How does climate change affect heatwaves?

Attribution studies investigate the links between extreme weather and climate change. World Weather Attribution, a science initiative led by the Grantham Institute's Dr Friederike Otto, carries out attribution studies to quantify how climate change influences the intensity and likelihood of an extreme weather event. Their recent studies have shown that:

• In the UK, the temperatures of 40°C seen in July 2022 would have been extremely unlikely to have happened without human-caused climate change.[1]
• The heatwaves across Europe, China and the USA in July 2023 would have been virtually impossible without human-caused climate change.[2]
• The extreme heat experienced in Spain, Portugal, Morocco and Algeria in April 2023 would have been almost impossible without human-caused climate change.[3]

Multiple independent science organisations, including NASA, the National Oceanic and Atmospheric Administration (NOAA) and the UK Met Office, have measured increases in the frequency and/or intensity and duration of heatwaves around the world.[4] The IPCC has concluded that it is virtually certain (that is, a 99-100% probability) that there have been increases in the intensity and duration of heatwaves and in the number of heatwave days at the global scale.[5]

In the UK, 14 of the 30 hottest days on record have occurred since 2003. The extreme heatwave in July 2022 saw temperatures in England exceed 40°C for the first time. This surpasses the temperatures reached during the 1976 heatwave (although the latter was more prolonged).[6]

Increased global average temperatures caused primarily by the burning of fossil fuels will continue to make heatwaves hotter, longer, and more frequent in every part of the world.[7]

Even if we stopped emitting all greenhouse gases tomorrow, we would not see a reduction in the increased global average temperatures and associated increased intensity and frequency of heatwaves that have already occurred. This is because the carbon dioxide that has already been emitted stays in the atmosphere for hundreds of years. Global average temperatures will only stop increasing and heatwaves will stop intensifying when the world reduces greenhouse gas emissions to ‘net zero’; an overall balance between emissions produced and emissions removed from the atmosphere. The level of warming reached when the world achieves net zero will be the level of warming that humans will have to live with for centuries.[8]

References

[1] Zachariah, M. et al (2022). Without human-caused climate change temperatures of 40 oC in the UK would have been extremely unlikely. World Weather Attribution  

[2] Zachariah, M. et al (2023). Extreme heat in North America, Europe and China in July 2023 made much more likely by climate change. World Weather Attribution

[3] Philip, S. et al (2023). Extreme April heat in Spain, Portugal, Morocco & Algeria almost impossible without climate change. World Weather Attribution

[4] Buis A (2021). Extreme Makeover: Human activities are making some extreme events more frequent or intense; United States Environmental Protection Agency Climate Change Indicators: Heat Waves (accessed 28 July 2023); Met Office UK and Global extreme events – Heatwaves (accessed 28 July 2023)

[5] Seneviratne, S.I., et al (2021): Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1513–1766, doi:10.1017/9781009157896.013.

[6] Met Office 2022. Unprecedented extreme heatwave, July 2022

[7] Seneviratne, S.I., et al 2021: Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1513–1766, doi:10.1017/9781009157896.013.

[8] Allen, M.R., Friedlingstein, P., Girardin, C.A., Jenkins, S., Malhi, Y., Mitchell-Larson, E., Peters, G.P. and Rajamani, L., (2022). Net zero: science, origins, and implications. Annual Review of Environment and Resources, 47, pp.849-887.

Heatwaves are deadly; a recent study has estimated that extreme heat caused more than 60,000 deaths in Europe in the summer of 2022.[1] In 2022, there were an estimated 2,985 excess deaths in England associated with 5 heat episodes across the summer.[2]

While people can die directly from heatstroke, many heat-related deaths occur when extreme heat compromises the human body’s ability to cope with existing health conditions like heart disease, meaning that heatwaves are particularly dangerous for elderly people. Heatwaves are also dangerous for homeless, outdoor workers and people living in low-income neighbourhoods without access to air conditioning, green spaces and medical care. [3]  

Like other weather extremes, the impacts of heatwaves are unequal, disproportionately affecting poorer and marginalised people.[4]

To prevent heatwave deaths, it is critical that heatwaves are well-forecast, the public are educated about the risks of extreme heat, and that governments and health providers look after the most vulnerable people.[5]

 References:

[1] Ballester, J., Quijal-Zamorano, M., Méndez Turrubiates, R.F., Pegenaute, F., Herrmann, F.R., Robine, J.M., Basagaña, X., Tonne, C., Antó, J.M. and Achebak, H., (2023). Heat-related mortality in Europe during the summer of 2022. Nature medicine, pp.1-10.

[2] Heat mortality monitoring report: 2022, UK Health Security Agency

[3] Kravchenko, J., Abernethy, A.P., Fawzy, M. and Lyerly, H.K., (2013). Minimization of heatwave morbidity and mortality. American journal of preventive medicine, 44(3), pp.274-282.

[4] Herold, N., Alexander, L., Green, D. and Donat, M., (2017). Greater increases in temperature extremes in low versus high income countries. Environmental Research Letters, 12(3), p.034007.

[5]Puley G, 2022, Extreme Heat Preparing for the Heatwaves of the Future, UN Office for the Coordination of Humanitarian Affairs, the International Federation of Red Cross and Red Crescent Societies, and the Red Cross Red Crescent Climate Centre

Heatwaves are broadly defined as extended periods of hot weather relative to the expected conditions of the area at that time of year. In the UK, a heatwave is defined as occurring when there are at least three consecutive days where daily maximum temperatures meet or exceed the heatwave temperature threshold. The temperature threshold is set at the county level and varies from 25-28°C across different parts of the UK, reflecting the different climate across different geographical locations.[1]

Other countries will have their own definitions of heatwaves according to the local climatic conditions.

References

[1] Met Office website What is a heatwave? (accessed 28 July 2023)

Attribution studies calculate if, and the degree to which, a specific extreme weather event was made more (or less) likely and/or intense because of climate change. To do this, climate scientists use three methods:

1. Using climate models to simulate the modern climate and the climate as it would be without any emissions from humans and comparing how likely heatwaves are to occur in each case.
2. Using real-world temperature records from the present and past to compare how the probability of similar events has changed.
3. Using climate models to simulate the climate over time with slowly rising human emissions to calculate how the overall probability of heatwaves changes.

Using more than one attribution method as well as multiple different climate models increases the reliability of the results.[1]

Attribution studies have overwhelmingly demonstrated that virtually every heatwave is made hotter, longer, and more likely because of climate change.

References

[1] Clarke B and Otto F, Reporting extreme weather and climate change: A guide for journalists, World Weather Attribution