Heat Waves (recent)

What is a heat wave? 

  • A heat wave is a period of unusually warm weather that lasts three or more days, during which the temperature is higher than the 90th percentile of the daily values at that location for that time of the year.
  • The frequency, duration and intensity of heat waves have increased recently with ongoing global warming (graphs below). 

Heat wave characteristics over the last six decades showing the increase in frequency, length, and intensity since the 1960s in the United States (from US EPA).

How do heat waves affect people?

  • Heat waves can create health risks such as cardiovascular issues, blood clots, respiratory problems, heat stroke, or even death. Extreme heat has been one of the leading causes of weather related deaths in the United States (graph below). 

Number of weather related fatalities in the United States 1991-2020. Note high average values for heat related fatalities (from National Weather Service, 2020).

How do heat waves affect the environment?

  • Heat waves can cause droughts, wildfires, and heat stress that can destroy crops and affect food supply and agriculture. 
  • Heat waves also impact wildlife severely because they alter habitats and damage the health of animals by causing heat stress that can lead to severe illness and death (image below). With destroyed habitats animals are displaced and ecosystems are ruptured, which can result in more dire consequences such as extinctions of species.

In 2018 about 23,000 animals, many belonging to endangered species, in Australia died due to heat waves, including the flying fox bat pictured here (from Yale School of the Environment).

  • Smith College Biology Professor Mariana Abarca studies the impact of heat waves on butterflies. Her research team found that winter warm spells, which are becoming increasingly common, result in drastic mortality of butterfly larvae in Maryland, USA. The researchers also discovered that feeding on invasive plants increases the vulnerability of butterflies to heat stress compared to feeding on native vegetation (image below).

Heat waves increase mortality of a wetland butterfly Baltimore Checkerspot (Euphydryas phaeton phaeton) (photo courtesy of Mariana Abarca).

Examples of major heat waves throughout history:

The Great North American heat wave of July 1936

  • In the summer of 1936 blistering heat tore through the Upper Midwest, the Great Plains, and Great Lake states, and persisted for ten days with temperatures over 90°F (32°C).  In some areas temperatures even reached a high of 108°F (42.2°C). This had devastating effects on crops in this region and resulted in over 5000 deaths. 

Newspaper from St. Paul, Minnesota reported deaths caused by the intense heat in 1936 (from Pioneer Press, 2016).

Heat waves in Greece 1987-1988

  • From 1987 to 1988 Greece experienced abnormally high summer temperatures that claimed the lives of many in a time and place where air conditioning wasn’t common. For 6-8 days straight temperatures reached a harsh 104°F (40°C) throughout the day and wouldn’t even drop below 86°F (30°C) at night. Many victims of the heat were elderly, and the deaths totaled to around 1300 people. 

The European heat wave of 2003

  • Record high temperatures, on average 20-30% higher than normal, swept across Europe in the summer of 2003. The UK and France reported extreme temperatures starting from a minimum of 95°F (35°C) to a high of 105.8°F (41°C). As a result, thousands of elderly people died, and 13 billion Euros worth of damages were reported due to droughts, forest fires, and affected ecosystems and agriculture. 

The India heat wave of 2015

  • During May of 2015, in anticipation of the upcoming monsoons, seasonal temperatures began to routinely increase to as high as 104°F (40°C) in India. However, unexpectedly temperatures soared past this average and peaked at around 117°F (47.2°C) in some areas for the entire week. The humidity made the high temperatures unbearable and as a result over 2000 human lives were lost. This was one of the worst heat waves of that decade and the one of the deadliest heat waves recorded to date. 

The extreme heat in 2015 in the capital of India, New Delhi, melted the roads (from DW News).

2022 heat wave in India and Pakistan 

  • India and Pakistan experienced an extended, three-month-long heat wave during the spring of 2022. In May, daytime temperatures soared above 110°F (43°C) and nighttime temperatures stayed in the 90s (~35°C). Temperatures this high are not usually seen until later in the summer. Surging electricity use led to widespread power outages. Schools and businesses adapted to the heat by closing earlier, with the Indian government encouraging people to stay inside during the afternoon. This, unfortunately, is not an option for unhoused people (image below).  
  • As of late May 2022, at least 90 people have died across India and Pakistan due to the heat wave, but the actual number of casualties is likely higher. The heat wave also triggered forest fires in India and an extreme flood in northern Pakistan when a lake overflowed with water from a rapidly melting glacier.  

Homeless people sleep in the shade of a highway overpass on a hot day in New Delhi, India in May 2022 (from Manish Swarup/Associated Press via the New York Times).

Climate inequality leads to disparity in who suffers from heat waves 

  • Large cities in particular suffer under heat waves due to the “urban heat island effect” (images below), which occurs when natural land cover is replaced with  pavement, buildings, and other surfaces that preferentially absorb and retain heat. As climate change worsens heat waves, the disparity in who suffers from these events increases as well. For example, in New York City (USA), the effects of the heat differ between neighborhoods: 
    • Wealthy neighborhoods have more green space, including tall trees that provide shade, while poor neighborhoods have fewer trees and less access to recreational green space. Without trees to provide shade and natural cooling, paved sidewalks and roads get extremely hot in summer months.
    • Wealthy New Yorkers are more likely to have air conditioning, as well as the means to escape the city for the beach, lakes, or forests. Public spaces in New York do not serve everyone equally, either: to access public pools in the summer, New Yorkers have to wait in line, sometimes for hours, and there are more pools per capita in white, wealthy boroughs. Black New Yorkers are more than twice as likely to die from heat as white residents. 

Artist Alisa Singer creates colorful visual art to showcase alarming climate trends, such as the unequal impact of heat waves in urban areas. Singer’s digital art piece “Urban Heat Islands” (above) is paired with graphs (below) explaining the science behind the issue. These graphs show elevated air and surface temperature in urban areas compared to rural areas (from Environmental Graphiti).

References and additional resources