Heat Waves

The southwest is experiencing a heat wave. Here in northern Virginia we’ve only had summer with highs dancing in the upper eighties and low nineties, but our turn seems to be coming. Unusually hot days and heat waves are a natural part of the variation in weather. Though according to the U.S. EPA, as the Earth’s climate warms, hotter-than-usual days and nights are becoming more common and heat waves are expected to become more frequent and intense in the future.

The EPA backs this up with four charts of the frequency; the duration; the number of days between the first and last heat wave of the year (season length); and how hot the heat waves were, compared with the local temperature threshold for defining a heat wave (intensity). The data were analyzed were for 50 large metropolitan areas over the period of 1961-2021. 

from EPA

The 50 metropolitan areas were selected because they had a data set over the time period. Though the data may reflect other factors, like the urban heat island effect and the characteristics of a certain time period. As cities develop, vegetation is often lost, and more surfaces are paved or covered with buildings. This type of development can lead to higher temperatures—part of an effect called an urban heat island. Compared with surrounding rural areas, built-up areas have higher temperatures, especially at night.  Urban growth since 1961 may have contributed to part of the increase in heat waves seen for the 50 cities. This indicator focuses on the temperatures, regardless of whether the trends reflect a combination of climate change and other factors.

Longer-term records show that heat waves in the 1930s remain the most severe in recorded U.S. history. The spike in chart below reflects extreme, persistent heat waves in the Great Plains region during the 1930’s when poor land use practices and many years of intense drought contributed to these heat waves by depleting soil moisture and reducing the moderating effects of evaporation. This would suggest an impact from land use, water use on weather or climate- an interesting line of research.

from EPA

The chart also show a period in the 1960’s and 1970’s  that had a low frequency and coverage of heat waves. An extremely low incidence of heat waves followed by a return to a more normal pattern could be the reason the frequency of heat waves appears to have increased when they were simply returning to normal after a period of extremely low frequency.  In finance they call this data mining. 

Nonetheless, increases in extreme heat events for more people as we continue to urbanize can lead to more heat-related illnesses and deaths if people and communities are unable to take steps to adapt. Even small increases in extreme heat can result in increased deaths and illnesses. We need to focus our action on adapting to the future this planet will experience.