Extreme Rain, Flooding, and Climate Change

Time to completion
5 min

Summary

Extreme rainfall events are increasing in intensity in North America and climate change is responsible for much of this increase.1 Heavy rain causes flooding that can, for example, damage buildings and roads, erode soil, and flush pollutants into waterways. There will be an increase in future extreme rainfall frequency and severity globally, including in Canada.2

The Essentials

  • Rising air temperatures, due primarily to growing concentrations of greenhouse gases in Earth’s atmosphere, are increasing evaporation from soil, plants, lakes, and oceans. Warm air also holds more moisture than cooler air, allowing more rain to fall during extreme rain storms.
  • There is an increasing trend in extreme rainfall globally3,4 and this increase is attributed to human activity.4
  • In North America, climate change has intensified extreme rainfall events and this trend is likely to continue in the future.1
  • Days with heavy rainfall* increased by more than 1 day per year over Canada as a whole, particularly in British Columbia, southern Ontario, southern Quebec and in eastern provinces between the mid-20th century and recent years.5
  • Climate scientists are confident that future extreme rainfall will increase in intensity globally, including in Canada.2 Supporting evidence includes:
    • physical understanding that warming causes atmospheric moisture increases;
    • observed increases in high-latitude total rainfall are attributable to human influence; and
    • climate models consistently project increases in future extreme rainfall.2

*Heavy rainfall refers to rainfall greater than the annual 90th percentile from all rainfall events greater than 1 mm per day.

  • A number of important climate processes determine changes in extreme rainfall, including changes to atmospheric temperatures, shifts in large-scale atmospheric circulation, and more intense evaporation.6,7
  • Global and regional climate models represent the climate at a scale of 100-250 km and 10-50 km, respectively. As a result, rainfall extremes from these models represent average conditions over hundreds to thousands of square kilometres. Because of this relatively coarse spatial scale, climate models do not capture many of the physical processes that produce local intense rain events in Canada.
  • Despite challenges in modelling extreme rain events, information from global and regional models – taken together with local historical data – is still very useful for climate change risk assessments efforts.
  • Emerging ultra-high-resolution climate models, with spatial scales typically between 1 and 4 km resolution, are able to explicitly resolve short-duration extreme rainfall events, like summertime thunderstorms.8,9 As computers become more powerful, these types of models will be increasingly used to project changes to future extreme rainfall conditions.
  • Floods caused by extreme rain are called ‘pluvial’ or ‘overland’ floods. These floods differ in their cause and effects from other flood mechanisms.  For example, river and lake flooding often occurs due to longer-term rainfall conditions, rapid snow melt, rain-on-snow events, and ice jams.10 In coastal areas, floods can arise from high tides combined with storm surge and waves. It is important to remember that information related to extreme rain events may not be appropriate for assessing climate change-driven shifts to these other flood types.
  • Pluvial flood severity depends most importantly on rainfall intensity, but also on whether or not the ground is already saturated, the size of the area affected by the event and the type of surface on which the rain falls.

Taking Action

There are many actions that can reduce damages from rain-driven pluvial flooding, for example:

  • Ensuring that infrastructure is not built in areas prone to pluvial floods;
  • Reducing/replacing hard surfaces (such as concrete, asphalt) with more natural landscaping that absorbs excess water;
  • Building pluvial flood protection measures;
  • Protecting existing, or restoring, wetlands, which can reduce flood severity by providing space for excess water and slowing the flood response.

 

 

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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. Kirchmeier-Young MC, Zhang X (2020): Human influence has intensified extreme precipitation in North America. PNAS 117: 13308-13313. https://doi.org/10.1073/pnas.1921628117
  2. 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 Bush E, Lemmen DS (Eds.) Canada’s Changing Climate Report. Government of Canada, Ottawa, Ontario, pp 112-193.
  3. Westra S, Alexander LV, Zwiers FW (2013): Global increasing trends in annual maximum daily precipitation. Journal of Climate 26: 3904-3918. DOI: https://doi.org/10.1175/JCLI-D-12-00502.1
  4. Dong S, Sun Y, Li C, Zhang X, Min S-K, Kim Y-H (2021): Attribution of extreme precipitation with updated observations and CMIP6 simulations. Journal of Climate 34: 871-881. https://doi.org/10.1175/JCLI-D-19-1017.1
  5. 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
  6. Trenberth KE (2011): Changes in precipitation with climate change. Climate Research 47: 123-138. Doi: 10.3354/cr00953
  7. Westra S, Fowler HJ, Evans JP, Alexander LV, Berg P, Johnson R, Kendon EJ, Lenderink G, Roberts NM (2014): Future changes to the intensity and frequency of short-duration extreme rainfall. Reviews of Geophysics 52: 522-555. https://doi.org/10.1002/2014RG000464
  8. Kendon EJ, Roberts NM, Fowler HJ, Roberts MJ, Chan SC, Senior CA (2014): Heavier summer downpours with climate change revealed by weather forecast resolution model. Nature Climate Change 4: 570-576.
  9. Cannon AJ, Innocenti S (2019): Projected intensification of sub-daily and daily rainfall extremes in convection-permitting climate model simulations over North America: implications for future intensity-duration-frequency curves. Natural Hazards and Earth System Sciences 19: 421-440. https://doi.org/10.5194/nhess-19-421-2019.
  10. Bonsal BR, Peters DL, Seglenieks F, Rivera A, Berg A (2019): Changes in freshwater availability across Canada. Chapter 6 in Bush E, Lemmen DS (Eds.) Canada’s Changing Climate Report. Government of Canada, Ottawa, Ontario p. 261–342.