David R. Easterling, NOAA National Centers for Environmental Information
David W. Fahey, NOAA Earth System Research Laboratory
Katharine Hayhoe, Texas Tech University
Sarah Doherty, University of Washington
James P. Kossin, NOAA National Centers for Environmental Information
William V. Sweet, NOAA National Ocean Service
Russell S. Vose, NOAA National Centers for Environmental Information
Michael F. Wehner, Lawrence Berkeley National Laboratory
Donald J. Wuebbles, University of Illinois
Linda O. Mearns, National Center for Atmospheric Research
Robert E. Kopp, Rutgers University
Kenneth E. Kunkel, North Carolina State University
John Nielsen-Gammon, Texas A&M University
David J. Dokken, Senior Program Officer
David Reidmiller, Director
<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
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.
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
Figure 2.1: Human and Natural Influences on Global Temperature
Figure 2.1: Both human and natural factors influence Earth’s climate, but the long-term global warming trend observed over the past century can only be explained by the effect that human activities have had on the climate.
Sophisticated computer models of Earth’s climate system allow scientists to explore the effects of both natural and human factors. In this figure, the black line shows the observed annual average global surface temperature for 1880–2017 as a difference from the average value for 1880–1910. The other lines show the contributions from individual natural and human factors, all natural factors, all human factors, and the combined effects of both natural and human drivers. Details on these factors are provided below (panel references and colors refer to the static version of the figure, available via the “View static image” link above):
The top panel (a) shows the temperature changes simulated by a climate model when only natural factors (yellow line) are considered. The other lines show the individual contributions to the overall effect from observed changes in Earth’s orbit (brown line), the amount of incoming energy from the sun (purple line), and changes in emissions from volcanic eruptions (green line). Note that no long-term trend in globally averaged surface temperature over this time period would be expected from natural factors alone.10
The middle panel (b) shows the simulated changes in global temperature when considering only human influences (dark red line), including the contributions from emissions of greenhouse gases (purple line) and small particles (referred to as aerosols, brown line) as well as changes in ozone levels (orange line) and changes in land cover, including deforestation (green line). Changes in aerosols and land cover have had a net cooling effect in recent decades, while changes in near-surface ozone levels have had a small warming effect.18 These smaller effects are dominated by the large warming influence of greenhouse gases such as carbon dioxide and methane. Note that the net effect of human factors (dark red line) explains most of the long-term warming trend.
The bottom panel (c) shows the temperature change (orange line) simulated by a climate model when both human and natural influences are included. The result matches the observed temperature record closely, particularly since 1950, making the dominant role of human drivers plainly visible.
Researchers do not expect climate models to exactly reproduce the specific timing of actual weather events or short-term climate variations, but they do expect the models to capture how the whole climate system behaves over long periods of time. The simulated temperature lines represent the average values from a large number of simulation runs. The orange hatching represents uncertainty bands based on those simulations. For any given year, 95% of the simulations will lie inside the orange bands. Source: NASA GISS.