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Heat Waves and Climate Change

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6 min

Summary

Climate change has increased the frequency, extent, and duration of extreme heat events. What were once very rare events are now becoming more common.

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The Essentials

  • There is a clear scientific consensus that human activities – primarily the burning of fossil fuels – are causing the Earth to warm, and Canada is warming about twice as fast as the globe as a whole.1
  • In Canada, the highest temperature on the hottest day of the year, averaged across the country, increased by 0.6°C between 1948 and 2016. Much of this warming can be attributed to human influence.1 The highest daily temperature that on average occurs only once in 20 years, also increased, as did the annual number of hot days (defined as days with a maximum temperature above 30°C) and humidity at a number of weather stations in southern Canada.2,3
  • Extreme heat is one of the deadliest forms of extreme weather in Canada. For example, the 2018 heat wave in Montreal – the hottest summer in 146 years – caused 86 possible excess heat-related deaths across the region.4
  • Those most vulnerable to heat-related illness and death include young children, the elderly, those with chronic illnesses, student athletes who practise outside, outdoor workers, city dwellers, and those without air conditioning (or who lose it during power outages).5
  • A heat wave is an extended period of unusually hot weather. Given Canada’s vastness, what constitutes a heat wave for one region would not necessarily be a heat wave in another (for example a heat wave in Toronto vs. Tuktoyaktuk). So, the temperature thresholds defining a heat wave vary from region to region depending on the average temperatures of the area.
  • Between 1953 and 2012, extreme heat events*, increased significantly at more than 35% of Canadian weather stations, most of which are located in southern Canada (south of 55°N).
  • Humidity takes a toll on people’s health because it interferes with the body’s ability to cool itself through the evaporation of sweat. A temperature of 32°C with 80 percent humidity feels like 45°C.6
  • Nights are getting warmer and more humid too in Canada. Between 1948 and 2016, very warm summer night temperatures increased more than those of very warm summer days in Canada (1.3°C versus 0.9°C, respectively)3 — a shift that exacerbates health impacts because hot nights reduce the body’s ability to cool down and recover from hot days.7 Night-time hourly humidex** values that remained above 20 increased significantly at more than half the weather stations.\

*Extreme heat events are defined as days with at least one hourly humidex** value (a combination of temperature and humidity) above 30.

**The humidex (short for humidity index) is an index which combines the effects of temperature and humidity to describe how hot the weather feels to the average person. While this index does not have any units (e.g., °C, %), it is generally interpreted as being equivalent to the temperature in °C.

  • Across the Northern Hemisphere, heat waves are not just happening more often than in the past but, on average, affecting a 25% larger land area than they did in 1980; including ocean areas, heat waves grew 50% larger.8
  • The Northern Hemisphere-wide string of extreme heat events in the summer of 2018 was the largest ever recorded and a new analysis concluded that it would not have occurred without human-caused global warming.9
  • Recent analysis suggests that climate change is altering atmospheric circulation, such as the jet stream, and may partially influence conditions favouring persistent weather patterns, increasing the duration and damaging effects of heat waves.10,11
  • It is virtually certain that Canada’s climate will warm further in the future.12 Scientists have a high level of confidence in projections of future temperature because temperature changes are controlled by basic rules of physics (thermodynamics) and are a direct result of the Earth’s response to changing greenhouse gas and aerosol emissions.
  • Annual and seasonal (winter, spring, summer, fall) average temperatures are projected to increase everywhere in Canada.1 While these increases are projected to be much larger in northern Canada in winter, even the smaller projected increases in summer temperature will exacerbate heat waves.
  • Averaged across the country, by 2050 warming is projected to be about 2°C higher than the 1986-2005 reference period and to remain relatively steady thereafter under a low emissions scenario. In a high emissions scenario, however, temperature increases will continue, increasing by more than 6°C by the late 21st century.1
  • Extreme hot temperatures are also projected to continue to increase in the future and become more frequent. Under a high emission scenario, the highest daily summer temperature, which is currently reached once every 10 years, on average, will occur every other year by 2050 in Canada.1

Taking Action

Extreme heat is likely to become more of a problem in urban areas, particularly in southeastern Canada where humidity levels also tend to be high in summer. Many urban areas already experience the urban heat island (UHI) effect, which keeps cities warmer than the surrounding countryside due to the building fabric (concrete, stone, asphalt etc.) absorbing and retaining heat. The UHI effect prevents cities from cooling down at night and thus exacerbates heat waves. There are measures which can be taken to reduce the impact of hot weather in cities, which some communities are already implementing. For example, the Montreal borough of Rosemont – La Petite Patrie revised its zoning bylaws in 2011 to combat the UHI effect, which is projected to worsen in the future.13,14 These bylaw revisions include:

  • The installation of green, or highly reflective, roofs on new buildings or for roof replacements;
  • The introduction of at least 15% vegetated area landscaped with plants, bushes and trees for all new parking lots of 10 or more spaces;
  • A minimum specified surface reflectivity rating for new paving materials;
  • The requirement for at least 20% of a new building site to remain open ground and be landscaped with plants, bushes and trees.

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These fact sheets are based on text from Susan Hassol, and ClimateData.ca would like to acknowledge her contribution, as well as those from the Climate Research Division (Environment and Climate Change Canada), the Canadian Forest Service (Natural Resources Canada) and our regional climate service partners.

References

  1. Zhang X, Flato G, Kirchmeier-Young M, Vincent L, Wan H, Wang X, Rong R, Fyfe J, Li G, Kharin VV (2019): Changes in Temperature and Precipitation Across Canada. Chapter 4 in: Canada’s Changing Climate Report [Eds. Bush E, Lemmen DS]. Government of Canada, Ottawa, Ontario, pp 112-193. http://www.changingclimate.ca/CCCR2019
  2. Mekis E, Vincent LA, Shephard MW, Zhang X (2015): Observed trends in severe weather conditions based on humidity, wind chill and heavy rainfall events in Canada for 1953-2012. Atmosphere-Ocean 53: 383-397. https://doi.org/10.1080/07055900.2015.1086970.
  3. Vincent LA, Zhang X, Mekis É, Wan H, Bush EJ (2018): Changes in Canada’s Climate: Trends in Indices Based on Daily Temperature and Precipitation Data. Atmosphere-Ocean 56: 332-349. https://doi.org/10.1080/07055900.2018.1514579
  4. Lebel G, Dubé M, Bustinza R. (2019) Surveillance des impacts des vagues de chaleur extrême sur la santé au Québec à l’été 2018, INSPQ. https://www.inspq.qc.ca/bise/surveillance-des-impacts-des-vagues-de-chaleur-extreme-sur-la-sante-au-quebec-l-ete-2018#ref
  5. Health Canada (2020): https://www.canada.ca/en/health-canada/services/publications/healthy-living/too-hot-protect-yourself-extreme-heat.html
  6. NOAA, NWS, What is the Heat Index? https://www.weather.gov/ama/heatindex
  7. U.S. Centers for Disease Control and Prevention (CDC), National Environmental Public Health Tracking Network.
  8. Skinner CB (2018) in: Northern Hemisphere heat waves covering more area than before. https://blogs.agu.org/geospace/2018/12/20/northern-hemisphere-heat-waves-covering-more-area-than-before/
  9. Vogel MM, Zscheischler J, Wartenburger R, Dee D, Seneviratne SI (2019): Concurrent 2018 hot extremes across Northern Hemisphere due to human-induced climate change. Earth’s Future 7: 692-703. https://doi.org/10.1029/2019EF001189
  10. Horton DE, Johnson NC, Singh D, Swain DL, Rajaratnam B, Diffenbaugh NS (2015): Contribution of changes in atmospheric circulation patterns to extreme temperature trends. Nature 522, 465-469. https://doi.org/10.1038/nature14550
  11. Mann ME, Rahmstorf S, Kornhuber K, Steinman BA, Miller SK, Coumou D (2017): Influence of anthropogenic climate change on planetary wave resonance and extreme weather events. Science Reports 7, 45242. https://doi.org/10.1038/srep45242
  12. Bush E, Lemmen DS (Eds) (2019): Canada’s Changing Climate Report. Government of Canada, Ottawa, ON. 444pp. http://www.changingclimate.ca/CCCR2019.
  13. http://ville.montreal.qc.ca/portal/page?_pageid=7357,82287591&_dad=portal&_schema=PORTAL
  14. Richardson GRA & Otero J (2012): Land use planning tools for local adaptation to climate change. Ottawa, ON, Government of Canada, 38pp.