions, including efforts in the private sector. Climate change is projected to significantly damage human health, the economy, and the environment in the United States, particularly under a future with high greenhouse gas emissions. A collection of frontier research initiatives is underway to improve understanding and quantification of climate impacts. These studies have been designed across a variety of sectoral and spatial scales and feature the use of internally consistent climate and socio-economic scenarios. Recent findings from these multi-sector modeling frameworks demonstrate substantial and far-reaching changes over the course of the 21st century, and particularly at the end of the century, with negative consequences for a large majority of sectors, including infrastructure and human health. For sectors where positive effects are observed in some regions or for specific time periods, the effects are typically dwarfed by changes happening overall within the sector or at broader scales. Recent studies also show that many climate change impacts in the United States can be substantially reduced over the course of the 21st century through global scale reductions in greenhouse gas emissions. While the difference in climate outcomes between scenarios is more modest through the first half of the century, the effect of mitigation in avoiding climate change impacts typically becomes clear by 2050 and increases substantially in magnitude thereafter. Research supports that early and substantial mitigation offers a greater chance of avoiding increasingly adverse impacts. The reduction of climate change risk due to mitigation also depends on assumptions about how adaptation changes to the exposure and vulnerability of the population. Physical damages to coastal property and transportation infrastructure are particularly sensitive to adaptation assumptions, with proactive measures estimated to be capable of reducing damages by large fractions. Because society is already committed to a certain amount of future climate change due to past and present emissions and because mitigation activities cannot avoid all climate-related risks, mitigation and adaptation activities can be considered complementary strategies. However, adaptation can require large upfront costs and long-term commitments for maintenance, and uncertainty exists in some sectors regarding the applicability and effectiveness of adaptation in reducing risk. Interactions between adaptation and mitigation strategies can result in benefits or adverse consequences. While uncertainties still remain, advancements in the modeling of climate and economic impacts, including current understanding of adaptation pathways, are increasingly providing new capabilities to understand and quantify future effects. For full chapter including references and traceable accounts, see HTTPS colon double backslash nca2018.globalchange.gov backslash chapter backslash mitigation. End of section 31. Section 32 of the Fourth National Climate Assessment, Volume 2 by USGCRP. This LibriVox recording is in the public domain. Recording by Warren Caudy, Gurney, Illinois. Frequently asked questions. Introduction to climate change. Question. How do we know Earth is warming? Short answer. Many indicators show conclusively that Earth has warmed since the 19th century. In addition to warming shown in the observational record of oceanic and atmospheric temperature, other evidence includes melting glaciers and continental ice sheets, rising global sea level, a longer frost-free season, changes in temperature extremes, and increases in atmospheric humidity, all consistent with long-term warming. Long answer. Observations of surface temperature taken over Earth's land and ocean surfaces since the 19th century show a clear warming trend. Temperature observations have been taken consistently since the 1880s or earlier at thousands of observing sites around the world. Additionally, instruments on ships, buoys, and floats together provide a more than 100-year record of sea surface temperature, showing that the top 6,500 feet of Earth's ocean is warming in all basins. These observations are consistent with readings from satellite instruments that measure atmospheric and sea surface temperatures from space. Used together, land, ocean, and space-based temperature observations show clear evidence of warming at Earth's surface over climatological timescales—http colon double backslash www.globalchange.gov backslash browse backslash indicators for more indicators of change. See also chapter two, climate. Scientists around the world have been measuring the extent and volume of ice contained in the same glaciers every few years since 1980. These measurements show that, globally, there is a large net volume loss in glacial ice since the 1980s. However, the rate of the ice loss varies by region, and in some cases, yearly glacier advances are observed. See frequently asked question, how does climate change affect mountain glaciers? Ice sheets on Antarctica and Greenland have been losing ice mass consistently since 2002, when advanced satellite measurements of their continental ice mass began. See frequently asked question, is Antarctica losing ice? What about Greenland? Arctic sea ice coverage has been monitored using satellite imagery since the late 1970s, showing consistent and large declines in September, the time of year when the minimum coverage occurs. There are additional observational lines of evidence for warming. For example, the area of land in the Northern Hemisphere, covered by snow each spring, is now smaller on average than it was in the 1960s. Tide gauges and satellites show that global sea level is rising, both as a result of the addition of water to the ocean from melting glaciers, and from the expansion of seawater as it warms. Lastly, as air warms, its capacity to hold water vapor increases, and measurements show that atmospheric humidity is increasing around the globe, consistent with a warming climate. See Chapter 3, Water. See also Chapter 1, Overview, Figure 1.2, for more indicators of a warming world. Question, what makes recent climate change different from warming in the past? Short answer. Increases in global temperature since the 1950s are unusual for two reasons. First, current changes are primarily the result of human activities, rather than natural physical processes. Second, temperature changes are occurring much faster than they did in the past. Long answer. Our planet's climate has changed before. Sedimentary rocks and fossils show clear evidence for a series of long, cold periods, called ice ages, followed by warm periods. Common archaeological and geological processes for dating past events show that these cycles of cooling and warming occurred about once every 100,000 years, for at least the last million years. Before major land use changes and industrialization, changes in global temperature were caused by natural factors, including regular changes in Earth's orbit around the Sun, volcanic eruptions, and changes in energy from the Sun. Major warming and cooling events were driven by natural variations of Earth's orbit that altered the amount of sunlight reaching Earth's arctic and Antarctic regions, resulting in the retreat and advance of massive ice sheets. Additionally, quiescent or active periods of volcanic eruptions also could contribute to warming or cooling events, respectively. Natural factors are still affecting the planet's climate today, see figure A5.5. Yet, since the beginning of the Industrial Revolution, human use of coal, oil, and gas has rapidly changed the composition of the atmosphere, figure A5.1. Land use changes, such as deforestation, cement production, and animal production for food, have also contributed to the increase in levels of greenhouse gases in the atmosphere. Unlike past changes in climate, today's warming is driven primarily by human activity, rather than by natural physical processes, see figure A5.5, see also chapter 2, climate. Current warming is also happening much faster than it did in the past. Scientific records from ice cores, tree rings, soil boreholes, and other natural thermometers, often called proxy climate data, show that the recent increase in temperature is unusually rapid compared to past changes, see figures A5.2 and A5.4. After an ice age, Earth typically took thousands of years to warm up again. The observed rate of warming over the last 50 years is about 8 times faster than the average rate of warming from a glacial maximum to a warm interglacial period. Question, what's the difference between global warming and climate change? Short answer. Though some people use the terms global warming and climate change interchangeably, their meanings are slightly different. Global warming refers only to Earth's rising surface temperature, while climate change includes temperature changes and a multitude of effects that result from warming, including melting glaciers, increased humidity, heavier rain storms, and changes in the patterns of some climate-related extreme events. Long answer. By itself, the phrase global warming refers to increases in Earth's annual average surface temperature. Today, however, when people use the phrase, they usually mean the recent warming that is due in large part to the rapid increase of greenhouse gases, GHGs, in the atmosphere from human activities such as deforestation and the burning of fossil fuels for energy. Thus, global warming has become a form of shorthand for a complex scientific process. The entire globe is not warming uniformly. Some areas may cool, such as the North Atlantic Ocean, while some may warm faster than the global average, such as the Arctic. The term climate change refers to the full range of consequences or impacts that occur as atmospheric levels of GHGs rise, and different parts of the Earth's system respond to a higher average surface temperature. For instance, observed long-term trends, such as increases in the frequency of drought and heavy precipitation events, are not technically warming trends, but they are related to current warming and our processes of climate change. End of Section 32. Section 33 of the Fourth National Climate Assessment, Volume 2 by USG-CRP. This LibriVox recording is in the public domain. Recording by Warren Caudy, Gurnee, Illinois. Frequently asked questions, climate science. Question. What are greenhouse gases, and what is the greenhouse effect? Short answer. Greenhouse gases, GHGs, are gases that absorb and emit thermal, heat, infrared radiation. Carbon dioxide, methane, nitrous oxide, ozone, and water vapor are the most prevalent GHGs in Earth's atmosphere. These gases absorb heat emitted by Earth's surface and re-emit that heat into Earth's atmosphere, making it much warmer than it would be otherwise. A process known as the greenhouse effect. Long answer. Most of Earth's atmosphere is made up of nitrogen, N2, and oxygen, O2, neither of which is considered a greenhouse gas. Other gases, known as greenhouse gases, GHGs, behave very differently from O2 and N2 when it comes to infrared radiation emitted from Earth. GHGs, such as water vapor, carbon dioxide, CO2, and methane, CH4, have a more complex molecular structure made up of three or more atoms, as opposed to the symmetrical two atom molecules of O2 and N2 that absorbs some of the energy emitted from Earth's surface and then re-radiates that energy in all directions, including back down towards the surface. This ultimately traps energy in the lower atmosphere in the form of heat, figure A5.3. This greenhouse effect makes the average temperature of Earth nearly 60 degrees Fahrenheit warmer than it would be in the absence of these GHGs.  Many GHGs, including CO2, CH4, water vapor, and nitrous oxide, N2O, occur naturally in the atmosphere. However, atmospheric concentrations of these GHGs have been rising over the last few centuries as a result of human activities. In addition, human activities have added new, entirely human-made GHGs to the atmosphere, including chlorofluorocarbons, CFCs, hydrofluorocarbons, HFCs, perfluorocarbons, PFCs, and sulfur hexafluoride, SF6. As the global population has increased, so have GHG emissions. This, in turn, makes the greenhouse effect stronger, resulting in higher average temperature around the globe. Question. Why are scientists confident that human activities are the primary cause of recent climate change? Short answer. Many independent lines of evidence support the finding that human activities are the dominant cause of recent, since 1950, climate change. These lines of evidence include changes seen in the observational records that are consistent with our understanding, based on physics, of how the climate system should change due to human influences. Other evidence comes from climate modeling studies that closely reproduce the observed temperature record. Long answer. The climate science special report concludes, quote, human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century, end quote. The Earth's climate only warms or cools significantly in response to changes that affect the balance of incoming and outgoing energy. Over long time scales, tens to hundreds of thousands of years, orbital cycles produce long periods of warming and cooling. Over shorter time scales, two factors could generally force changes in Earth's temperature to a measurable degree. One, changes in the amount of energy put out by the sun, and two, changes in the concentrations of greenhouse gases, GHGs, in Earth's atmosphere. Recent measurements of the sun's energy show no trend over the last 50 years. Additionally, observations show that the lower atmosphere, troposphere, has warmed, while the upper atmosphere, stratosphere, has cooled. If the observed warming had been due to an increase in energy from the sun, then all layers of Earth's atmosphere would have warmed, which is not what scientists observe. Thus, we can eliminate changes in the energy received from the sun as a major factor in the warming observed since about 1950. This leaves the possibility that changes in GHG concentrations in the atmosphere are the primary cause of recent warming. Atmospheric carbon dioxide, CO2, levels have increased from approximately 270 parts per million PPM during pre-industrial times to the current 408 parts per million observed in 2018. In addition, atmospheric concentrations of other GHGs, including methane and nitrous oxide, have increased over the same period. This increase in GHG concentrations has coincided with the observed increase in global temperature. Scientists use methods that provide chemical fingerprints of the source of these increased emissions and have shown that the 40% increase in atmospheric CO2 levels since the Industrial Revolution is due mainly to human activities, primarily the combustion of fossil fuels, and not due to natural carbon cycle processes. Other evidence attributing human activities as the dominant driver of observed warming comes from climate modeling studies. Computer simulations of Earth's climate, based on historical data of observed changes in natural and human influences, accurately reproduce the observed temperature record over the last 120 years. These results show that without human influences, such as the observed increases in GHG emissions, Earth's surface would have cooled slightly over the past half century. The only way to closely replicate the observed warming is to include both natural and human forcing changes in climate models, figure A5.5. Thus, the observational record and modeling studies both point to human factors being the main cause for their recent warming, Chapter 2 Climate. Question. What role does water vapor play in climate change? Short answer. Water vapor is the most abundant greenhouse gas, GHG, in the atmosphere, and plays an important role in Earth's climate, significantly increasing Earth's temperature. However, unlike other GHGs, water vapor can condense and precipitate, so water vapor has a short lifespan in the atmosphere. Air temperature, and not emissions, controls the amount of water vapor in the lower atmosphere. For this reason, water vapor is considered a feedback agent and not a driver of climate change. Long answer. Water vapor is the primary GHG in the atmosphere, and its contribution to Earth's greenhouse effect is about two or three times that of carbon dioxide, CO2. Human activities directly add water vapor to the atmosphere, primarily through increasing evaporation from irrigation, power plant cooling, and combustion of fossil fuels. Other GHGs, such as CO2, are not condensable at atmospheric temperatures and pressures, so they will continue to build up in the atmosphere as long as their emissions continue. The amount of water vapor in the lower atmosphere, troposphere, is mainly controlled by the air temperature and proximity to a water source, such as an ocean or large lake, rather than by emissions from human activities. Fluctuations in air temperature change the amount of water vapor that the air can hold, with warmer air capable of holding more moisture. Increases in water vapor levels in the lower atmosphere are considered a positive feedback, or self-reinforcing cycle, in the climate system. As increasing concentrations of other GHGs, for example carbon dioxide, methane, and nitrous oxide, warm the atmosphere, atmospheric water vapor concentrations increase, thereby amplifying the warming effect, figure A5.6. If atmospheric concentrations of CO2 and other GHGs decreased, air temperature would drop, decreasing the ability of the atmosphere to hold water vapor, further decreasing temperature. Question, how are El Nino and climate variability related to climate change? Short answer. El Nino and other forms of natural climate variability are not caused by humans, but their frequency, duration, extent, or intensity might be affected by greenhouse gas emissions from human activities. Natural climate variability produces short-term regional changes in temperature and weather patterns, whereas human-caused climate change is a persistent long-term phenomenon. Long answer. Climate variability refers to the natural changes in climate that fall within the observed range of extremes for a particular region, as measured by temperature, precipitation, and frequency of events. Drivers of climate variability include the El Nino Southern Oscillation, or ENSO, and other phenomena. ENSO is a quasi-periodic warming or cooling of the sea surface temperatures in the tropical eastern Pacific and is often referred to by its phase of El Nino, warm phase, or La Nina, cool phase. These different ENSO phases can have varying ecosystem and economic effects, especially in certain fishing communities, while also influencing weather worldwide, figure A5.7. In the United States, El Nino conditions generally correspond with warmer-than-average sea surface and air temperatures along the west coast, wetter conditions in the southwest, cooler temperatures in the southeast, and warmer conditions in the northeast. In contrast, the La Nina phase of ENSO corresponds to cooler temperature in the U.S. northwest and drier and warmer conditions in the southeast, along with increased upwelling along the west coast. Evidence from paleoclimate records suggests that there have been changes in the frequency and intensity of ENSO events in the past. Human-caused climate change might also affect the frequency and magnitude of ENSO events and can exacerbate or ameliorate regional ENSO impacts. For example, if there is a strong La Nina event that results in dry conditions in the southwest, those conditions may be exacerbated by additional drying due to climate change. ENSO is a complex phenomenon, but new research is shedding light on the many factors influencing how climate change affects the ENSO cycle. End of Section 33. Section 34 of the Fourth National Climate Assessment, Volume 2 by USG-CRP. This Slibervox recording is in the public domain. Recording by Warren Coddy, Gurney, Illinois. Frequently asked questions. Temperature and climate projections. Question. What methods are used to record global surface temperatures and measure changes in climate? Short answer. Global surface temperatures are measured by using data from weather stations over land and by ships and buoys over the ocean. Global surface temperature records date back more than 300 years in some locations and near global coverage has existed since the late 1800s. Multiple research groups have examined US and global temperature records in great detail, taking into account changes in instruments, the time of observations, station location, and any other potential sources of error. Although there are slight differences among datasets due to choices in data selection, analysis, and averaging techniques, these differences do not change the clear result that global surface temperature is rising. Long answer. Climate change is best measured by assessing trends over long periods of time, generally greater than 30 years, which means we need global surface temperature records that include data from before the satellite age. Scientists who obtain, digitize, and collate long-term temperature records take great care to ensure that any potentially skewed measurements, such as a change in instrument method or location, or a change in the time of day a recording is made, do not affect the integrity of the dataset. Researchers rigorously examined the data to identify and adjust for any such effects before using it to evaluate long-term climate trends. Different choices in data selection, analysis, and averaging techniques by multiple independent research teams mean that each dataset varies slightly. Even with these variations, however, multiple independently produced results are in very good agreement at both global and regional scales. All global surface temperature datasets indicate that the vast majority of Earth's surface has warmed since 1901, figure A5.8. Scientists also consider other influences that could impact temperature records, such as whether data from thermometers located in cities are skewed by the urban heat island effect, where heat absorbed by buildings in asphalt makes cities warmer than the surrounding countryside. When determining climate trends, data corrections to these temperature records have adequately accounted for this effect. At the global scale, evidence of global warming over the past 50 years is still observed, even if all of the urban stations are removed from the global temperature record. Studies have also shown that the warming trends of rural and urban areas that are in close proximity essentially match, even though the urban areas may have higher temperatures overall. Question. Were there predictions of global cooling in the 1970s? Short answer. No. A review of the scientific literature from the 1970s shows that the broad climate science community did not predict global cooling or an imminent ice age. On the contrary, even then, discussions of human-related warming dominated scientific publications on climate and human influences. Long answer. Scientific understanding of what are called the Milankovitch cycles, cyclical changes in Earth's orbit that can explain the onset and ending of ice ages, led a few scientists in the 1970s to contemplate that the current warm interglacial period might be ending soon, leading to a new ice age over the next few centuries. These few speculations were picked up and amplified by the media. But at that time there were far more scientific articles describing how warming would occur from the increase in atmospheric concentrations of greenhouse gases from human activities, including the burning of fossil fuels. Figure A5.9. The latest information suggests that if Earth's climate was being controlled primarily by natural factors, the next cooling cycle would begin sometime in the next 1500 years. However, humans have so altered the composition of the atmosphere that the next ice age has likely now been delayed. That delay could potentially be tens of thousands of years. Question. How are temperature and precipitation patterns projected to change in the future? Short answer. Our world will continue to warm in the future because of historic emissions of greenhouse gases, GHGs. But the amount of warming will depend largely on the level of future emissions of GHGs and the choices humans make. If humans continue burning fossil fuels at or above our current rate through the end of the century, scientists project Earth will warm about 9 degrees Fahrenheit relative to pre-industrial times prior to 1750. Precipitation is projected to still be seasonally and regionally variable, but on average projections show high latitude areas getting wetter and subtropical areas getting drier. The frequency and intensity of very heavy precipitation are expected to increase, increasing the likelihood of flooding. Climate change will not affect all places in the same way or to the same degree, but will vary at regional levels. Long answer. In the coming decades, scientists project that global average temperature will continue to increase, Chapter 2, Climate. Although natural variability will continue to play a significant role in year to year changes, sizable variations from global average changes are possible at the regional level. Even if humans drastically reduce levels of GHG emissions, near-term warming will still occur because there is a lag in the temperature response to changes in atmospheric composition, Figure A5.10. Over the next couple decades, natural variability and the response of Earth's climate system to historic emissions will be the primary determinants of observed warming. After about 2050, however, the rate and amount of emissions of GHGs released by human activities, as well as the response of Earth's climate system to those emissions, will be the primary determining factors in changes in global and regional temperature, Figure A5.13, see also Chapter 2, Climate. Efforts to rapidly and significantly reduce emissions of GHGs can still limit the global temperature increase to 3.6 degrees Fahrenheit, 2 degrees Celsius, by the end of the century relative to pre-industrial levels. Precipitation patterns are also expected to continue to change throughout this century and beyond. The trends observed in recent decades are expected to continue, with more precipitation projected to fall in the form of heavier precipitation events. Such events increase the likelihood of flooding, even in drought-prone areas. As with increases in global average temperature, large-scale shifts toward wetter or drier conditions and the projected increases in heavy precipitation are expected to be greater under higher GHG emissions scenarios, for example, RCP 8.5, versus lower ones, for example, RCP 4.5. Projected warming is also expected to lead to an increase in the fraction of total precipitation falling as rain rather than snow, which reduces snowpack on the margins of areas that now have reliable snowpack accumulation during the cold season. See, for example, Chapter 24, Northwest, Key Message 2. Question, how do computers model Earth's climate? Short answer. Global climate models enable scientists to create virtual Earths, where they can analyze causes and effects of past changes in temperature, precipitation, and other climate variables. Today's climate models can accurately reproduce broad features of past and present climate, such as the location and strength of the jet stream, the spatial distribution and seasonal cycle of precipitation, and the natural occurrence of extreme weather events, such as heat and cold waves, droughts and floods, and hurricanes. They can also reproduce historic natural cycles, such as the periodic occurrence of ice ages and interglacial warm periods, as well as the human-caused warming that has occurred over the last 50 years. While uncertainties remain, scientists have confidence in model projections of how climate is likely to change in the future in response to key variables, such as an increase in human-caused emissions of greenhouse gases, in part because of how accurately they can represent past climate changes. Long answer. Climate models are based on equations that represent fundamental laws of nature and the many processes that affect Earth's climate system. By dividing the atmosphere, land, and ocean into smaller spatial units to solve the equations, climate models capture the evolving patterns of atmospheric pressures, winds, temperatures, and precipitation. Over longer time frames, these models simulate wind patterns, high and low pressure systems, ocean currents, ice and snowpack accumulation and melting, soil moisture, extreme weather occurrences, and other environmental characteristics that make up the climate system. Figure A5.11. Some important processes, including cloud formation and atmospheric mixing, are represented by approximate relationships, either because the processes are not fully understood or they are at a scale that a model cannot directly represent. These approximations lead to uncertainties in model simulations of climate. Approximations are not the only uncertainties associated with climate models, as discussed in the frequently asked question, what are key uncertainties when projecting climate change? Question. Can scientists project the effects of climate change for local regions? Short answer. Yes, though there are limitations. With advances in computing power, the future effects of climate change can be projected more accurately for local communities. Local high resolution, downscaled climate modeling can be used to produce data at a scale of 1 to 20 miles. These downscaled projections show climate related impacts at the local level and can be an important tool for community planners and decision makers. Long answer. One significant research focus recently has been to develop models of climate impacts on a relatively small geographic scale. Most global climate projections use grid units that may be too coarse to properly represent mountains, coastlines, and other important features of a local landscape. Recently, two different approaches have been used by scientists to project local climate conditions. The first is a statistical approach that uses local observations in conjunction with global models to project future changes. The local observations required for this approach are available only for limited regions and for a few climate variables, mainly temperature and precipitation, figure A5.12. The second method is a so-called dynamical approach that uses an additional high-resolution computer model, similar to a weather prediction model, to account for complex topography and varying land cover that can impact climate on the local level. High resolution dynamical models are complete enough to simulate numerous climate variables, temperature, precipitation, winds, humidity, surface sunlight, etc., and do not require the local observations required for the statistical approach. However, these models require an immense amount of computing power. Today's most powerful supercomputers enable climate scientists to examine the effects of climate change in ways that were impossible just five years ago. Over the next decade, computer speeds are predicted to increase 100-fold or more, improving climate projections and models on both the global and local levels. It should also be noted that both statistical and dynamical approaches have biases and errors that, when combined with uncertainties from global model simulations, can reduce the level of confidence in these more localized projections. See Hayhoe et al. 2017 for more details. Question, what are key uncertainties when projecting climate change? Short answer. The precise amount of future climate change that will occur over the rest of this century is uncertain, mainly due to uncertainties in emissions, natural variability, and differences in scientific models. Long answer. First, projections of future climate changes are usually based on scenarios or sets of assumptions regarding how future emissions may change due to changes in population, energy use, technology, and economics. Society may choose to reduce emissions or continue on a pathway of increasing emissions. The differences in projected future climate under different scenarios are generally small for the next few decades. By the second half of the century, however, human choices, as reflected in these scenarios, become the key determinant of future climate change. Figure A5.13. A second source of uncertainty is natural variability, which affects the climate over timescales from months to decades. These natural variations are largely unpredictable, such as a volcanic eruption, and are superimposed on the warming from increasing greenhouse gases, GHGs. A third source of uncertainty involves limitations in our current scientific knowledge. Climate models differ in the way they represent various processes, for example, cloud properties, ocean circulation, and aerosol effects. Additionally, climate sensitivity, or how much the climate will warm with a given increase in GHGs, often a doubling of GHG from pre-industrial levels, is still a major source of uncertainty. As a result, different models produce small differences in projections of global average change. Scientists often use multiple models to account for the variability and represent this as a range of projected outcomes. Finally, there is always the possibility that there are processes and feedbacks not yet being included in projections of climate in the future. For example, as the Arctic warms, carbon trapped in permafrost may be released into the atmosphere, increasing the initial warming due to human-caused emissions of GHGs, or an ice sheet may collapse, leading to faster than expected sea level rise. However, for a given future scenario, the amount of future climate change can be specified within plausible bounds, with those bounds determined not only from the differences in how climate responds to a doubling of GHG concentrations among models, but also by utilizing information about climate changes in the past. See Hayhoe et al. 2017 for more details. Question. Is it getting warmer everywhere at the same rate? Short answer. Our world is warming overall, but temperatures are not increasing at the same rate everywhere. The average global temperature is projected to continue increasing throughout the remainder of this century due to greenhouse gas GHG emissions from human activities. Generally, high latitudes are expected to continue warming more than lower latitudes. Coastal and island regions are expected to warm less than interior continent regions. Long answer. Temperature changes at a given location are a function of multiple factors, including global and local forces, and both human and natural influences. Though Earth's average temperature is rising, some locations could be cooling due to local factors. In some places, including the U.S. Southeast, temperatures do not show a warming trend over the last century as a whole, although they have been increasing since the 1960s, Chapter 19, Southeast. Possible causes of the observed lack of warming in the Southeast during the 20th century include increased cloud cover and precipitation, increases in the presence of fine particles, called aerosols, in the atmosphere, expanding forests, decreases in the amount of heat conducted from land due to increases in irrigation, and multi-decadal variability in sea surface temperatures in both the North Atlantic and the tropical Pacific oceans. At smaller geographic scales and time intervals, the relative influence of natural variations in climate compared to the human contribution is larger than at the global scale. A lack of warming or a decrease in temperature at an individual location does not negate the fact that, overall, the planet is warming. Alaska, in contrast to the U.S. Southeast, has been warming twice as fast as the global average since the middle of the 20th century, Chapter 26, Alaska. State-wide average temperatures for 2014 through 2016 were notably warmer as compared to the last few decades, with 2016 being the warmest on record. Daily record high temperatures in the contiguous United States are now occurring twice as often as record low temperatures. In Alaska, starting in the 1990s, record high temperatures occurred three times as often as record lows, and in 2015 an astounding nine times as often, Chapter 26, Alaska. Because Earth's climate system still has more energy entering than leaving, global warming has not yet equilibrated to the load of increased GHGs that have already accumulated in the atmosphere. For example, the oceans are still warming over many layers from surface to depth. Some GHGs have long lifetimes. For example, carbon dioxide can reside in the atmosphere for a century or more. Thus, even if the emissions of GHGs were to be sharply curtailed to bring them back to natural levels, it is estimated that Earth is committed to continued warming of more than one degree Fahrenheit by 2100. At the global scale, some future years will be cooler than the preceding year. Some decades could even be cooler than the preceding decade, figure A5.14. Brief periods of faster temperature increases and also temporary decreases in global temperature can be expected to continue into the future as a result of natural variability and other factors. Nonetheless, each successive decade in the last 30 years has been the warmest in the period of reliable instrumental records, going back to 1850, figure A5.15. In fact, the rate of warming has accelerated in the past several decades and 17 of the 18 warmest years have occurred since 2001. See frequently asked question, what do scientists mean by the warmest year on record? Based on this historical record and assessed scenarios for the future, it is expected that future global temperatures, averaged over climate time skills of 30 years or more, will be higher than preceding periods as a result of emissions of CO2 and other GHGs from human activities. Chapter 2 Climate. Question, what do scientists mean by the warmest year on record? Short answer. When scientists declare it the warmest year on record, they mean it's the warmest year since modern global surface temperature record keeping began in 1880. Global temperature data from NASA show that 2016 marked the sixth time this century that a new record high annual average temperature was set, along with 2002, 2005, 2010, 2014 and 2015, and that 17 of the 18 warmest years have occurred since 2001. Long answer. The warmest year on record means it is the warmest year in more than 130 years of modern record keeping of global surface temperature. Prior to 1880, observations did not cover a large enough area of Earth's surface to enable an accurate calculation of the global average temperature. To calculate the value in recent times, scientists evaluate data from roughly 6,300 stations around the world on land, ships and buoys. The year the last national climate assessment was published, 2014, was the warmest year on record at the time, but it was surpassed by 2015, which was then surpassed by 2016. Data from NASA shows that 17 of the 18 warmest years have occurred since 2001, and the six warmest years on record have occurred this century, figure A5.16. However, the global surface temperature is affected by natural variability in addition to climate change, so it is not expected that each year will set a new temperature record. Question. How do climate projections differ from weather predictions? Short answer. The range of possible weather conditions at a specific location on any given day can vary considerably. The climate varies far less for that same location, because it is a measure of weather conditions averaged over 30 years or more. Because the range of possible climate conditions at a given location is much smaller than the range of possible weather conditions, scientists are able to project climate conditions decades into the future. Long answer. Projecting how climate may change decades in the future is a different scientific issue than forecasting weather a few days from now. Weather prediction means determining the exact location, time, and magnitude of specific events. Because the range of possible weather conditions can vary so widely, the weather forecast is extremely sensitive to even the smallest uncertainties or errors in our description of the state of the atmosphere at the start of a forecast. The impact of those uncertainties magnifies over time, which makes it very difficult to predict specific weather events at a given location more than a week or two into the future. Because climate is the average weather at a given location over long periods of time, three decades or more, the range of possible climate conditions at a given location is much smaller than the range of possible weather conditions. For example, the daytime high temperature at a given location may vary by 30 degrees Fahrenheit or more over the course of a day, while the annual average temperature over 30 years may vary by no more than a few degrees. Figure A5.17 We can project how climate may change over time in response to natural forces, such as changes in incoming solar radiation and in response to human activities, such as increasing the abundance of greenhouse gases, GHGs, or decreasing particle pollution. These projections are usually expressed in terms of probabilities, describing a range of possible outcomes, not in the sort of exact, deterministic language of many weather forecasts. The difference between predicting weather and projecting climate is sometimes illustrated with a public health analogy. While it is impossible for us to determine the exact date and time when a particular individual will die, we can easily calculate the average age of death of all Americans for a time period in the past. In this case, weather is like the individual while climate is like the average. To extend this analogy into the realm of climate change, we can also calculate the average life expectancy of Americans who smoke. We can predict that, on average. Smokers will not live as long as non-smokers. Similarly, we can project what the climate will be like if we emit lower levels of GHGs and what it will be like if we emit more.