Tides of Change: Climate Change and Flooding in Canada

The negative effects of climate change and progressively severe and frequent extreme weather events are becoming harder to ignore.

Submerged wooden fence on a river in heavy flood after a storm

The negative effects of climate change and progressively severe and frequent extreme weather events are becoming harder to ignore. According to the World Economic Forum in its 2023 Global Risks Report, climate change and environmental damage are some of the most severe risks expected to negatively impact the planet, both in the short-term and long-term. Unfortunately, these may also be the risks for which we the least prepared.

In this article — second in a three-part series on flooding in Canada — we examine the impact of climate change on flooding in Canada. Click here to read part one in our series, which outlines the main types of flooding and mitigation techniques.

The greater the warming, the worse the impacts

The past eight years have been the warmest on record. Despite 196 parties committing to the Paris Agreement’s goal to limit the rise in mean global temperature to preferably below 1.5°C, there is now a 50% chance of average global temperatures reaching this level in the next five years. In Canada, the rate of warming is about double the global average, with warming occurring even faster in northern Canada. This has significant effects on weather patterns and natural catastrophes across the world.

With every additional 0.5°C rise in global temperature, there are clearly discernible increases in the intensity and frequency of hot extremes, which affect not only heatwaves, wildfire, and droughts, but also extreme precipitation.

Climate change has already produced observable increases in the frequency and severity of extreme precipitation. As average air temperature rises, its capacity to hold moisture grows. The intensity of downpours depends in part on how much water the air can hold at a given time. According to Canada’s Changing Climate Report 2019, Canada’s annual mean precipitation could increase by as much as 24% by the end of the 21st century. Extreme precipitation events that were once expected to occur every 20 years on average are now projected to happen every 10 years by 2050 and every 5 years by the end of the century.

This projected upsurge in the intensity of extreme precipitation will likely lead to an increase in the frequency and magnitude of pluvial flooding, as pluvial flooding mainly results from precipitation intensity exceeding the capacity of that area’s natural and artificial drainage systems. Perhaps counterintuitively, the effects of extreme precipitation are exacerbated by heat and drought conditions. Soil is most absorbent when it is a bit damp, whereas dry soils are almost impermeable. Even though a rainstorm after a drought can seem like a godsend, when extreme precipitation follows extreme heat, the water droplets may fail to penetrate the desperately dry soil and instead quickly run off, causing flooding and potentially landslides.

Rising temperatures will likely cause a shift from snow to rain in the spring and fall seasons. As a result, spring freshets (i.e., annual snow or ice melt) are expected to happen earlier, which may lower peak annual river flows and lead to decreases in fluvial flooding. However, there remains uncertainty on how extreme precipitation, which frequently accompanies or contributes to freshet-related flooding, may also affect its frequency and severity.

Rising temperatures bring rising sea levels

In addition to influencing the frequency and severity of extreme precipitation, climate change has also impacted coastal flooding through rising sea levels and changes in storm activity. Global mean sea levels have increased by about 21 cm since the late 19th century and are projected to rise by an additional 28 to 98 cm during this century due largely in part to the thermal expansion of oceans and melting land ice.

While a global phenomenon, Canada’s exposure to sea level rise varies significantly across the country, owing to differences in vertical land motion. For regions where the land is experiencing uplift, such as in Hudson Bay, relative sea level is projected to fall. However, in parts of the country that are sinking naturally due to land subsidence, such as in the Maritimes and portions of British Columbia, the effects of global sea level rise will be compounded. These areas will see a greater escalation in both the frequency and severity of extreme high-water-level events caused by storm surges, waves, and high tides.

*Graphic representation of sea-level change is by approximation only and not exact

Exceptionally high tide events can cause tidal flooding, also called nuisance flooding, where coastal areas are inundated temporarily. High-tide floods are expected to increase significantly in the mid-2030s, as the effects of climate change will be amplified by a lunar cycle that occurs approximately every 19 years. When this precession effect combines with the rising sea levels caused by climate change, Costal communities may expect nuisance flooding to occur in clusters — in some places, potentially 10 to 15 floods a month.

Coastal Atlantic communities will also see increases in storm surge flooding, caused by stronger hurricanes. Although hurricanes are only occasionally responsible for devastating levels of damage in Canada due to its cooler waters offshore, ocean warming may lead to increasing intensity of storms. In September 2022, Canada was hit by post-tropical cyclone Fiona — its costliest and most intense cyclone on record. As ocean temperatures continue to rise, the likelihood Canada will experience similar storms increases.

What to do in the meantime

In the face of increasing climate risks, it is crucial to take steps to protect your business before disaster strikes. Leveraging climate experts, such as the Marsh Climate and Sustainability Team, can help you assess physical risks brought on by climate change, including riverine, rainfall, and coastal flooding, severe wildfires, and record-breaking heat waves. In the changing regulatory landscape, Marsh has helped clients with climate transition risk assessments and Task Force on Climate-Related Financial Disclosures (TCFD) reporting.

To effectively respond to the changing physical climate risk landscape, organizations require data-led insights. By combining our property engineers’ asset-level knowledge with our climate modeling expertise, Marsh Advisory can help assess your facility’s flood vulnerability and sensitivity, and propose adaptation measures to reduce these risks now and in the future.

The flood risk in Canada will no doubt be exacerbated by climate change in the coming decades, but by taking proactive action and collaborating with experts, this risk can be controlled and minimized.

In part three of our series, we will dive deeper into how you can improve your business’s flood resilience.

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George Fan

Vice President, Senior Risk Consultant, Marsh Advisory