Global climate is changing rapidly compared to the pace of natural variations in climate that have occurred throughout Earth’s history. Global average temperature has increased by about 1.8°F from 1901 to 2016, and observational evidence does not support any credible natural explanations for this amount of warming; instead, the evidence consistently points to human activities, especially emissions of greenhouse or heat-trapping gases, as the dominant cause.
Earth’s climate will continue to change over this century and beyond. Past mid-century, how much the climate changes will depend primarily on global emissions of greenhouse gases and on the response of Earth’s climate system to human-induced warming. With significant reductions in emissions, global temperature increase could be limited to 3.6°F (2°C) or less compared to preindustrial temperatures. Without significant reductions, annual average global temperatures could increase by 9°F (5°C) or more by the end of this century compared to preindustrial temperatures.
The world’s oceans have absorbed 93% of the excess heat from human-induced warming since the mid-20th century and are currently absorbing more than a quarter of the carbon dioxide emitted to the atmosphere annually from human activities, making the oceans warmer and more acidic. Increasing sea surface temperatures, rising sea levels, and changing patterns of precipitation, winds, nutrients, and ocean circulation are contributing to overall declining oxygen concentrations in many locations.
Global average sea level has risen by about 7–8 inches (about 16–21 cm) since 1900, with almost half this rise occurring since 1993 as oceans have warmed and land-based ice has melted. Relative to the year 2000, sea level is very likely to rise 1 to 4 feet (0.3 to 1.3 m) by the end of the century. Emerging science regarding Antarctic ice sheet stability suggests that, for higher scenarios, a rise exceeding 8 feet (2.4 m) by 2100 is physically possible, although the probability of such an extreme outcome cannot currently be assessed.
Annual average temperature over the contiguous United States has increased by 1.2ºF (0.7°C) over the last few decades and by 1.8°F (1°C) relative to the beginning of the last century. Additional increases in annual average temperature of about 2.5°F (1.4°C) are expected over the next few decades regardless of future emissions, and increases ranging from 3°F to 12°F (1.6°–6.6°C) are expected by the end of century, depending on whether the world follows a higher or lower future scenario, with proportionally greater changes in high temperature extremes.
Annual precipitation since the beginning of the last century has increased across most of the northern and eastern United States and decreased across much of the southern and western United States. Over the coming century, significant increases are projected in winter and spring over the Northern Great Plains, the Upper Midwest, and the Northeast. Observed increases in the frequency and intensity of heavy precipitation events in most parts of the United States are projected to continue. Surface soil moisture over most of the United States is likely to decrease, accompanied by large declines in snowpack in the western United States and shifts to more winter precipitation falling as rain rather than snow.
In the Arctic, annual average temperatures have increased more than twice as fast as the global average, accompanied by thawing permafrost and loss of sea ice and glacier mass. Arctic-wide glacial and sea ice loss is expected to continue; by mid-century, it is very likely that the Arctic will be nearly free of sea ice in late summer. Permafrost is expected to continue to thaw over the coming century as well, and the carbon dioxide and methane released from thawing permafrost has the potential to amplify human-induced warming, possibly significantly.
Human-induced change is affecting atmospheric dynamics and contributing to the poleward expansion of the tropics and the northward shift in Northern Hemisphere winter storm tracks since 1950. Increases in greenhouse gases and decreases in air pollution have contributed to increases in Atlantic hurricane activity since 1970. In the future, Atlantic and eastern North Pacific hurricane rainfall and intensity are projected to increase, as are the frequency and severity of landfalling “atmospheric rivers” on the West Coast.
Regional changes in sea level rise and coastal flooding are not evenly distributed across the United States; ocean circulation changes, sinking land, and Antarctic ice melt will result in greater-than-average sea level rise for the Northeast and western Gulf of Mexico under lower scenarios and most of the U.S. coastline other than Alaska under higher scenarios. Since the 1960s, sea level rise has already increased the frequency of high tide flooding by a factor of 5 to 10 for several U.S. coastal communities. The frequency, depth, and extent of tidal flooding are expected to continue to increase in the future, as is the more severe flooding associated with coastal storms, such as hurricanes and nor’easters.
The climate change resulting from human-caused emissions of carbon dioxide will persist for decades to millennia. Self-reinforcing cycles within the climate system have the potential to accelerate human-induced change and even shift Earth’s climate system into new states that are very different from those experienced in the recent past. Future changes outside the range projected by climate models cannot be ruled out, and due to their systematic tendency to underestimate temperature change during past warm periods, models may be more likely to underestimate than to overestimate long-term future change.
Global climate is changing rapidly compared to the pace of natural variations in climate that have occurred throughout Earth’s history. Global average temperature has increased by about 1.8°F from 1901 to 2016, and observational evidence does not support any credible natural explanations for this amount of warming; instead, the evidence consistently points to human activities, especially emissions of greenhouse or heat-trapping gases, as the dominant cause.
Earth’s climate will continue to change over this century and beyond. Past mid-century, how much the climate changes will depend primarily on global emissions of greenhouse gases and on the response of Earth’s climate system to human-induced warming. With significant reductions in emissions, global temperature increase could be limited to 3.6°F (2°C) or less compared to preindustrial temperatures. Without significant reductions, annual average global temperatures could increase by 9°F (5°C) or more by the end of this century compared to preindustrial temperatures.
The world’s oceans have absorbed 93% of the excess heat from human-induced warming since the mid-20th century and are currently absorbing more than a quarter of the carbon dioxide emitted to the atmosphere annually from human activities, making the oceans warmer and more acidic. Increasing sea surface temperatures, rising sea levels, and changing patterns of precipitation, winds, nutrients, and ocean circulation are contributing to overall declining oxygen concentrations in many locations.
Global average sea level has risen by about 7–8 inches (about 16–21 cm) since 1900, with almost half this rise occurring since 1993 as oceans have warmed and land-based ice has melted. Relative to the year 2000, sea level is very likely to rise 1 to 4 feet (0.3 to 1.3 m) by the end of the century. Emerging science regarding Antarctic ice sheet stability suggests that, for higher scenarios, a rise exceeding 8 feet (2.4 m) by 2100 is physically possible, although the probability of such an extreme outcome cannot currently be assessed.
Annual average temperature over the contiguous United States has increased by 1.2ºF (0.7°C) over the last few decades and by 1.8°F (1°C) relative to the beginning of the last century. Additional increases in annual average temperature of about 2.5°F (1.4°C) are expected over the next few decades regardless of future emissions, and increases ranging from 3°F to 12°F (1.6°–6.6°C) are expected by the end of century, depending on whether the world follows a higher or lower future scenario, with proportionally greater changes in high temperature extremes.
Annual precipitation since the beginning of the last century has increased across most of the northern and eastern United States and decreased across much of the southern and western United States. Over the coming century, significant increases are projected in winter and spring over the Northern Great Plains, the Upper Midwest, and the Northeast. Observed increases in the frequency and intensity of heavy precipitation events in most parts of the United States are projected to continue. Surface soil moisture over most of the United States is likely to decrease, accompanied by large declines in snowpack in the western United States and shifts to more winter precipitation falling as rain rather than snow.
In the Arctic, annual average temperatures have increased more than twice as fast as the global average, accompanied by thawing permafrost and loss of sea ice and glacier mass. Arctic-wide glacial and sea ice loss is expected to continue; by mid-century, it is very likely that the Arctic will be nearly free of sea ice in late summer. Permafrost is expected to continue to thaw over the coming century as well, and the carbon dioxide and methane released from thawing permafrost has the potential to amplify human-induced warming, possibly significantly.
Human-induced change is affecting atmospheric dynamics and contributing to the poleward expansion of the tropics and the northward shift in Northern Hemisphere winter storm tracks since 1950. Increases in greenhouse gases and decreases in air pollution have contributed to increases in Atlantic hurricane activity since 1970. In the future, Atlantic and eastern North Pacific hurricane rainfall and intensity are projected to increase, as are the frequency and severity of landfalling “atmospheric rivers” on the West Coast.
Regional changes in sea level rise and coastal flooding are not evenly distributed across the United States; ocean circulation changes, sinking land, and Antarctic ice melt will result in greater-than-average sea level rise for the Northeast and western Gulf of Mexico under lower scenarios and most of the U.S. coastline other than Alaska under higher scenarios. Since the 1960s, sea level rise has already increased the frequency of high tide flooding by a factor of 5 to 10 for several U.S. coastal communities. The frequency, depth, and extent of tidal flooding are expected to continue to increase in the future, as is the more severe flooding associated with coastal storms, such as hurricanes and nor’easters.
The climate change resulting from human-caused emissions of carbon dioxide will persist for decades to millennia. Self-reinforcing cycles within the climate system have the potential to accelerate human-induced change and even shift Earth’s climate system into new states that are very different from those experienced in the recent past. Future changes outside the range projected by climate models cannot be ruled out, and due to their systematic tendency to underestimate temperature change during past warm periods, models may be more likely to underestimate than to overestimate long-term future change.
Virtually Certain | Extremely Likely | Very Likely | Likely | About as Likely as Not | Unlikely | Very Unikely | Extremely Unlikely | Exceptionally Unlikely |
---|---|---|---|---|---|---|---|---|
99%–100% | 95%–100% | 90%–100% | 66%-100% | 33%-66% | 0%-33% | 0%-10% | 0%-5% | 0%-1% |
Very High | Hoch | Medium | Niedrig |
---|---|---|---|
Strong evidence (established theory, multiple sources, consistent results, well documented and accepted methods, etc.), high consensus | Moderate evidence (several sources, some consistency, methods vary and/or documentation limited, etc.), medium consensus | Suggestive evidence (a few sources, limited consistency, models incomplete, methods emerging, etc.), competing schools of thought | Inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.), disagreement or lack of opinions among experts |
Documenting Uncertainty: This assessment relies on two metrics to communicate the degree of certainty in Key Findings. See Guide to this Report for more on assessments of likelihood and confidence.
<b>Hayhoe</b>, K., D.J. Wuebbles, D.R. Easterling, D.W. Fahey, S. Doherty, J. Kossin, W. Sweet, R. Vose, and M. Wehner, 2018: Our Changing Climate. In <i>Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II</i> [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 72–144. doi: 10.7930/NCA4.2018.CH2
This chapter is based on the Climate Science Special Report (CSSR), which is Volume I of the Fourth National Climate Assessment (available at science2017.globalchange.gov). The Key Messages and the majority of the content represent the highlights of CSSR, updated with recent references relevant to these topics. The interested reader is referred to the relevant chapter(s) in CSSR for more detail on each of the Key Messages that follow.
Long-term temperature observations are among the most consistent and widespread evidence of a warming planet. Global annually averaged temperature measured over both land and oceans has increased by about 1.8°F (1.0°C) according to a linear trend from 1901 to 2016, and by 1.2°F (0.65°C) for the period 1986–2015 as compared to 1901–1960. The last few years have also seen record-breaking, climate-related weather extremes. For example, since the Third National Climate Assessment was published,1 2014 became the warmest year on record globally; 2015 surpassed 2014 by a wide margin; and 2016 surpassed 2015.2,3 Sixteen of the last 17 years have been the warmest ever recorded by human observations.
For short periods of time, from a few years to a decade or so, the increase in global temperature can be temporarily slowed or even reversed by natural variability (see Box 2.1). Over the past decade, such a slowdown led to numerous assertions that global warming had stopped. No temperature records, however, show that long-term global warming has ceased or even substantially slowed over the past decade.4,5,6,7,8,9 Instead, global annual average temperatures for the period since 1986 are likely much higher and appear to have risen at a more rapid rate than for any similar climatological (20–30 year) time period in at least the last 1,700 years.10,11
While thousands of studies conducted by researchers around the world have documented increases in temperature at Earth’s surface, as well as in the atmosphere and oceans, many other aspects of global climate are also changing12,13 (see also EPA 2016, Wuebbles et al. 201710,14). Studies have documented melting glaciers and ice sheets, shrinking snow cover and sea ice, rising sea levels, more frequent high temperature extremes and heavy precipitation events, and a host of other climate variables or “indicators” consistent with a warmer world (see Box 2.2). Observed trends have been confirmed by multiple independent research groups around the world.
Many lines of evidence demonstrate that human activities, especially emissions of greenhouse gases from fossil fuel combustion, deforestation, and land-use change, are primarily responsible for the climate changes observed in the industrial era, especially over the last six decades. Observed warming over the period 1951–2010 was 1.2°F (0.65°C), and formal detection and attribution studies conclude that the likely range of the human contribution to the global average temperature increase over the period 1951–2010 is 1.1°F to 1.4°F (0.6°C to 0.8°C;15 see Knutson et al. 201716 for more on detection and attribution).
Human activities affect Earth’s climate by altering factors that control the amount of energy from the sun that enters and leaves the atmosphere. These factors, known as radiative forcings, include changes in greenhouse gases, small airborne soot and dust particles known as aerosols, and the reflectivity (or albedo) of Earth’s surface through land-use and land-cover changes (see Ch. 5: Land Changes).17,18 Increasing greenhouse gas levels in the atmosphere due to emissions from human activities are the largest of these radiative forcings. By absorbing the heat emitted by Earth and reradiating it equally in all directions, greenhouse gases increase the amount of heat retained inside the climate system, warming the planet. Aerosols produced by burning fossil fuels and by other human activities affect climate both directly, by scattering and absorbing sunlight, as well as indirectly, through their impact on cloud formation and cloud properties. Over the industrial era, the net effect of the combined direct and indirect effects of aerosols has been to cool the planet, partially offsetting greenhouse gas warming at the global scale.17,18
Over the last century, changes in solar output, volcanic emissions, and natural variability have only contributed marginally to the observed changes in climate (Figure 2.1).15,17 No natural cycles are found in the observational record that can explain the observed increases in the heat content of the atmosphere, the ocean, or the cryosphere since the industrial era.11,19,20,21 Greenhouse gas emissions from human activities are the only factors that can account for the observed warming over the last century; there are no credible alternative human or natural explanations supported by the observational evidence.10,22
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Beyond the next few decades, how much the climate changes will depend primarily on the amount of greenhouse gases emitted into the atmosphere; how much of those greenhouse gases are absorbed by the ocean, the biosphere, and other sinks; and how sensitive Earth’s climate is to those emissions.23 Climate sensitivity is typically defined as the long-term change that would result from a doubling of carbon dioxide in the atmosphere relative to preindustrial levels; its exact value is uncertain due to the interconnected nature of the land–atmosphere–ocean system. Changes in one aspect of the system can lead to self-reinforcing cycles that can either amplify or weaken the climate system’s responses to human and natural influences, creating a positive feedback or self-reinforcing cycle in the first case and a negative feedback in the second.18 These feedbacks operate on a range of timescales from very short (essentially instantaneous) to very long (centuries). While there are uncertainties associated with modeling some of these feedbacks,24,25 the most up-to-date scientific assessment shows that the net effect of these feedbacks over the industrial era has been to amplify human-induced warming, and this amplification will continue over coming decades18 (see Box 2.3).
Because it takes some time for Earth’s climate system to fully respond to an increase in greenhouse gas concentrations, even if these concentrations could be stabilized at their current level in the atmosphere, the amount that is already there is projected to result in at least an additional 1.1°F (0.6°C) of warming over this century relative to the last few decades.24,26 If emissions continue, projected changes in global average temperature corresponding to the scenarios used in this assessment (see Box 2.4) range from 4.2°–8.5°F (2.4°–4.7°C) under a higher scenario (RCP8.5) to 0.4°–2.7°F (0.2°–1.5°C) under a very low scenario (RCP2.6) for the period 2080–2099 relative to 1986–2015 (Figure 2.2).24 However, these scenarios do not encompass all possible futures. With significant reductions in emissions of greenhouse gases, the future rise in global average temperature could be limited to 3.6°F (2°C) or less, consistent with the aim of the Paris Agreement (see Box 2.4).27 Similarly, without major reductions in these emissions, the increase in annual average global temperatures relative to preindustrial times could reach 9°F (5°C) or more by the end of this century.24 Because of the slow timescale over which the ocean absorbs heat, warming that results from emissions that occur during this century will leave a multi-millennial legacy, with a substantial fraction of the warming persisting for more than 10,000 years.28,29,30
Oceans occupy over 70% of the planet’s surface and host unique ecosystems and species, including those important for global commercial and subsistence fishing. For this reason, it is essential to highlight the fact that observed changes in the global average temperature of the atmosphere represent only a small fraction of total warming. Since the 1950s, the oceans have absorbed 93% of the excess heat in the earth system that has built up as a result of increasing concentrations of greenhouse gases in the atmosphere.31,32 Significant increases in heat content have been observed over the upper 6,560 feet (2,000 m) of the ocean since the 1960s, with surface oceans warming by about 1.3° ± 0.1°F (0.7° ± 0.1°C) globally from 1900 to 2016.20,31,33,34
Oceans’ net uptake of CO2 each year is approximately equal to a quarter of that emitted to the atmosphere annually from human activities.35,36 It is primarily controlled by the difference between CO2 concentrations in the atmosphere and ocean, with small variations from year to year due to changes in ocean circulation and biology. This carbon uptake is making near-surface ocean waters more acidic, which in turn can harm vulnerable marine ecosystems (see Ch. 9: Oceans; Ch. 26: Alaska; Ch. 27: Hawai‘i & Pacific Islands). Although tropical coral reefs are the most frequently cited casualties of ocean warming and acidification, ecosystems at higher latitudes can be more vulnerable than those at lower latitudes as they typically have a lower buffering capacity against changing acidity. Regionally, acidification is greater along the U.S. coast than the global average, as a result of upwelling (for example, in the Pacific Northwest), changes in freshwater inputs (such as in the Gulf of Maine), and nutrient input (as in urbanized estuaries).34,37,38,39,40,41,42
In addition to higher temperatures and increasing acidification, ocean oxygen levels are also declining in various ocean locations and in many coastal areas.43,44 This decline is due to a combination of increasing sea surface temperatures (SSTs), rising sea levels inundating coastal wetlands, and changing patterns of precipitation, winds, nutrients, and ocean circulation. Over the last 50 years, declining oxygen levels have been observed in many inland seas, estuaries, and nearshore coastal waters.43,45,46,47,48,49,50,51,52 This is a concern because oxygen is essential to most life in the ocean, governing a host of biogeochemical and biological processes that ultimately shape the composition, diversity, abundance, and distribution of organisms from microbes to whales.34
By 2100, under a higher scenario (RCP8.5; see Box 2.4), average SST is projected to increase by 4.9° ± 1.3°F (2.7° ± 0.7°C) as compared to late 20th-century values, ocean oxygen levels are projected to decrease by 3.5%,53 and global average surface ocean acidity is projected to increase by 100% to 150%.32 This rate of acidification would be unparalleled in at least the past 66 million years.34,54,55
Since 1900, global average sea level has risen by about 7–8 inches (about 16–21 cm). The rate of sea level rise over the 20th century was higher than in any other century in at least the last 2,800 years, according to proxy data such as salt marsh sediments and fossil corals.58 Since the early 1990s, the rate of global average sea level rise has increased due to increased melting of land-based ice.56,68,69,70,71,72 As a result, almost half (about 0.12 inches [3 mm] per year) of the observed rise of 7–8 inches (16–21 cm) has occurred since 1993.73,74,75
Over the first half of this century, the future scenario the world follows has little effect on projected sea level rise due to the inertia in the climate system. However, the magnitude of human-caused emissions this century significantly affects projections for the second half of the century and beyond (Figure 2.3). Relative to the year 2000, global average sea level is very likely to rise by 0.3–0.6 feet (9–18 cm) by 2030, 0.5–1.2 feet (15–38 cm) by 2050, and 1–4 feet (30–130 cm) by 2100.56,57,58,59,76,77,78,79 These estimates are generally consistent with the assumption—possibly flawed—that the relationship between global temperature and global average sea level in the coming century will be similar to that observed over the last two millennia.58 These ranges do not, however, capture the full range of physically plausible global average sea level rise over the 21st century. Several avenues of research, including emerging science on physical feedbacks in the Antarctic ice sheet (e.g., DeConto and Pollard 2016, Kopp et al. 201780,81) suggest that global average sea level rise exceeding 8 feet (2.5 m) by 2100 is physically plausible, although its probability cannot currently be assessed (see Sweet et al. 2017, Kopp et al. 201757,25).
Regardless of future scenario, it is extremely likely that global average sea level will continue to rise beyond 2100.82 Paleo sea level records suggest that 1.8°F (1°C) of warming may already represent a long-term commitment to more than 20 feet (6 meters) of global average sea level rise;83,84 a 3.6°F (2°C) warming represents a 10,000-year commitment to about 80 feet (25 m), and 21st-century emissions consistent with the higher scenario (RCP8.5) represent a 10,000-year commitment to about 125 feet (38 m) of global average sea level rise.30 Under 3.6°F (2°C), about one-third of the Antarctic ice sheet and three-fifths of the Greenland ice sheet would ultimately be lost, while under the RCP8.5 scenario, a complete loss of the Greenland ice sheet is projected over about 6,000 years.30
Over the contiguous United States, annual average temperature has increased by 1.2°F (0.7°C) for the period 1986–2016 relative to 1901–1960, and by 1.8°F (1.0°C) when calculated using a linear trend for the entire period of record.85 Surface and satellite data both show accelerated warming from 1979 to 2016, and paleoclimate records of temperatures over the United States show that recent decades are the warmest in at least the past 1,500 years.86
At the regional scale, each National Climate Assessment (NCA) region experienced an overall warming between 1901–1960 and 1986–2016 (Figure 2.4). The largest changes were in the western half of the United States, where average temperature increased by more than 1.5°F (0.8°C) in Alaska, the Northwest, the Southwest, and also in the Northern Great Plains. Over the entire period of record, the Southeast has had the least warming due to a combination of natural variations and human influences;87 since the early 1960s, however, the Southeast has been warming at an accelerated rate.88,89
Over the past two decades, the number of high temperature records recorded in the United States far exceeds the number of low temperature records. The length of the frost-free season, from the last freeze in spring to the first freeze of autumn, has increased for all regions since the early 1900s.85,90 The frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has increased since the mid-1960s. Over timescales shorter than a decade, the 1930s Dust Bowl remains the peak period for extreme heat in the United States for a variety of reasons, including exceptionally dry springs coupled with poor land management practices during that era.85,91,92,93
Over the next few decades, annual average temperature over the contiguous United States is projected to increase by about 2.2°F (1.2°C) relative to 1986–2015, regardless of future scenario. As a result, recent record-setting hot years are projected to become common in the near future for the United States. Much larger increases are projected by late century: 2.3°–6.7°F (1.3°–3.7°C) under a lower scenario (RCP4.5) and 5.4°–11.0°F (3.0°–6.1°C) under a higher scenario (RCP8.5) relative to 1986–2015 (Figure 2.4).85
Extreme high temperatures are projected to increase even more than average temperatures. Cold waves are projected to become less intense and heat waves more intense. The number of days below freezing is projected to decline, while the number of days above 90°F is projected to rise.85
Annual average precipitation has increased by 4% since 1901 across the entire United States, with strong regional differences: increases over the Northeast, Midwest, and Great Plains and decreases over parts of the Southwest and Southeast (Figure 2.5),94 consistent with the human-induced expansion of the tropics.95 In the future, the greatest precipitation changes are projected to occur in winter and spring, with similar geographic patterns to observed changes: increases across the Northern Great Plains, the Midwest, and the Northeast and decreases in the Southwest (Figure 2.5, bottom). For 2070–2099 relative to 1986–2015, precipitation increases of up to 20% are projected in winter and spring for the north central United States and more than 30% in Alaska, while precipitation is projected to decrease by 20% or more in the Southwest in spring. In summer, a slight decrease is projected across the Great Plains, with little to no net change in fall.
The frequency and intensity of heavy precipitation events across the United States have increased more than average precipitation (Figure 2.6, top) and are expected to continue to increase over the coming century, with stronger trends under a higher as compared to a lower scenario (Figure 2.6).94 Observed trends and model projections of increases in heavy precipitation are supported by well-established physical relationships between temperature and humidity (see Easterling et al. 2017,94 Section 7.1.3 for more information). These trends are consistent with what would be expected in a warmer world, as increased evaporation rates lead to higher levels of water vapor in the atmosphere, which in turn lead to more frequent and intense precipitation extremes.
For heavy precipitation events above the 99th percentile of daily values, observed changes for the Northeast and Midwest average 38% and 42%, respectively, when measured from 1901, and 55% and 42%, respectively, when measured with the more robust network available from 1958. The largest observed increases have occurred and are projected to continue to occur in the Northeast and Midwest, where additional increases exceeding 40% are projected for these regions by 2070–2099 relative to 1986–2015. These increases are linked to observed and projected increases in the frequency of organized clusters of thunderstorms and the amount of precipitation associated with them.96,97,98
Trends in related types of extreme events, such as floods, are more difficult to discern (e.g., Hirsch and Ryberg 2012, Hodgkins et al. 201799,100). Although extreme precipitation is one of the controlling factors in flood statistics, a variety of other compounding factors, including local land use, land-cover changes, and water management also play important roles. Human-induced warming has not been formally identified as a factor in increased riverine flooding and the timing of any emergence of a future detectable human-caused change is unclear.101
Declines have been observed in North America spring snow cover extent and maximum snow depth, as well as snow water equivalent (a measurement of the amount of water stored in snowpack) in the western United States and extreme snowfall years in the southern and western United States.102,103,104 All are consistent with observed warming, and of these trends, human-induced warming has been formally identified as a factor in earlier spring melt and reduced snow water equivalent.101 Projections show large declines in snowpack in the western United States and shifts to more precipitation falling as rain rather than snow in many parts of the central and eastern United States. Under higher future scenarios, assuming no change to current water resources management, snow-dominated watersheds in the western United States are more likely to experience lengthy and chronic hydrological drought conditions by the end of this century.105,106,107
Across much of the United States, surface soil moisture is projected to decrease as the climate warms, driven largely by increased evaporation rates due to warmer temperatures. This means that, all else being equal, future droughts in most regions will likely be stronger and potentially last longer. These trends are likely to be strongest in the Southwest and Southern Great Plains, where precipitation is projected to decrease in most seasons (Figure 2.5, right) and droughts may become more frequent.101,108,109,110,111,112 Although recent droughts and associated heat waves have reached record intensity in some regions of the United States, the Dust Bowl of the 1930s remains the benchmark drought and extreme heat event in the historical record, and though by some measures drought has decreased over much of the continental United States in association with long-term increases in precipitation (e.g., see McCabe et al. 2017113), there is as yet no detectable change in long-term U.S. drought statistics. Further discussion of historical drought is provided in Wehner et al. (2017).101
Few analyses consider the relationship across time and space between extreme events; yet it is important to note that the physical and socioeconomic impacts of compound extreme events can be greater than the sum of the parts.25,114 Compound extremes can include simultaneous heat and drought such as during the 2011–2017 California drought, when 2014, 2015, and 2016 were also the warmest years on record for the state; conditions conducive to the very large wildfires, that have already increased in frequency across the western United States and Alaska since the 1980s;115 or flooding associated with heavy rain over snow or waterlogged ground, which are also projected to increase in the northern contiguous United States.116
The Arctic is particularly vulnerable to rising temperatures, since so much of it is covered in ice and snow that begin to melt as temperatures cross the freezing point. The more the Arctic warms, the more snow and ice melts, exposing the darker land and ocean underneath. This darker surface absorbs more of the sun’s energy than the reflective ice and snow, amplifying the original warming in a self-reinforcing cycle, or positive feedback.
Some of the most rapid observed changes are occurring in Alaska and across the Arctic. Over the last 50 years, for example, annual average air temperatures across Alaska and the Arctic have increased more than twice as fast as the global average temperature.117,118,119,120,121,122 As surface temperatures increase, permafrost—previously permanently frozen ground—is thawing and becoming more discontinuous.123 This triggers another self-reinforcing cycle, the permafrost–carbon feedback, where carbon previously stored in solid form is released from the ground as carbon dioxide and methane (a greenhouse gas 35 times more powerful than CO2, on a mass basis, over a 100-year time horizon), resulting in additional warming.25,122 The overall magnitude of the permafrost–carbon feedback is uncertain, but it is very likely that it is already amplifying carbon emissions and human-induced warming and will continue to do so.124,125,126 Permafrost emissions imply an even greater decrease in emissions from human activities would be required to hold global temperature below a given amount of warming, such as the levels discussed in Box 2.4.
Most arctic glaciers are losing ice rapidly, and in some cases, the rate of loss is accelerating.127,128,129,130 This contributes to sea level rise and changes in local salinity that can in turn affect local ocean circulation. In Alaska, annual average glacier ice mass for each year since 1984 has been less than the year before, and glacial ice mass is declining in both the northern and southern regions around the Gulf of Alaska.131 Dramatic changes have occurred across the Greenland ice sheet as well, particularly at its edges. From 2002 to 2016, ice mass was lost at an average rate of 270 billion tons per year on average, or about 0.1% per decade, a rate that has increased in recent years.131 The effects of warmer air and ocean temperatures on the melting ice sheet can be amplified by other factors, including dynamical feedbacks (faster sliding, greater calving, and increased melting for the part of the ice that is underwater), near-surface ocean warming, and regional ocean and atmospheric circulation changes.132,133,134,135
Finally, much of the Arctic region is ocean that is covered by sea ice, and like land ice, sea ice is also melting (Figure 2.7).122 Since the early 1980s, annual average arctic sea ice extent has decreased by 3.5%–4.1% per decade.127,136 The annual minimum sea ice extent, which occurs in September of each year, has decreased at an even greater rate of 11%–16% per decade.137 Remaining ice is also, on average, becoming thinner (Figure 2.7), as less ice survives to subsequent years, and average ice age declines.137 The sea ice melt season—defined as the number of days between spring melt onset and fall freeze-up—has lengthened across the Arctic by at least five days per decade since 1979.
Melting sea ice does not contribute to sea level rise, but it does have other climate effects. First, sea ice loss contributes to a positive feedback, or self-reinforcing cycle, through changing the albedo or reflectivity of the Arctic’s surface. As sea ice, which is relatively reflective, is replaced by darker ocean, more solar radiation is absorbed by the ocean surface. This contributes to a greater rise in Arctic air temperature compared to the global average and affects formation of ice the next winter. Ice loss also acts to freshen the Arctic Ocean, affecting the temperature of the ocean surface layer and how surface heat is distributed through the ocean mixed layer. This also affects ice formation in subsequent seasons, as well as regional wind patterns, clouds, and ocean temperatures. And finally, sea ice loss also impacts key marine ecosystems and species that depend on the ice, from the polar bear to the ring seal,138,139,140 and the Alaska coastline becomes more vulnerable to erosion when it is not shielded from storms and waves by sea ice.141
It is virtually certain that human activities have contributed to arctic surface temperature warming, sea ice loss, and glacier mass loss.122,142,143,144,145,146,147,148 Observed trends in temperature and arctic-wide land and sea ice loss are expected to continue through the 21st century. It is very likely that by mid-century the Arctic Ocean will be almost entirely free of sea ice by late summer for the first time in about 2 million years.26,149As climate models have tended to under-predict recent sea ice loss,143 it is possible this will happen before mid-century.
Changes that occur in one part or region of the climate system can affect others. One of the key ways this is happening is through changes in atmospheric circulation patterns. While the Arctic may seem remote to many, for example, disruptions to the natural cycles of arctic sea ice, land ice, surface temperature, snow cover, and permafrost affect the amount of warming, sea level change, carbon cycle impacts, and potentially even weather patterns in the lower 48 states. Recent studies have linked record warm temperatures in the Arctic to changes in atmospheric circulation patterns in the midlatitudes.122,150
Observed changes in other aspects of atmospheric circulation include the northward shift in winter storm tracks since detailed observations began in the 1950s and an associated poleward shift of the subtropical dry zones.151,152,153 In the future, some studies show increases in the frequency of the most intense winter storms over the northeastern United States (e.g., Colle et al. 2013154). Regarding the influence of arctic warming on midlatitude weather, two studies suggest that arctic warming could be linked to the frequency and intensity of severe winter storms in the United States;155,156 another study shows an influence of arctic warming on summer heat waves and large storms.157 Other studies show mixed results (e.g., Barnes and Polvani 2015, Perlwitz et al. 2015, Screen et al. 2015158,159,160), however, and the nature and magnitude of the influence of arctic warming on U.S. weather over the coming decades remain open questions.
There is no question, however, that the effects of human-induced warming have the potential to affect weather patterns around the world. Changes in the subtropics can also impact the rest of the globe, including the United States. There is growing evidence that the tropics have expanded poleward by about 70 to 200 miles in each hemisphere since satellite measurements began in 1979, with an accompanying shift of the subtropical dry zones, midlatitude jets, and both midlatitude and tropical cyclone tracks.153,161,162 Human activities have played a role in the change, and although it is not yet possible to separate the magnitude of the human contribution relative to natural variability,15 these trends are expected to continue over the coming century.
Landfalling “atmospheric rivers” are narrow streams of moisture that account for 30%–40% of precipitation and snowpack along the western coast of the United States. They are associated with severe flooding events in California and other western states. As the world warms, the frequency and severity of these events are likely to increase due to increasing evaporation and higher atmospheric water vapor levels in the atmosphere.101,163,164,165
Human-caused emissions of greenhouse gases and air pollutants have also affected observed ocean–atmosphere variability in the Atlantic Ocean, and these changes have contributed to the observed increasing trend in North Atlantic tropical cyclone activity since the 1970s166 (see also review by Sobel et al. 2016167). In a warmer world, there will be a greater potential for stronger tropical cyclones (also known as hurricanes and typhoons, depending on the region) in all ocean basins.15,166,168,169,170,171 Climate model simulations indicate an increase in global tropical cyclone intensity in a warmer world, as well as an increase in the number of very intense tropical cyclones, consistent with current scientific understanding of the physics of the climate system.15,166,168,169,170,172 In the future, the total number of tropical storms is generally projected to remain steady, or even decrease, but the most intense storms are generally projected to become more frequent, and the amount of rainfall associated with a given storm is also projected to increase.170 This in turn increases the risk of freshwater flooding along the coasts and secondary effects such as landslides. Though scientific confidence in changes in the projected frequency of very strong storms is low to medium, depending on ocean basin, it is important to note that these storms are responsible for the vast majority of damage and mortality associated with tropical storms.
Extreme events such as tornadoes and severe thunderstorms occur over much shorter time periods and smaller areas than other extreme phenomena such as heat waves, droughts, and even tropical cyclones. This makes it difficult to detect trends and develop future projections172,173 (see Box 2.6). Compared to damages from other types of extreme weather, those occurring due to thunderstorm-related weather hazards have increased the most since 1980,174 and there is some indication that, in a warmer world, the number of days with conditions conducive to severe thunderstorm activity is likely to increase.175,176,177
Observed trends and projections of future changes in severe thunderstorms, tornadoes, hail, and strong wind events are uncertain.
Observed and projected future increases in certain types of extreme weather, such as heavy rainfall and extreme heat, can be directly linked to a warmer world. Other types of extreme weather, such as tornadoes, hail, and thunderstorms, are also exhibiting changes that may be related to climate change, but scientific understanding is not yet detailed enough to confidently project the direction and magnitude of future change.172
For example, tornado activity in the United States has become more variable, particularly over the 2000s (e.g., Tippett 2014, Elsner et al. 2015239,240), with a decrease in the number of days per year with tornadoes and an increase in the number of tornadoes on these days.241 Although the United States has experienced several significant thunderstorm wind events (sometimes referred to as “derechos”) in recent years, there are not enough observations to determine whether there are any long-term trends in their frequency or intensity.242
Modeling studies consistently suggest that the frequency and intensity of severe thunderstorms in the United States could increase as climate changes,177,243,244,245 particularly over the U.S. Midwest and Southern Great Plains during spring.177 There is some indication that the atmosphere will become more conducive to severe thunderstorm formation and increased intensity, but confidence in the model projections is low. Similarly, there is only low confidence in observations that storms have already become stronger or more frequent. Much of the lack of confidence comes from the difficulty in both monitoring and modeling small-scale and short-lived phenomena.
Along U.S. coastlines, how much and how fast sea level rises will not just depend on global trends; it will also be affected by changes in ocean circulation, land elevation, and the rotation and the gravitational field of Earth, which are affected by how much land ice melts, and where.
The primary concern related to ocean circulation is the potential slowing of the Atlantic Ocean Meridional Overturning Circulation (AMOC). An AMOC slowdown would affect poleward heat transport, regional climate, sea level rise along the East Coast of the United States, and the overall response of the Earth’s climate system to human-induced change.34,178,179,180,181
The AMOC moves warm, salty water from lower latitudes poleward along the surface to the northern Atlantic. This aspect of the AMOC is also known as the Gulf Stream. In the northern Atlantic, the water cools, sinks, and returns southward as deep waters. AMOC strength is controlled by the rate of sinking within the North Atlantic, which is in turn affected by the rate of heat loss from the ocean to the atmosphere. As the atmosphere warms, surface waters entering the North Atlantic may release less heat and become diluted by increased freshwater melt from Greenland and Northern Hemisphere glaciers. Both of these factors would slow the rate of sinking and weaken the entire AMOC.
Though observational data have been insufficient to determine if a long-term slowdown in the AMOC began during the 20th century,31,182 one recent study quantifies a 15% weakening since the mid-20th century183 and another, a weakening over the last 150 years.184 Over the next few decades, however, it is very likely that the AMOC will weaken. Under the lower RCP4.5 scenario, climate model simulations suggest the AMOC might ultimately stabilize, though bias-corrected simulations continue to show a long-term risk.180 Under the higher RCP8.5 scenario, projections suggest the AMOC would continue to weaken throughout the century, increasing the probability of an AMOC shutdown (see Box 2.4).26,180,185
For almost all future global average sea level rise scenarios of the Interagency Sea Level Rise Taskforce,76 relative sea level rise is projected to be greater than the global average along the coastlines of the U.S. Northeast and the western Gulf of Mexico due to the effects of ocean circulation changes and sinking land. In addition, with the exception of Alaska, almost all U.S. coastlines are projected to experience higher-than-average sea level rise in response to Antarctic ice loss. Higher global average sea level rise scenarios imply higher levels of Antarctic ice loss; under higher scenarios, then, it is likely that sea level rise along all U.S. coastlines, except Alaska, would be greater than the global average. Along portions of the Alaska coast, especially its southern coastline, relative sea levels are dropping as land uplifts in response to glacial isostatic adjustment (the ongoing movement of land that was once burdened by ice-age glaciers) and retreat of the Alaska glaciers over the last several decades. Future rise amounts are projected to be less than along other U.S. coastlines due to continued uplift and other effects stemming from past and future glacier shrinkage.
Due to sea level rise, daily tidal flooding events capable of causing minor damage to infrastructure have already become 5 to 10 times more frequent since the 1960s in several U.S. coastal cities, and flooding rates are accelerating in over 25 Atlantic and Gulf Coast cities.186,187,188 For much of the U.S. Atlantic coastline, a local sea level rise of 1.0 to 2.3 feet (0.3 to 0.7 m) would be sufficient to turn nuisance high tide events into major destructive floods.189 Coastal risks may be further exacerbated as sea level rise increases the frequency and extent of extreme coastal flooding and erosion associated with U.S. coastal storms, such as hurricanes and nor’easters. For instance, the projected increase in the intensity of hurricanes in the North Atlantic could increase the probability of extreme flooding along most U.S. Atlantic and Gulf Coast states beyond what would be projected based on relative sea level rise alone—although it is important to note that this risk could be either offset or amplified by other factors, such as changes in storm frequency or tracks (e.g., Knutson et al. 2013, 2015170,190).
Humanity’s effect on Earth’s climate system since the start of the industrial era, through the large-scale combustion of fossil fuels, widespread deforestation, and other activities, is unprecedented. Atmospheric carbon dioxide concentrations are now higher than at any time in the last 3 million years,191 when both global average temperature and sea level were significantly higher than today.24 One possible analog for the rapid pace of change occurring today is the relatively abrupt warming of 9°–14°F (5°–8°C) that occurred during the Paleocene-Eocene Thermal Maximum (PETM), approximately 55–56 million years ago.192,193,194,195 Although there were significant differences in both background conditions and factors affecting climate during the PETM, it is estimated that the rate of maximum sustained carbon release was less than 1.1 gigatons of carbon (GtC) per year (about a tenth of present-day emissions rates). Present-day emissions of nearly 10 GtC per year suggest that there is no analog for this century any time in at least the last 50 million years. Moreover, continued growth in carbon emissions over this century and beyond would lead to atmospheric CO2 concentrations not experienced in tens to hundreds of millions of years55,195 (see Hayhoe et al. 201724 for further discussion of paleoclimate analogs for present and near-future conditions).
Most of the climate projections used in this assessment are based on simulations by global climate models (GCMs). These comprehensive, state-of-the-art mathematical and computer frameworks use fundamental physics, chemistry, and biology to represent many important aspects of Earth’s climate and the processes that occur within and between them (see Box 2.7).24 However, there are still elements of the earth system that GCMs do not capture well.196 Self-reinforcing cycles or feedbacks within the climate system have the potential to amplify and accelerate human-induced climate change. As discussed in Kopp et al. (2017),25 they may even shift Earth’s climate system, in part or in whole, into new states that are very different from those experienced in the recent past. Tipping elements are subcomponents of the earth system that can be stable in multiple different states and can be “tipped” between these states by small changes in forcing, amplified by self-reinforcing cycles. Tipping point events may occur when such a threshold is crossed in the climate system (e.g., Lenton et al. 2008, Kopp et al. 2016197,198). Some of the self-reinforcing cycles that lead to potential state shifts, such as an ice-free Arctic, can be modeled and quantified; others can be identified but have not yet been quantified, such as changes to cloudiness driven by changes in large-scale patterns of atmospheric circulation;199 and some are probably still unknown.25
Projections of future changes are based on simulations from global climate models, downscaled to higher resolutions more relevant to local- to regional-scale impacts.
The projections of future change used in this assessment come from global climate models (GCMs) that reproduce key processes in Earth’s climate system using fundamental scientific principles. GCMs were previously referred to as “general circulation models” when they included only the physics needed to simulate the general circulation of the atmosphere. Today, global climate models simulate many more aspects of the climate system: atmospheric chemistry and particles, soil moisture and vegetation, land and sea ice cover, and increasingly, an interactive carbon cycle and/or biogeochemistry. Models that include this last component are also referred to as Earth System Models (ESMs), and climate models are constantly being expanded to include more of the physics, chemistry, and increasingly, the biology and biogeochemistry at work in the climate system (Figure 2.10; see also Hayhoe et al. 2017,24 Section 4.3).
The ability to accurately reproduce key aspects of Earth’s climate varies across climate models. In addition, many models share model components or code, so their simulations do not represent entirely independent projections. The Coupled Model Intercomparison Project, Phase 5 (CMIP5) provides a publicly available dataset of simulations from nearly all the world’s climate models. As discussed in CSSR,246 most NCA4 projections use a weighted multimodel average of the CMIP5 models based on a combination of model skill and model independence to provide multimodel ensemble projections of future temperature, precipitation, and other climate variables.
The resolution of global models has increased significantly over time. Even the latest experimental high-resolution simulations, however, are unable to simulate all of the important fine-scale processes occurring at regional to local scales. Instead, a range of methods, generally referred to as “downscaling,” are typically used to correct systematic biases in global projections and generate the higher-resolution information required for some impact assessments.24
There are two main types of downscaling: 1) dynamical downscaling, which uses regional climate models (RCMs) to calculate the response of regional climate processes to global change over a limited area and 2) empirical statistical downscaling models (ESDMs), which develop statistical relationships between real-world observations and historical global model output, then use these relationships to downscale future projections. Although dynamical and statistical methods can be combined into a hybrid framework, many assessments still tend to rely on one or the other type of downscaling, where the choice is based on the needs of the assessment. Many of the projections shown in this report, for example, are either based on the original GCM simulations, or on the latest CMIP5 simulations that have been statistically downscaled using the LOcalized Constructed Analogs (LOCA) ESDM.247 It is important to note that while ESDMs effectively remove bias and increase spatial resolution, and while RCMs add additional physical insight at smaller spatial scales by resolving processes such as convection (e.g., Prein et. al 2015248), they do not include all the processes relevant to climate at local scales. For further discussion, see Hayhoe et al. (2017),24 Section 4.3.
While climate models incorporate important climate processes that can be well quantified, they do not include all of the processes that can contribute to feedbacks, compound extreme events, and abrupt and/or irreversible changes, including key ice sheet processes and arctic carbon reservoirs.25,185,200 The systematic tendency of climate models to underestimate temperature change during warm paleoclimates201 suggests that climate models are more likely to underestimate than to overestimate the amount of long-term future change; this is likely to be especially true for trends in extreme events. For this reason, there is significant potential for humankind’s planetary experiment to result in surprises—and the further and faster Earth’s climate system is changed, the greater the risk of unanticipated changes and impacts, some of which are potentially large and irreversible.
This chapter is based on the collective effort of 32 authors, 3 review editors, and 18 contributing authors comprising the writing team for the Climate Science Special Report (CSSR),208 a featured U.S. Global Change Research Project (USGCRP) deliverable and Volume I of the Fourth National Climate Assessment (NCA4). An open call for technical contributors took place in March 2016, and a federal science steering committee appointed the CSSR team. CSSR underwent three rounds of technical federal review, external peer review by the National Academies of Sciences, Engineering, and Medicine, and a review that was open to public comment. Three in-person Lead Authors Meetings were conducted at various stages of the development cycle to evaluate comments received, assign drafting responsibilities, and ensure cross-chapter coordination and consistency in capturing the state of climate science in the United States. In October 2016, an 11-member core writing team was tasked with capturing the most important CSSR key findings and generating an Executive Summary. The final draft of this summary and the underlying chapters was compiled in June 2017.
The NCA4 Chapter 2 author team was pulled exclusively from CSSR experts tasked with leading chapters and/or serving on the Executive Summary core writing team, thus representing a comprehensive cross-section of climate science disciplines and supplying the breadth necessary to synthesize CSSR content. NCA4 Chapter 2 authors are leading experts in climate science trends and projections, detection and attribution, temperature and precipitation change, severe weather and extreme events, sea level rise and ocean processes, mitigation, and risk analysis. The chapter was developed through technical discussions first promulgated by the literature assessments, prior efforts of USGCRP,208 e-mail exchanges, and phone consultations conducted to craft this chapter and subsequent deliberations via phone and e-mail exchanges to hone content for the current application. The team placed particular emphasis on the state of science, what was covered in USGCRP,208 and what is new since the release of the Third NCA in 2014.1