Extreme weather event attribution

Extreme weather events

An extreme weather event is when we get unusual or unseasonal extremes compared to historical data. Extreme weather events include:

• heat waves, which can drive wildfires and drought
• extreme rainfall, which can cause flooding
• extreme cold

Human-caused climate change

Human activities, mainly greenhouse gas emissions, are causing the global climate to change. This leads to changes including:

• more extreme heat
• less extreme cold
• shorter snow and ice-cover seasons
• thinning glaciers
• thawing permafrost^{Footnote 1} 

Our warming environment affects the strength, number, and risks of extreme weather events.

Why we calculate the effects of climate change on extreme weather

When we calculate how much human-caused climate change affects extreme weather events, we:

• help educate Canadians about the importance of continuing to fight climate change
• give Canadians more information to use to prepare for and adapt to the risks and effects of a changing climate

This information helps us plan for, respond to, and rebuild from weather emergencies. It also supports informed decision making to protect health, safety, and property.

The science of extreme weather event attribution

Extreme weather event attribution brings together meteorology, weather observation, and climate science. Climate scientists study how human-caused climate change is linked to the frequency and severity of extreme weather events.

Research on past extreme weather event attribution

As part of our mandate, we conduct and share the results of extensive climate science research in Canada and internationally.

Our research helps decision-makers, leaders, and professionals to make science-based, informed decisions.

Canada’s National Adaptation Strategy is the long-term vision to reduce the damaging effects and risks of climate-related disasters. Our goal is to help prepare our communities for the effects of climate change and give them information that helps protect their health, well-being, and livelihoods.

A pilot system for rapid heat wave attribution

The pilot system can analyse the link between a recent heat wave in Canada and human-caused climate change.  In the future, the system will also include extreme cold weather events and extreme precipitation.

How it works: comparing two different climates

The Rapid Extreme Weather Event Attribution pilot system uses climate models to represent two different climates:

• the climate of the 1800s, based on levels of atmospheric gases that existed before the Industrial Revolution
• the climate of today, based on observed levels of greenhouse gases and other results of human activity

Several days after a heat wave, scientists can compare the number of heat waves in the two different climates.  Then, they calculate the difference between the two to find how much human activity has changed the chances of such a heat wave happening.^{Footnote 2}

Plans for the pilot stage

During this pilot stage, scientists will analyze the temperatures in 17 regions covering all of Canada.

Regions

Importance of the pilot system

When we can understand the causes and calculate the risks of extreme events such as heat waves, wildfires, drought, and floods, we can:

• better plan for, respond to, and rebuild from weather emergencies
• support informed decision making to protect health, safety, and property
• inspire environmental learning
• help Canadians deal with biodiversity loss, pollution, and climate change

Statement of likelihood

We use seven set statements to describe how much more likely an extreme weather event was because of human influence on the climate.

On one end of the scale, the extreme weather event was:

• far less likely because of human influence on the climate
• much less likely because of human influence on the climate
• less likely because of human influence on the climate

In the middle:

• no evidence of an attributable change in likelihood for this event

On the other end of the scale, the extreme weather event was:

• more likely because of human influence on the climate
• much more likely because of human influence on the climate
• far more likely because of human influence on the climate

Probability/risk range

The probability/risk range is a scale that describes how much more likely it was for an extreme weather event to happen because of human influence on the climate.

On one end of the scale, if the event was:

• far less likely: at least 10X less likely to have happened
• much less likely: at least 2X to 10X less likely to have happened
• less likely: at least 1X to 2X less likely to have happened due to human influence on the climate

In the middle, if there was:

• no evidence: no change due to human influence on the climate identified

On the other end of the scale, if the event was:

• more likely: at least 1X to 2X more likely to have happened
• much more likely: at least 2X to 10X more likely to have happened
• far more likely: at least 10X more likely to have happened due to human influence on the climate

Graphs and visuals

We also use a variety of graphs to demonstrate this work visually.

Probability distribution graphs

Probability distribution graphs can help illustrate how much more or less likely an extreme weather event was made because of human influence on the climate.

The first curve (blue) shows the likelihood of temperatures in the past.

The dots that shade the end of the blue curve show the chances of an extreme event occurring in the past.

The second curve (red) shows the likelihood of temperatures today.

The dots and lines that shade the end of the red curve show the chances of an extreme event happening today.

When we compare the two curves, we see the climate of today, the red curve, has shifted to the right. This shift shows that we have more chances of extreme heat.

*Illustrative. Not an actual probability distribution graph.

Atlantic Canada

• Dates: August 24 to August 27, 2024
• Peak daily high temperature during the heat wave: 23.9 °C
• Degrees above normal daily high temperature: 6.7 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Eastern Ontario

• Dates: July 31 to August 2, 2024
• Peak daily high temperature during the heat wave: 29.2 °C
• Degrees above normal daily high temperature: 5.8 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Fort Smith

• Dates: August 8 to August 13, 2024
• Peak daily high temperature during the heat wave: 26.9 °C
• Degrees above normal daily high temperature: 9.2 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Inuvik

• Dates: August 6 to August 10, 2024
• Peak daily high temperature during the heat wave: 26.5 °C
• Degrees above normal daily high temperature: 13.0 °C*
• Statement of likelihood: The event was far more likely because of human influence on the climate.

Kitikmeot

• Dates: August 7 to August 12, 2024
• Peak daily high temperature during the heat wave: 25.0 °C
• Degrees above normal daily high temperature: 12.4 °C*
• Statement of likelihood: The event was far more likely because of human influence on the climate.

Kivalliq

• Dates: August 8 to August 14, 2024
• Peak daily high temperature during the heat wave: 27.1 °C
• Degrees above normal daily high temperature: 12.2 °C*
• Statement of likelihood: The event was far more likely because of human influence on the climate.

Manitoba

• Dates: July 30 to August 1, 2024
• Peak daily high temperature during the heat wave: 28.3 °C
• Degrees above normal daily high temperature: 6.4 °C*
• Statement of likelihood: The event was more likely because of human influence on the climate.

Northern British Columbia

• Dates: August 7 to August 11, 2024
• Peak daily high temperature during the heat wave: 23.6 °C
• Degrees above normal daily high temperature: 7.0 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Northern Quebec

• Dates: August 13 to August 16, 2024
• Peak daily high temperature during the heat wave: 23.6 °C
• Degrees above normal daily high temperature: 7.8 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Southern Quebec

• Dates: July 27 to August 3, 2024
• Peak daily high temperature during the heat wave: 28.3 °C
• Degrees above normal daily high temperature: 7.6 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Yukon

• Dates: August 5 to August 9, 2024
• Peak daily high temperature during the heat wave: 24.1 °C
• Degrees above normal daily high temperature: 8.9 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

* Normal daily high temperature = average of high temperatures observed from 1991-2020 in the month surrounding the heat wave, for that region.

Atlantic Canada

• Dates: July 28 to July 31, 2024
• Peak daily high temperature during the heat wave: 26.1 °C
• Degrees above normal daily high temperature: 6.7 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Alberta:

• Dates: July 7 to 11, 2024
• Peak daily high temperature during the heat wave: 31.5 °C
• Degrees above normal daily high temperature: 9.8 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Fort Smith:

• Dates: July 15 to July 20, 2024
• Peak daily high temperature during the heat wave: 28.4 °C
• Degrees above normal daily high temperature: 9.2 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Northern British Columbia:

• Dates: July 17 to 22, 2024
• Peak daily high temperature during the heat wave: 24.2 °C
• Degrees above normal daily high temperature: 7.2 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Northern Quebec:

• Dates: July 28 to 30, 2024
• Peak daily high temperature during the heat wave: 24.2 °C
• Degrees above normal daily high temperature: 7.0 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Southern British Columbia:

• Dates: July 14 to 22, 2024
• Peak daily high temperature during the heat wave: 29.2 °C
• Degrees above normal daily high temperature: 9.2 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Saskatchewan:

• Dates: July 17 to 22, 2024
• Peak daily high temperature during the heat wave: 30.9 °C
• Degrees above normal daily high temperature: 8.0 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Western Ontario:

• Dates: July 26 to July 28, 2024
• Peak daily high temperature during the heat wave: 27.9 °C
• Degrees above normal daily high temperature*: 6.1 °C*
• Statement of likelihood: The event was more likely because of human influence on the climate.

Yukon:

• Dates: July 21 to July 26, 2024
• Peak daily high temperature during the heat wave: 23.0 °C
• Degrees above normal daily high temperature: 6.5 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

* Normal daily high temperature = average of high temperatures observed from 1991-2020 in the month surrounding the heat wave, for that region.

Atlantic Canada

• Dates: June 18 to 20, 2024
• Peak daily high temperature during the heat wave: 26.1 °C
• Degrees above normal daily high temperature: 10.6 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Eastern Ontario

• Dates: June 17 to 19, 2024
• Peak daily high temperature during the heat wave: 29.0 °C
• Degrees above normal daily high temperature: 7.4 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Fort Smith

• Dates: June 29 to July 1, 2024
• Peak daily high temperature during the heat wave: 24.9 °C
• Degrees above normal daily high temperature: 6.7 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Inuvik

• Dates: June 28 to 30, 2024
• Peak daily high temperature during the heat wave: 22.0 °C
• Degrees above normal daily high temperature: 7.2 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Kivalliq

• Dates: June 30 to July 2, 2024
• Peak daily high temperature during the heat wave: 22.1 °C
• Degrees above normal daily high temperature: 7.5 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Northern Quebec

• Dates: June 18 to 19, 2024
• Peak daily high temperature during the heat wave: 21.5 °C
• Degrees above normal daily high temperature: 7.2 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Southern Quebec

• Dates: June 18 to 20, 2024
• Peak daily high temperature during the heat wave: 29.5 °C
• Degrees above normal daily high temperature: 10.7 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

Yukon

• Dates: June 26 to 30, 2024
• Peak daily high temperature during the heat wave: 21.4 °C
• Degrees above normal daily high temperature: 5.3 °C*
• Statement of likelihood: The event was much more likely because of human influence on the climate.

* Normal daily high temperature = average of high temperatures observed from 1991-2020 in the month surrounding the heat wave, for that region.

News releases

• October 25, 2024 - Climate change sets and breaks heat records across Canada this summer
• September 25, 2024 - Climate change fuels August heat waves across Canada
• August 12, 2024 - Science shows climate change made summer heat waves much more likely
• July 9, 2024 – Canada releases first results from rapid extreme weather event attribution system
• June 11, 2024 – Environment and Climate Change Canada presents summer seasonal outlook and outlines new heat wave attribution system

Resources for climate research

• Climate change science, research, and data
• Canadian Climate Normals

Resources for extreme weather events

• Prepare for weather events
• Wildfire smoke, air quality and your health: Overview - Canada.ca
• Air Quality Health Index - Canada.ca
• Prepare for floods (getprepared.gc.ca)
• Canadian Drought Outlook - agriculture.canada.ca
• Severe weather - Canada.ca
• Seasonal weather hazards - Canada.ca

Resources for adapting to climate change

• Canada’s National Adaptation Strategy: Building Resilient Communities and a Strong Economy
• Causes of climate change - Canada.ca