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Heat-Related Illness Increase

Exposure to extreme heat is already a significant public health problem and the primary cause of weather-related mortality in the U.S.[1] As temperatures continue to increase due to climate change, heat-related illness is expected to worsen.

The atmosphere can also hold more water, reducing our bodies’ ability to cool off and increasing our risk of heat-related illness. The Heat Index, also known as the "real feel" temperature, is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature.

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Climate science at a glance

  • Challenges to human and livestock health are growing due to the increased frequency and intensity of high temperature extremes that cause heat-related illness.[1]
  • Heat stress can exacerbate preexisting health conditions and lead to an increase in human mortality.[2][3]
  • As air warms, its capacity to hold water vapor increases, and measurements show that atmospheric humidity is increasing around the globe, consistent with a warming climate.[1]
  • The fingerprint of climate change has been found in the increase of wet bulb temperature since 1973, driving heat stress globally.[4]

Background information

What is the Heat Index?

The National Weather Service uses a measure called “maximum heat index“—which takes into account both air temperature and humidity to calculate how hot it truly feels outside—to warn people of extreme heat. The group typically issues a “heat advisory” when a maximum heat index is expected to hit at least 100°F for two or more days, and an “excessive heat warning” when it will hit at least 105°F for two or more days. At these levels, prolonged heat exposure can lead to health risks including dehydration, worsening of chronic conditions, and heat stroke, especially for children and the elderly.

What is relative versus specific humidity?

Specific humidity increases as the climate warms. It is an absolute measure of the amount of water in a given amount of air. Just as the absolute amount of water in a given amount of air increases as the climate warms, the amount of water the air can hold increases. Relative humidity looks at how much water is in the air relative to how much could be in the air. This ratio—between how much water is in the air versus how much the air could hold— is expected to remain constant as the climate warms.

What is "wet-bulb temperature"?

Instead of simply looking at the thermometer to see how hot it is, a wet-bulb temperature is taken by first wrapping the bulb of a thermometer in a wet cloth – hence, “wet bulb”. Wet-bulb temperature accounts for both heat and humidity. It reflects what that combination means for the human body’s ability to cool down.

“The idea here is that you and I are essentially wet bulbs. We cool ourselves by evaporation. If the wet-bulb temperature reading is higher than our body temperature, that means that we cannot cool ourselves to a temperature tolerable for humans by evaporating sweat, and that basically means you can’t survive,” said Tapio Schneider, a California Institute of Technology climate scientist and professor.

As the globe warms and water evaporates at higher rates than before, raising humidity levels, wet bulb temperatures will continue to rise.

U.S. humidity and heat-related illness trends and climate change

  • (NCA 4, 2018): Elevated temperature, combined with increased salinity and humidity, accelerates deterioration in bridges and roads constructed with concrete.[1]
  • (Feng et al. 2016): Absolute humidity is increasing significantly in the Midwest,[6] creating favorable conditions for pests and pathogens and degrading the quality of stored grain.[1][7]
  • (Brown and DeGaetano, 2013): In the Midwest, daily minimum temperatures have increased in all seasons due to increasing humidity.[7]
  • (Karl et al. 2008): Increasing humidity is contributing to higher nighttime temperatures in the U.S.[5]

U.S. studies attribute increased humidity and heat stress to climate change

  • (Lehner et al. 2018): Warming in the southwestern U.S. is largely due to greenhouse gas forcing.[8]
  • (Diffenbaugh et al. 2017): The study formally identifies the influence of anthropogenic warming in the observed trend of increasing heat extremes over the western U.S.[9]
  • (Shiogama et al. 2014): Anthropogenic warming made a severe heat wave that occurred in the southwestern United States during June and July 2013 more likely.[10]
  • (Diffenbaugh and Scherer, 2013): Extreme July temperatures like those observed in 2012 are more than four times as frequent over the north-central and northeastern United States due to anthropogenic climate change.[11]
  • (Morak et al. 2011): Human-caused climate change is at least partly responsible for a strong increase in the number of warm nights in western North America.[12]

Global humidity and heat stress trends and climate change

  • (Speizer et al. 2022): Humid heat extremes have intensified in most regions, with some areas having experienced greater increases than others.
  • (Raymond et al. 2020): The frequency of humid heat so high it comes close to overwhelming the human body’s ability to regulate its temperature has more than doubled in some coastal subtropical regions of the world since 1979.[13]
  • (Grumm and Balogh, 2016): The northern hemisphere has had increasingly warmer and more humid summers, and the global area covered by extreme water vapor is increasing significantly.[15]

Global studies attribute increased humidity and heat stress to climate change

  • (IPCC AR6, 2021): "Global total column water vapour content has very likely increased since the 1980s, and it is likely that human influence has contributed to tropical upper tropospheric moistening. Near-surface specific humidity has increased over the ocean (likely) and land (very likely) since at least the 1970s, with a detectable human influence (medium confidence)." (Technical Summary, p. 51)
  • (Wehner et al. 2018): The study looks at increases in the frequency and intensity of extremely hot three day periods due to human-caused climate change. The authors find that the shift in the distribution of surface air temperature since the mid 20th century has been profound and most regions have experienced increases in the frequency an intensity of extremely hot three day periods. Temperatures that were rare prior to the 1980's now occur with regularity.[16]
  • (Diffenbaugh et al. 2017): Anthropogenic global warming had a significant hand in the temperatures seen during the hottest month and on the hottest day on record throughout much of the world from 1931–2016. They find that the global trend in warming contributed to the record for hottest day of the year in at least 82 percent of the records over the 1961-2010 period.[9]
  • (Knutson and Ploshay 2016): There has been a detectable human-caused increase in mean summertime heat stress since 1973, both globally and in most land regions analyzed.[4]

Select a pillar to filter signals

Air Mass Temperature Increase
Arctic Amplification
Extreme Heat and Heat Waves
Glacier and Ice Sheet Melt
Global Warming
Greenhouse Gas Emissions
Land Ice and Snow Cover Decline
Land Surface Temperature Increase
Permafrost Thaw
Precipitation Falls as Rain Instead of Snow
Sea Ice Decline
Sea Surface Temperature Increase
Season Creep/ Phenology Change
Snowpack Decline
Snowpack Melting Earlier and/or Faster
Atmospheric Moisture Increase
Extreme Precipitation Increase
Runoff and Flood Risk Increase
Total Precipitation Increase
Atmospheric Blocking Increase
Atmospheric River Change
Extreme El Niño Frequency Increase
Gulf Stream System Weakening
Hadley Cell Expansion
Large Scale Global Circulation Change/ Dynamical Changes
North Atlantic Surface Temperature Decrease
Ocean Acidification Increase
Southwestern US Precipitation Decrease
Surface Ozone Change
Surface Wind Speed Change
Drought Risk Increase
Land Surface Drying Increase
Intense Atlantic Hurricane Frequency Increase
Intense Cyclone, Hurricane, Typhoon Frequency Increase
Intense Northwest Pacific Typhoon Frequency Increase
Tropical Cyclone Steering Change
Wildfire Risk Increase
Coastal Flooding Increase
Sea Level Rise
Air Mass Temperature Increase
Storm Surge Increase
Thermal Expansion of the Ocean
Winter Storm Risk Increase
Coral Bleaching Increase
Habitat Shift or Decline
Parasite, Bacteria and Virus Population Increase
Pine Beetle Outbreaks
Heat-Related Illness Increase
Infectious Gastrointestinal Disease Risk Increase
Respiratory Disease Risk Increase
Vector-Borne Disease Risk Increase
Storm Intensity Increase
Tornado Risk Increase
Wind Damage Risk Increase
What are Climate Signals?