Climate Change Threatens Health: Extreme Heat

Extreme Heat: More Intense Hot Days and Heat Waves

Across the nation, climate change is making hot summer days hotter and stretching their numbers into heat waves that never seem to end. And the heat is causing more than just discomfort - as temperatures rise, so are the number of illnesses, emergency room visits, and deaths.

At least 37 states saw record highs in the summer of 2010, and in many regions, it didn't cool off at night. Nationwide, over 28.5 million people lived in counties where 2010's average temperature set records, and over 36 million people lived in counties where the hottest summer nights ever were recorded.

The record heat experienced in the United States in the summer of 2010 was no isolated event. Global temperature data compiled by NASA show that 2010 was tied with 2005 as the hottest year on record. This comes on top of the warmest decade on record (2000-2009).

Extreme heat waves cause the most harm among elderly people and young children. City dwellers are at particular risk because of elevated temperatures in cities, known as the "urban heat island effect" due to the magnifying effect of paved surfaces and the lack of tree cover.

In the United States, an average of 400 deaths per year are directly related to heat, and an estimated 1,800 die from illnesses made worse by heat - including heat exhaustion, heat stroke, cardiovascular disease, and kidney disease. Deadly heat waves swept across most of the nation in 2006, hitting California the hardest; the state saw an additional 16,000 emergency room visits during the two-week heat wave.

Scientists predict that average temperatures in the United States will rise between 5 and 9°F (3-5°C) over the next century; these hot summer days and heat waves could be the norm by 2100.

Communities across the nation must educate themselves about the risks from extreme heat and learn how to protect their most vulnerable residents.

Twelve states and several local governments have developed preparedness measures to prevent the health impacts of increasing extreme heat associated with climate change. The most frequent recommendation is developing or strengthening statewide heat early warning systems, with an emphasis on alerting the most vulnerable populations.

How are states addressing the threat of extreme heat?

  • California's plan includes measures to develop heat warning systems, improve outreach systems, and identify and reduce vulnerabilities to extreme heat. The cities of Berkeley and Los Angeles have adopted a strategy that includes measures to protect and increase urban trees to help cool the city in the face of extreme heat threats. Find out more >>
  • Florida's plan only addresses the interaction of heat waves and existing air quality problems in major urban areas and does not describe other measures to protect public health from extreme heat. Find out more >>
  • Maine's plan includes measures to prevent the build-up of extreme heat in urban areas and to track the health impacts of extreme heat. Find out more >>
  • Maryland's plan includes measures to prevent the worsening impacts of extreme heat by evaluating currents threats and vulnerabilities, improving alert systems and increasing the response capacity of vulnerable populations. Find out more >>
  • Michigan's plan includes measures that identify vulnerable populations, improve warning and emergency response systems and conduct outreach and education about the dangers of heat waves. Find out more >>
  • New York's plan includes measures to strengthen the ability of local emergency services to respond to heat waves and temperature extremes. Find out more >>
  • New Hampshire's plan includes a measure to strengthen local emergency services' ability to respond to heat waves and temperature extremes. The city of Keene's strategy includes measures to create cooling shelters and emergency response policies for extreme weather events. Find out more >>
  • Oregon's plan includes measures to respond to heat waves, and improve delivery of information on cooling centers, especially for isolated and vulnerable populations. Find out more >>
  • Pennsylvania's plan includes a measure to develop a statewide early-warning system for heat waves modeled on the lifesaving Philadelphia program. Find out more >>
  • Washington's plan includes measures to address health threats from extreme heat by improving preparedness and emergency response systems. King County is also directing efforts to improve capacity to respond to extreme heat emergencies. Find out more >>
  • Wisconsin's plan includes measures to prevent the worsening impacts of extreme heat by improving heat response and early warning systems. Find out more >>
  • Virginia's plan includes a measure to ensure that every Health District in Virginia has a heat emergency response plan. Find out more >>

Map Methods

Background: The frequency, duration, and intensity of heat waves in the U.S. are projected to increase substantially by 2090 due to climate change.1 Extreme heat is a significant public health threat and has been linked to increases in premature mortality, hospitalizations and emergency room visits.

Extreme Heat Vulnerability Indicator: The percentage of the U.S. affected by heat waves has risen since the 1970s, distinguished by a rise in extremely high nighttime temperatures, as well as daily high temperatures well above normal.2 We applied the 90th percentile value of daily maximum summer temperatures as a measure of extreme heat, that is, daily temperatures exceeding those values were considered "extreme." Summer was calculated as June, July and August (JJA) temperatures at each meteorological station for which data were available, as is common practice in climate analyses. In addition, a 30-year period was used as a baseline, in this case a 1961-1990 reference period against which the most recent decade of data was compared, namely 2000-2009 summer daily temperatures.

Data Source: Data from all cooperative weather stations for all years historically from the National Climatic Data Center was collected. The data through 2008 was purchased from EarthInfo, a private vendor that collects NCDC data and makes it available on DVDs. 2009 data was downloaded manually from NCDC. Geographic detail for sites was also from NCDC. The NCDC defines cooperative stations as: "U.S. stations operated by local observers which generally report max/min temperatures and precipitation. National Weather Service (NWS) data are also included in this dataset. The data receive extensive automated + manual quality control."

Data Preparation: Data was assembled in an SQL data base. Each record represented a single site-month of data. The total was 24 million site-month records or a total of more than 720 million site-days of data.

Calculations: For each site we calculated the 90th percentile for the maximum temperature during the reference period of June, July and August of 1961-1990. All days June, July and August 2000-2009 at the same site were then compared against the reference period 90th percentile value. The total number of days that exceeded the 90th percentile reference value was computed. County-level averages were then computed by averaging site-level data within the county.

Sites were excluded from the analysis if A) they had less than 75% of possible reference days; B) one entire year from the 2000-2009 period was missing; or C) they had less than 75% of possible current period days.

Map: In a summer defined as June, July and August, there are a total of 92 days, so the "expected" number of days that would exceed the 90th percentile is 10% of 92, or 9.2 days. Rounding to the nearest whole number of days, locations with over 9 days on average per summer with temperatures above that station's 90th percentile reference value had "more than expected" number of days of extreme heat. The highest category (greater than 13.8 days) gives a sense of those locations with over approximately two weeks of these "more than expected" hot days in a recent decade, relative to temperatures in the 30 years from 1961-1990.

Note, too, that "extreme heat" is defined by local temperatures at each site; the map does not compare temperatures in one part of the U.S. to those in another region.


Related Links


  1. Karl TR, Melillo JM, Peterson TC, editors. Global climate change impacts in the United States. New York: Cambridge University Press; 2009.
  2. EPA "Climate Change Indicators in the United States" (2010), EPA 430-R-1—007. www.epa.gov/climatechange/indicators.html
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