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
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.
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
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.