Wildfire smoke blanketed skies across North America, hail caused significant damage in Calgary, flash floods swept through parts of Manitoba and a tropical storm threatened Atlantic Canada – all in the span of just a few weeks during the summer season. These events reflect a new reality: Hydroclimatic extremes are increasingly common, disrupting lives and livelihoods in Canada and around the world.
But what are hydroclimatic extremes, and how is climate change accelerating their intensity? Engineering professor Reza Najafi studies these events. He spoke with Western News about their impacts and whether we’re prepared for what lies ahead.
What are hydroclimatic extremes?
Reza Najafi: Hydroclimatic extremes refer to extreme weather and climate events linked to the movement of water and energy. Common examples include floods, droughts, heat waves, extreme winds, tornadoes and hailstorms. Importantly, these hazards can also interact, creating what we call compound events, where multiple extremes occur together or in sequence, amplifying their overall impact.
Is there increased risk associated with compound events?
RN: The combined impact of compound events is generally much more severe than those of individual events. Think of rain on saturated soil. If the soil is already saturated, whether from snowmelt or several days of rain, then additional rain produces more runoff and a faster rise in water levels, resulting in flooding and less lead time for communities to respond. Other examples include storm surges coinciding with heavy rainfall, or a heat wave followed by intense convective storms, such as thunderstorms, over very dry soils.
What kind of extremes can Canada expect in the coming years?
RN: Canada’s diverse climate and geography make it vulnerable to a wide range of natural hazards. Flooding, droughts, heat waves and wildfires are already being observed more frequently, with many communities experiencing increasingly severe impacts.
For hazards such as hail, tornadoes and other severe storms, long-term projections remain uncertain. However, the underlying ingredients for severe storms, such as higher atmospheric moisture and instability, are projected to increase in many regions, while changes in wind shear (speed and direction) are expected to vary by location. The key message is that Canada’s past climate is no longer a reliable guide for future risk.
Reza Najafi (Jake Arts/Western Engineering)
How is climate change impacting hydroclimatic extremes?
RN: Western Canada experienced an unprecedented heat wave in July 2021 that fueled devastating wildfires. Then the following November, the province faced severe flooding linked to an atmospheric river. Attribution studies show human-caused climate change significantly increased the likelihood and intensity of both events.
These examples illustrate how a warming climate exacerbates multiple hazards at once. Warmer air speeds up evaporation from soils and vegetation, drying fuels and raising drought and wildfire risk. At the same time, a warmer atmosphere holds about seven per cent more water vapour per degree Celsius (the Clausius–Clapeyron relationship). This often translates into heavier short-duration downpours and greater flash flood risk. Changes in atmospheric circulation and large slow moving weather systems, also known as blocking patterns, can further lock regions into prolonged wet or dry spells, amplifying extremes. These processes are driving more frequent, intense and compound hydroclimatic extremes.
Are we prepared?
RN: Scientifically, we have made significant advancements in understanding the climate and hydroclimate system. Many initiatives are underway to improve resilience and preparedness. The challenge is whether these efforts are keeping pace with the rapidly changing risks. Much of Canada’s infrastructure was designed for historical climate conditions, and with aging systems and outdated design standards, the pace of adaptation is not fast enough. Even where planning and preparation are in place, each new extreme event reveals vulnerabilities across multiple sectors.
What can be done?
RN: Preparing for hydroclimatic extremes requires action at multiple levels, from households to municipalities to entire regions. No single measure is enough. Resilience comes from combining infrastructure upgrades, nature-based solutions and community preparedness.
For flooding, our review of 11 cities worldwide, including Toronto and Ottawa, found a strong emphasis on Low Impact Development (LID) strategies, such as green roofs, rain gardens and permeable pavements, that help slow, store and infiltrate runoff. At the municipal scale, cities are moving toward Blue-Green Infrastructure, which integrates natural and engineered systems. Copenhagen’s “cloudburst” design is a well-known example, using parks and streets to temporarily store stormwater with controlled release or reuse. To keep pace with climate change, municipalities are beginning to update rainfall design standards so that stormwater systems reflect projected future conditions rather than relying only on historical data.
When extremes do occur, preparedness is critical. Following official alerts, knowing evacuation routes, avoiding floodwaters and seeking shelter during severe storms can reduce risk. During heat waves and droughts, conserving water, accessing cooling spaces and checking on vulnerable neighbours are vital steps. Prepared households, supported by up-to-date emergency plans, consistently save lives.
