The planet is getting warmer, and extreme heat events are becoming more common. Roads are melting, birds are falling from the sky, and cities are going up in flames. Around the world, human morbidity and mortality from extreme heat is rising. What is extreme heat? Generally, extreme heat is defined as temperatures that are much higher than the typical averages for that region at that time of year, persisting for 2 days or more. When we refer to 'much higher than average' temperatures, we mean temperatures that exceed the typical upper limit for that region. In statistical terms, this refers to temperatures that are two-and-a-half or three standard deviations above the mean for that location and time of year. Regarding recurrence, such extreme temperatures are likely to occur only once every 50 years, 100 years, or even less frequently.
Extreme heat events can occur in any region.
Although the reported temperatures may not seem extreme to people living in other parts of the world, they can be significantly beyond the normal range for that specific location and time of year. In March 2022, for example, a heatwave over the East Antarctic brought temperatures to -9°C (16°F), about 40°C (72°F) higher than the average over that region for that time of year. Despite the remote location and seemingly cold temperatures, this was one of the most extreme heatwaves ever recorded on the planet because it was so far outside the norm of what was expected over Antarctica in March. (Statistically, it was 4 standard deviations from the norm, making it equivalent to a 1-in-1000 year event.) Similarly, a heatwave over the Pacific Northwest in June 2021 brought temperatures of nearly 50°C (122°F) to a region accustomed to summer temperatures closer to 20°C (68°F), leading to increased human mortality and the widespread death of marine organisms. While such temperatures may not seem too extreme for someone living in the American Southwest, communities and ecosystems in the Pacific Northwest were unaccustomed to such warm temperatures, resulting in significant impacts on life and health.
Not surprisingly, heatwaves are becoming increasingly common as our planet warms.
There are several reasons for this.
First, baseline temperatures are rising around the globe as greenhouse gases build up in our atmosphere. Temperatures that are much hotter than the norm are becoming more frequent as this baseline shifts. Today's record high ocean temperatures and persisting Super El Nino will also contribute to warmer-than-average temperatures this summer that can turn into extreme heat events.
Second, meteorological and hydrological factors often work together to amplify warm temperatures, which can result in hotter, more persistent extreme heat events. From a meteorological perspective, extreme heat is often associated with heat domes, zones of high atmospheric pressure at the surface associated with advection of warm subtropical air and sinking of upper atmospheric air that warms as it becomes pressurized in the lower atmosphere. As the planet warms, these heat domes tend to stay put for longer periods (known as 'blocking highs' – high pressure centers that can stubbornly sit over a location for days and stall the usual movement of atmospheric waves).
To compound the effects of heat domes, drier soils can also act to amplify extreme heat in many regions. When the sun warms a dry land surface, surface temperatures can skyrocket because there is little moisture in the soil or vegetation to buffer the heating through evaporation from the soil or transpiration from plants. Even in many regions where annual precipitation is increasing, soils are becoming drier in summer as temperatures rise (hotter temperatures more effectively dry soils) and the dry season lengthens.
Furthermore, deforestation and urbanization contribute to lower surface moisture levels, which can amplify temperatures and exacerbate extreme heat events.
Not all types of heat are equal.
As we all know, high temperatures combined with high relative humidity can feel much hotter than high temperatures with low relative humidity. This is because our bodies cool evaporatively: we sweat, and this sweat evaporates, cooling our skin. When humidity is high, we still sweat, but the sweat has a hard time evaporating because there's already so much water vapor in the air. As a result, our bodies are much less able to cool through sweating when the humidity is high. Metrics like the Heat Index take into account the effect of humidity on assessments of extreme heat: 30°C (86°F) has a heat index of a comfortable 27°C (80°F) when humidity is near 0%, but has a heat index of 44°C (111°F) when the humidity is near 100%, which is extreme enough to cause heat cramps and heat exhaustion, particularly for vulnerable folks like children and the elderly. The Wet Bulb Temperature is another way to measure the combined effects of temperature and humidity, and represents the lowest temperature that a body can cool to with evaporation (e.g. sweating). A wet bulb temperature of 35°C (95°F) is the highest temperature where humans can survive outdoors without air conditioning – i.e. the limits of human survivability*. Around the world, as temperature records are broken, many regions of our planet are approaching this dangerous wet bulb temperature threshold, putting huge swaths of the human population at high risk for heat illnesses and death.
Dangerous heat extremes are increasing around the globe.
Now, more than ever, adaptation measures for extreme heat are crucial. These may include early extreme heat warning systems for communities, cooling centers for vulnerable populations, reinforcements to the electricity grid to prevent blackouts when both temperatures and electricity demand are high, and workplace safety measures and infrastructure updates to keep temperatures at safe levels for workers. In some cases, workplaces may need to have insurance to account for days (or weeks) when outdoor work cannot be done safely due to dangerous heat. In some areas of the world, parametric heat insurance products are already being deployed to account for days when outdoor workers are unable to work due to heat extremes. Planette's month-ahead, season-ahead, and year-ahead forecasts for extreme heat can provide many sectors and industries the critical information needed to put such life-saving adaptive measures in place.
The bottom line: on a hotter planet, extreme heat events are more common and persistent, and may become more deadly as critical thresholds are bypassed. Even as we all work together to decrease global emissions to stabilize the climate, adaptation measures will be crucial to ensure that lives and livelihoods are protected.
