Why Heat Risks Are Increasing

A changing environment means higher temperatures and higher humidity, compounding the effects of extreme heat events.

Global temperatures are rising, resulting in extreme heat events around the world. In the United States, this has resulted in hotter daily high temperatures in the summertime, longer summers, and deadly heat waves in areas that are not acclimated to heat risks.

Temperatures are increasing.

Over the last 30 years, the average temperature in the contiguous US has increased by 1.2°F compared to the first half of the twentieth century, which has driven an increase in both the frequency and intensity of hot days. Heat waves and heat advisories are more frequent as a result, and this warming trend is expected to increase over the next 30 years. Conservative estimates project that temperatures will continue to rise by a minimum of 2.5°F over the next 30 years compared to current averages, with the biggest increases occurring in historically cool areas such as the mountain west and northern plains regions.


    Change in daily high temperature (ºF) this year to 30 years

    As average temperatures increase around the globe, hot days increase in both frequency and intensity.

    Change in temperature (°F) in 30 years

    Change in temperature (°F) in 30 years
    1°F
    2°F
    3°F
    4°F
    Climatology Lab MACAv2 downscaled GCM, based on historical period 1950-2005 and adjusted to future conditions using the RCP4.5 emissions scenario.

    The impact of increasing temperatures

    While the expectation may be that increasing average temperatures will result in proportionately hotter days, the effects of heat are less evenly distributed nationwide. Areas that once experienced so few hot days that heat could be considered a passing nuisance will experience prolonged periods of uncomfortable and even harmful heat, such as the Pacific Northwest Heat Wave in 2021 that killed over 500 people in Washington, Oregon, Idaho and Canada.

    Areas that are more acclimated to heat risks will experience longer, more intense hot periods and as a result will see dramatic increases in energy consumption. This will not only increase the costs to homeowners, but without proper planning, would also put power grids at risk of failures that would disrupt economic activity and put a wide range of individuals at risk of heat stroke or worse.

    Changing humidity patterns are worsening the effects of heat.

    Another consequence of increasing temperatures is increased air humidity. Hot air has a greater capacity for holding moisture, which creates a vapor-pressure deficit between surface air and water stored in the ground and in plant-life. This causes a greater amount of moisture to be released into the air, and because moist, humid air has a greater capacity for holding heat, this results in a cycle of building heat that prolongs the duration and intensity of heat events.


      Changing humidity
      An increase in temperature also leads to an increase in the rate of water evaporation, causing changes to atmospheric humidity that affect the way heat impacts communities.
      Change in relative humidity in 30 years
      Change in relative humidity in 30 years
      -2%
      -1%
      0%
      1%
      2%
      Climatology Lab MACAv2 downscaled GCM, based on historical period 1950-2005 and adjusted to future conditions using the RCP4.5 emissions scenario.

      The impact of changing humidity

      Humidity increases will be felt most acutely in places that already have moist climates such as the Mid-Atlantic and Great Lakes regions. Not only will the result be longer, more intense heat events, but this will also result in hotter nighttime periods, increasing the need for temperature control and therefore energy consumption, putting power grids and people at risk as temperatures rise.

      Additional factors increasing risk

      As the environment changes in our communities, human activity will adapt in kind by using more energy to control indoor temperatures, creating another cycle of increasing heat. Most methods of generating electric energy for the power grid create heat as a by-product, so as our energy consumption to combat the effects of heat increases, so too will heat itself.

      Development patterns will also worsen the effects of heat, as abundant community greenspace and wild areas absorb and act as a control against heat. Both the expansion of suburbs and increasing density within cities will likely increase the effects of heat as a result.

      How does Heat Factor® incorporate changing heat risks?

      The data on Risk Factor™ considers how changing temperature and humidity patterns will increase heat indexes and affect the likelihood of risky heat events.

      What can be done to mitigate heat?

      Although heat risk can never be eliminated, there are a wide range of protection measures that reduce risk.

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