Both temperature and humidity are increasing, making risky heat events more possible.
As the global temperature rises at a faster rate than ever recorded, it's important to know what factors contribute to heat risk. 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 used to heat risks.
Understanding what areas will face the largest increase in extreme heat events, allows individuals and communities to prepare for and mitigate the negative effects associated with these dangerously hot days through various heat solutions.
Temperatures are increasing
In the US the average temperature has increased by 1.2°F over the last 30 years, causing an increase in both the intensity and frequency of hot days and, as a result, the frequency of heat waves and heat advisories. This warming trend is expected to increase over the next 30 years. Conservative estimates project temperatures will rise by a minimum of 2.5°F over the next 30 years, with the biggest increases occurring in the mountain west and northern plains regions.
Extreme heat belt - Concentration of extreme danger days of temperatures above 125°F in 2023 vs. 2053 (at least 1 day > 125°F).
The impact of increasing temperatures
Increasing average temperatures will not result in proportionately hotter days across the U.S. How high average temperatures increase is influenced by local factors such as an area's landscape, vegetation, elevation, urbanization, and distance to water bodies and coastlines. These factors explain why some places will experience large changes in heat risk while others will experience more mild changes.
Areas such as the south may not consider 90°F dangerously hot because they are more acclimated to heat risks. However, 90°F is potentially very dangerous for areas in cooler states in the Northeast, Upper Midwest, and Northwest of the country. Areas that are more accustomed to high temperatures can expect 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. Areas that once experienced a few hot days will experience prolonged periods of uncomfortable and even harmful heat, where the risk of heat-related illnesses is expected to increase for vulnerable populations.
Local Hot Days are used to understand how rising temperatures will uniquely affect different areas of the country. The First Street Foundation Extreme Heat Model identifies the temperature on the hottest 7 days in a given area to define what someone in that specific area would reasonably call a “hot day.” The term “hot day” is used to represent local heat index temperatures. The RCP4.5 carbon emissions scenario is used to predict the number of consecutive hot days 30 years from now. This allows individuals to understand how changing environmental factors will uniquely affect the felt experience of heat in their area based on a property’s surroundings.
The number of days in a row the temperature will be above 100°F in 2023 compared to the number of consecutive days above 100°F in 2053 (~30 years from now).
Changing humidity patterns
Increasing temperatures will also cause air humidity to increase. Because warmer air can hold more water vapor than cooler air, warmer temperatures speed up evaporation causing a greater amount of moisture to be released into the air. Moist humid air has a greater capacity for holding heat. The increased moisture in the air and a greater capacity to hold heat result in a cycle of building heat that prolongs the duration and intensity of heat events. Increased average temperatures and humidity have a compounding effect on heat indexes, which makes risky health impacts more likely.
Changing humidity patterns worsen the effects of heat
Moist climates such as the Mid-Atlantic and Great Lakes regions will experience the greatest effects of increased humidity. Humidity increases will result in longer, more heat events, and even hotter nighttime periods. Hotter nighttime periods increase the need for temperature control and therefore energy consumption, putting power grids and people at risk as temperatures rise.
Additional factors increasing risk
The increase in extreme heat conditions is a cause for concern, impacting everything from personal health to electricity costs associated with increased air conditioning usage, to physical infrastructure, and even public transport. When there is more moisture in the air or higher humidity, the body’s primary coping mechanism is unable to work properly; this is why dry heat is typically thought of as more comfortable than humid conditions.
When temperatures reach these extremes, people will seek relief in air-conditioned areas, 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. Increased air conditioning use increases energy consumption, putting a strain on energy grids, which is likely to be exacerbated by future use as temperatures rise. As a result, rolling blackouts may become more common as extreme heat days increase in frequency, intensity, and duration over the next 30 years.