Extreme weather event attribution

Extreme weather event attribution is the science of calculating how much human-caused climate change influences extreme weather events. The Rapid Extreme Weather Event Attribution system shows how human-caused climate change changes the likelihood of extreme weather events happening. It examines the extreme heat, cold, and precipitation events in regions covering all of Canada. 

The system uses climate models to compare three 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 
-a projected future climate reflecting additional global warming.
Several days after a heat wave, extreme cold, or extreme precipitation event, scientists can compare the number of those extreme events in the three different climates. Then, they calculate the ratios of the three climates to find how much human activity has changed the chances of such an event happening.

We use seven set statements to describe the likelihood that an extreme weather event was influenced by climate change from human activities. 
On one end of the scale, the event was:
•	far less likely (at least 10X less likely) because of human influence on the climate
•	much less likely (at least 2X to 10X less likely) because of human influence on the climate
•	less likely (at least 1X to 2X less likely) because of human influence on the climate
In the middle:
•	no evidence (no change) of an attributable change in likelihood
On the other end of the scale, the event was:
•	more likely (at least 1X to 2X more likely) because of human influence on the climate
•	much more likely (at least 2X to 10X more likely) because of human influence on the climate
•	far more likely (at least 10X more likely) because of human influence on the climate
Events that are more, much more, or far more likely, happen more often today or in the future than they did in the pre-industrial era (1850-1900). Events that are less, much less, or far less likely are becoming more rare.

Extreme heat: 
During the warm season, we share detailed analysis about the most intense heat waves in 17 regions covering Canada. The results show how human-caused climate change influenced the likelihood of heat waves. Each analysis shows:
•	the dates of each analysis period
•	the peak daily high temperature averaged over the region during the heat wave
•	degrees above normal daily high temperature
•	a statement of current likelihood: the event was xxx likely because of human influence on the climate, in today's climate compared to the pre-industrial climate
•	a statement of future likelihood: the event would be xxx likely because of human influence on the climate, in an even warmer future climate compared to the pre-industrial climate.
We determine the normal daily temperature from an average of the historical temperatures (1991-2020) of the month around the event.

Extreme cold:
During the cold season, we share detailed analysis about the coldest extreme temperatures in 17 regions covering Canada. The results show how much human-caused climate change influenced the coldest extreme temperatures. Each analysis shows:
•	the dates of each analysis period
•	the coldest daily temperature averaged over the region during the extreme cold event
•	degrees below normal daily low temperature
•	a statement of current likelihood: the event was xxx likely because of human influence on the climate, in today's climate compared to the pre-industrial climate
•	a statement of future likelihood: the event would be xxx likely because of human influence on the climate, in an even warmer future climate compared to the pre-industrial climate.
We determine the normal daily temperature from an average of the historical temperatures (1991-2020) of the month around the event.

Extreme precipitation:
Throughout the year, we share detailed analysis about the most intense precipitation events in 17 regions (subdivided into 89 smaller sections) covering Canada. The results show how human-caused climate change influenced the likelihood of these events. Each analysis shows:
•	the date of each event
•	the total accumulated precipitation averaged over the region during the event
•	the percentage of normal monthly precipitation that accumulated during the event
•	a statement of current likelihood: the event was xxx likely because of human influence on the climate, in today's climate compared to the pre-industrial climate
•	a statement of future likelihood: the event would be xxx likely because of human influence on the climate, in an even warmer future climate compared to the pre-industrial climate.
We determine normal monthly precipitation using historical data. It is an average of total precipitation accumulations for the month of the event (over 1991-2020).

For information on the regions, see reference by Stone (2019).

Visit Environment and Climate Change Canada’s Extreme Weather Event Attribution website to learn more: https://www.canada.ca/en/environment-climate-change/services/climate-change/science-research-data/extreme-weather-event-attribution.html

Supplemental Information

Information on the interpretation of the dataset can be found in the following publication: https://doi.org/10.1007/s10584-019-02479-6

Organization: Environment and Climate Change Canada | Environnement et Changement climatique Canada