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
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.
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