Killer Heat in the United States

The Union of Concerned Scientists (UCS) has evaluated how climate change will contribute to increasing incidence of dangerous high heat days across the U.S. This includes an analysis of the growing number of high heat days across various regions of the country, described under three climate change scenarios. The report also details the public health consequences of extreme heat and the populations that are particularly vulnerable to these threats. Policy recommendations are offered with adaptation measures that can be implemented at all levels of government to address rising temperatures.

This resource was featured in the July 19, 2019, ASAP Newsletter.

This new UCS report "forecasts the number of days per year across the United States when the heat index or “feels like” temperature will exceed 90°F, 100°F, 105°F, and “off-the-charts” heat thresholds based on different global warming emission scenarios. The report finds that without action to reduce emissions, the number of days when the heat index exceeds 105 degrees Fahrenheit will more than quadruple to 24 by mid-century and increase eight-fold to 40 by late century. Contact Alicia Race, Outreach Coordinator, Union of Concerned Scientists and ASAP Member at for more information."

UCS also developed interactive maps to help users assess how climate change will affect the number of high-heat days in their regions. 

The report discusses how climate change will affect the number of high heat days in the lower 48 states in the U.S. (excluding Hawaii and Alaska). Using the heat index, this factors in ambient air temperatures and humidity to specify "feels like" temperatures. How climate change will increase the number of days where the heat index exceeds four thresholds is described for: 90 degrees, 100 degrees, 105 degrees, and "off the charts." The number of high heat days under three climate change scenarios is also detailed for: a "no action" scenario resulting in an average 7.7°F warming, a "slow action" scenario resulting in an average 4.3°F of warming, and a "rapid action" scenario resulting in an average of 3.6°F/2 °C warming. The report also includes a summary of the increased number of high heat days that different regions of the country will experience based upon each climate change scenario, and for a few time-scales (mid-century and late century). 

The report offers an analysis of the change in the number of people that will be exposed to dangerous high heat conditions as a result of climate change. For example, historically only 29 of 481 U.S. cities experienced 30 or more days when the heat index exceeded 100 degrees. With climate change, it is estimated that number will grow to 251 cities by mid-century. Regional profiles are included that describe how the number of high heat days will increase under different climate change scenarios across regions.

The report discusses the public health consequences of heat, including heat-related illnesses, and how heat affects vulnerable populations including children, the elderly, and people with medical conditions that make them more vulnerable to heat related illnesses (such as respiratory or cardiovascular disease) (Chapter 3).

The unique risks heat poses for different vulnerable populations are detailed - including outdoor workers; city dwellers (due to the urban heat island effect), rural residents (due to lack of access to health care, air conditioning, and the prevalence of outdoor labor); low-income communities; and people exposed to other extremes (such as flooding, wildfires, or power outages) (Chapter 5).


The report concludes with adaptation strategies that policymakers at all levels of government can deploy to enhance resilience to increasing heatwaves (Chapter 6). Strategies include:

  • increasing access to services that help people cope with heat (such as heat warning systems, cooling centers, energy cost assistance programs, and medical care);
  • increasing investment in "heat-smart infrastructure" (such as climate-resilient design of public housing and critical infrastructure to accommodate higher temperatures, passive cooling of buildings, street trees, and other investments);
  • investing in "climate-smart power systems" (such as microgrids and more efficient cooling systems); and
  • rapid reductions in the greenhouse gas emissions causing climate change.


Publication Date: July 2019

Authors or Affiliated Users:

  • Kristina Dahl
  • Erika Spanger-Siegfried
  • Rachel Licker
  • Astrid Caldas
  • John Abatzoglou
  • Nicholas Mailloux
  • Rachel Cleetus
  • Shana Udvardy
  • Juan Declet-Barreto
  • Pamela Worth

Related Organizations:


Resource Category:

Resource Types:

  • Assessment
  • Policy analysis/recommendations


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