Heatwaves: An Effect of Global Warming

What Causes Heatwaves? What are the health effects of extreme heat? What are urban heat islands? How does heat affect crops? How hot will heatwaves become? How to adapt to extreme temperatures? We answer these questions and more.
Heatwaves in Los Angeles are forecast to rise significantly.
Central L.A. is forecast to experience 3 times more days of temperatures over 95°F. Photo: Marshall Astor CC BY 2.0

Heatwaves Are Getting Hotter

As Earth’s temperature continues to rise, each year seems to present yet another scorching heatwave record. And it’s not our imagination. The science shows that heatwaves are becoming hotter, they’re occurring more often and lasting longer.

In this article, we discuss how climate change is supercharging heatwaves and the dangers this poses for the biosphere and all its inhabitants.

What Exactly Is A Heatwave?

A heatwave is one of several extreme weather events, like hurricanes, flooding and droughts. However, in contrast to most natural disasters, a heatwave does not cause the sort of dramatic damage that makes front page news.

Yet, heatwaves are disasters and have recently been classified as such. 1 They are more deadly and pose a significant threat to our environment and society. According to the Centers for Disease Control and Prevention, extreme heat now causes more deaths in American cities than all other weather events combined.

Heatwaves take place in the summer months, which vary depending upon where you live in the world. Climate science generally defines a heatwave as a prolonged period of excessive heat, usually lasting 3 or more consecutive days, with daily temperatures above a certain threshold for the location and time of year. Such a threshold temperature is technically called an ‘apparent temperature’, calculated with the Apparent Heat Wave Index (AHWI).

However, there is no set international definition, because it can vary by region. A few days at 35 C (95 F) in July in London would be something to note, but the same temperature in Texas may be usual in that state at that time of year. In other words, the same meteorological conditions can constitute a heatwave in one place, but not in another.

There are two broad types of heatwaves:

  1. Dry heatwaves: Associated with clear skies and lots of sun. Sometimes dry heatwaves can be accompanied by windy conditions which increases heat stress. Usually dry heatwaves occur in continental or Mediterranean climates.
  2. Moist heatwaves: Associated with very warm, oppressive and humid conditions throughout the day and night. Often there is cloud cover that prevents heat escaping at night. Such heatwaves are more frequent to maritime and mid-latitude temperate climates 2 like Midwestern and Eastern U.S., China, Malaysia and Northern Latin America.

Is Climate Change Making Heatwaves Worse?

Yes. According to the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), over the previous 50 years, excessive hot days and nights have become ever more frequent. Furthermore, the report predicts that the length, frequency and/or intensity of heatwaves will increase this century. This is in line with global temperature projections: the warmer the planet becomes, the more extreme the heatwaves are likely to be.

In the U.S, for example, heatwaves occur 3 times more often than they did in the 1960s. Now Americans can expect 6 heatwaves a year instead of twice a year. 3

It’s all linked to global warming, as the science shows. Studies demonstrate that the climate crisis is making record-breaking hot months 5 times more likely. In other words, extreme heat events are yet another consequence of the Anthropocene epoch of human dominance over the global environment.

What’s worse, heatwaves tend to regularly align with other catastrophic events like wildfires and drought. This is why the effects of global warming are so far reaching. As the planet burns, other problems are triggered and amplified in our forests and oceans. For example, marine heatwaves have worsened to such an extent, that we are experiencing a cataclysmic loss of biodiversity in marine life.

Worth Reading: Effects of Global Warming On Oceans.

What Causes Heatwaves?

Heatwaves occur when a band of high pressure becomes stuck over a region for several days without moving on. This pressure seals the area, like a cap, trapping heat and causing temperatures to rise.

A stationary and persistent weather pattern like this is an example of atmospheric blocking, so called because it blocks other jet streams from moving in and bringing rain or storms. 4

Jet streams are high winds in the atmosphere, near the tropopause. When a jet stream develops Rossby waves, these determine where it moves to, and how long it stays. Recent research published in Proceedings of the National Academy of Sciences of the USA, discovered that Rossby waves are becoming much bigger and getting stuck, particularly in July and August. This could be what’s causing heatwaves to last longer.

Scientists have linked this imbalance in Earth’s climate system to unprecedented high temperatures in the Arctic. The Arctic region is warming twice as fast as the rest of the globe and it’s all down to the huge amount of fossil fuels we’re burning, which is ramping up the greenhouse effect in the lower atmosphere.

Rising temperatures around the world are also causing a number of positive climate feedbacks: in summer, dry soil induces fewer clouds because there is less evaporation. For more, see: What is the Water Cycle?

Atmospheric Climate Variability
Regional weather cycles – such as the El Nino-Southern Oscillation in the Pacific, the Indian Ocean Dipole (IOD), and the Southern Annular Mode (SAM) around Antarctica – exert a significant influence over atmospheric winds and climate conditions. Their mechanisms are not thought to be directly controlled by climate change, but their frequency and their overall effects probably are.

Did You Know?

In the past, climate change denial lobbyists used heatwaves to back up their claim that the planet was not in fact warming.

They cited the fact that there was a dip in the number of heatwaves in Europe which occurred during the 1970s and 1980s. They said: ‘If Earth’s temperature was rising, then why the dip?’

Since then however, scientists have proven that the blip was caused by the shading effect of industrial aerosols, which blocked sunlight from reaching Earth’s surface. 5 Essentially air pollution exerted a cooling influence for a number of years. As the air has become cleaner in Europe, heatwaves have returned with a vengeance.

Examples of Deadly Heatwaves

According to Carbon Brief, 69 percent of 355 extreme weather events recently tracked were found to be made more likely or more severe by human-caused climate change. Heatwaves account for 47 percent of such events.

Siberian Heatwave 2020

In the first 6 months of 2020, Siberia experienced a period of unusually high temperatures, including a record-breaking 38 C (100.4 F) in the town of Verkhoyansk 6. Research shows that global warming made this heatwave 600 times more likely. While a hotter climate in places like Siberia can have devastating effects on local wildlife and people, it also impacts the world’s climate system through the thawing of permafrost and a lower albedo effect due to reduced ice and snow cover.

European Heatwaves

In 2003, a scorching heat wave swept across Europe, accounting for the deaths of 70,000 people. Every summer since then, despite precautions, the continent has recorded between 500 and 3,500 fatalities from extreme heat. There were 700 deaths in 2016; 475 in 2017; and 1,500 in 2018. 7

In June 2019 France set a new record as temperatures climbed to 45.9 C (114 F). Research shows that global warming made this heatwave 100 times more likely. Furthermore, scientists say without climate mitigation actions, French temperatures could rise to 50 C (122 F) or more by 2100. Researchers say if global warming exceeds the 2 degrees Celsius limit laid down in the Paris Climate Agreement, such heatwaves will become the norm.

Urban Heat Islands

Urban heat islands (UHI) are urban areas that are warmer than their suburban or rural surroundings. Observed all over the world, UHIs expose people living in urban areas to more heat related deaths and illnesses. For example, research shows that UK cities like London, Birmingham and Manchester have been 5 C degrees warmer than surrounding areas in the past twenty years.

One study attributed over half of the heat-related deaths that occurred during the August 2003 heatwave in the West Midlands to the UHI effect. 8 Given that by 2050, nearly 90 percent of UK’s population is projected to live in urban areas, this is a cause for grave concern.

Dangers of Heatwaves to Humans

Heat Stroke

Extreme heat causes heat stress and in some cases death. In Australia, for example, heatwaves have killed more people than wildfires, floods, cyclones or any other natural disaster. See also: Australian Bushfires and Climate Change.

Our core normal body temperature is 37 C. Heat stress occurs when the body temperatures rises above this for more than several hours, causing heat exhaustion. If the body temperature rises to 42 C or above, heatstroke and death can occur within hours.

Normally the body can cool itself by sweating. But when humidity is high, sweat does not evaporate as quickly. High humidity and high night-time temperatures are the worst combination for heat stress.

Certain groups of people – those with chronic health conditions like high blood pressure, and construction workers, farmers and other outdoor laborers – are at greater risk of suffering heat stress and heat stroke during heat waves. Continuous days and nights of heat and humidity sap a workers’ strength, exacerbate underlying health conditions, and can lead to heat stress and increased risk of death.

Full List of Heat Related Medical Conditions

Heat ConditionSymptoms
Heat rashSmall, red and itchy bumps appear on the face, neck, chest, under breast and groin areas. Heat rash can affect any age, but is particularly common in young children.
Heat edemaHeat-related edema (or oedema) is a swelling of the lower limbs, usually ankles, appears at the start of the hot season. Heat causes blood vessels to swell and so fluid moves to the lower body because of gravity.
Heat syncopeThis involves fainting or dizziness. It is common in patients with cardiovascular diseases or taking diuretics before acclimatization takes place. It occurs when the body tries to cool itself and blood flow to the brain is restricted in the process.
Heat crampsPainful muscular spasms occur, most often in the legs, arms or abdomen, usually at the end of an exercise session. Heat cramps are usually caused by dehydration and loss of electrolytes through heavy sweating and muscle fatigue.
Heat exhaustionCore temperature is usually slightly elevated (less than 40 C). Symptoms include intense thirst, weakness, anxiety, dizziness, fainting and headache. Pulse is thready and rapid shallow breathing. This can be attributed to water and/or salt depletion resulting from exposure to high environmental heat or strenuous physical exercise.
Life-threatening heatstrokeExposure to heat stress usually during a heatwave and/or strenuous exercise. Body temperature rapidly increases to greater than 40 C and is associated with stupor, confusion or coma. Sweating, vomiting, hypotension, tachycardia and tachypnoea are often present.
Source: World Health Organization


In cities, hot, sunny days can increase the amount of ground level ozone in the air triggering photochemical smog, which poses a serious threat to the same risk groups outlined above.

Mental Health

For those whose livelihoods depend on predictable weather patterns, heatwaves can prove a challenge to mental health. A 2017 study linked the impact of climate change to the suicides of nearly 60,000 Indian farmers. 9

Read also: What Is The Difference Between Climate And Weather?

Heatwaves And Agriculture

One of the major effects of climate change on plants, is to do with heat. Normally heat causes plants to grow faster, but if temperatures exceed certain thresholds, this will eventually cause higher desiccation rates. A number of plant organisms like soy and corn are already close to their heat tolerance limits and some may not be able to maintain growth if temperatures rise much higher.

Grain yield and biomass typically decline significantly during periods of drought when temperatures exceed critical limits. A recent study 10 looking at 20 years of crop and weather data in the Corn Belt showed that corn yields in Illinois dip by 3.6 bushels/acre for every 1 degree increase in average nighttime temperature.

Furthermore, heat-stressed cattle can experience declines in milk production, slower growth and reduced conception rates.

Energy Demands

Higher summer temperatures increase electricity demand as people turn to air conditioning for cooling. This is associated with electrical outages. Although warmer winters will reduce the need for heating, modeling suggests that total U.S. energy use will increase in a warmer future. In addition, as rivers and lakes warm, their capacity for absorbing waste heat from power plants declines. This can reduce the thermal efficiency of power production, making it difficult for power plants to comply with cooling water environmental regulations.

How Hot Will Heatwaves Become?

Increasingly more sophisticated climate modelling are being used to estimate future climate change outcomes for different emissions scenarios.

Under the worst scenario (RCP8.5), very extreme heat waves (much stronger than any of the heatwaves on record to date) are projected to occur as often as every two years by 2050.

Europe is warming faster than the global average. The hottest summer days in the UK could be 12 degrees warmer than today, with peak summer temperatures in London regularly topping 40 C (104 F). Heatwaves are predicted to be particularly strong in Southern Europe.

In the U.S. the coldest and warmest days could be 5 degrees hotter, rising to 10 degrees by 2100. 11

If people cannot adapt to future climate change, heatwave-related deaths are expected to increase in the most in tropical and subtropical countries/regions, while European countries and the United States will have smaller increases. 12. And the higher the greenhouse gas emissions, the higher the death rate.

Adapting To Heatwaves

World-wide countries, cities and communities are reacting to the challenges of rapidly changing climatic conditions. Just some climate change adaptation strategies include:

  • Better urban planning design. Installing sponge city concepts such as green roofs and cool pavements to reduce urban heat island effect.
  • Planting trees to provide shade and transpiration to cool the air.
  • Ensuring that outdoor construction workers can rest properly during extreme temperatures.
  • Providing greater access to air conditioned cooling centers for the elderly and poor in the worst affected communities.

At the end of the day, or course – climate mitigation would be a better solution. We need to deal with the root cause of climate change – namely our massive over-consumption of fossil fuels.


  1. “Characteristics of Heat Waves From a Disaster Perspective”. Do-Woo Kim. January 2020 []
  2. “Humid heat waves at different warming levels.” Simone Russo, Jana Sillmann & Andreas Sterl August 2017 []
  3. “Multi-day extreme heat events in cities across the United States.” GlobalChange.gov []
  4. “Jet stream: Is climate change causing more ‘blocking’ weather events? Carbon Brief. 2020 []
  5. Understanding the rapid summer warming and changes in temperature extremes since the mid-1990s over Western Europe by Buwen Dong, Rowan T. Sutton & Len Shaffrey. May 2016. []
  6. “Siberian heatwave of 2020 almost impossible without climate change” July 2020. World Weather Attribution []
  7. “Europe’s killer heat waves are a new norm.” Washington Post. July 26, 2019 []
  8. Attribution of mortality to the urban heat island during heatwaves in the West Midlands, UK”. Clare Heaviside, Sotiris Vardoulakis & Xiao-Ming Cai 2016 []
  9. Crop-damaging temperatures increase suicide rates in India-July 31, 2017 Tamma A. Carleton []
  10. “Mapping twenty years of corn and soybean across the US Midwest using the Landsat archive.” Sherrie Wang et al. September 2020. []
  11. Climate Science Special Report. Fourth National Climate Assessment (NCA4), Volume I []
  12. “Quantifying excess deaths related to heatwaves under climate change scenarios: A multicountry time series modelling study.” Peer reviewed. 2018. Yuming Guo et al. []
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