Low Confidence prevents ascribing Heavy Precipitation causing Canada Floods to climate change

Press Summary 

• Depressions similar to those producing the Canada Floods show locally increased  precipitation (0-6 mm/day) while most of the region analysed does not show significant changes in precipitation. 

• This was a very uncommon event.

• Although natural climate variability likely played a modest role, for this event, we have  medium-low confidence in the robustness of our approach given the available climate data and that changes detected are local and highly dependent on the region chosen to perform the analysis.

Event Description

On August 18th, 2024, extreme weather in parts of Canada led to significant flooding and power outages, affecting areas such as Toronto, Mississauga, and North Dumfries Township. Environment Canada had issued weather alerts, including rainfall warnings and a severe thunderstorm watch. In Toronto, heavy rainfall persisted throughout the day, with amounts ranging from 100 to 200 mm. Mississauga also faced continuous heavy rain, expected to last until Sunday. In the Waterloo Region, emergency services responded to a tornado warning after reports of a tornado touching down in North Dumfries Township around 11 a.m. The storm caused property damage, downed trees, and disrupted power for about 3,000 customers in Ayr. Fortunately, no injuries were reported. Mississauga firefighters assisted pedestrians in flooded areas, and several roads were closed due to localized flooding. Additionally, Toronto Pearson Airport experienced service interruptions due to the weather.

The Surface Pressure Anomalies show a negative (cyclonic) anomaly, centred over southern Ontario. This depression was the main driver of the storms that caused the Canada floods. Temperature anomalies show positive values over the same areas affected by negative pressure anomalies, with up to 3 °C in southeastern Canada. Precipitation data show several areas of accumulation across Canada, with daily values up to about 20 mm. Wind speed data show that most of the domain was affected by low or  moderate winds..

Climate and Data Background for the Analysis

The IPCC AR6 report highlights that climate change is likely to lead to more intense and frequent precipitation in Canada, particularly in the form of heavy rainfall events. This shift is expected to contribute to an increased risk of flooding, especially in regions where the infrastructure may not be prepared for such extremes. IPCC AR6 Chapter 11 emphasises that as temperatures rise, the atmosphere can hold more moisture, leading to heavier and more sustained rainfall during storms.

In Canada, this trend is concerning for areas that are already prone to flooding. The report notes that the combination of increased precipitation, snowmelt, and the potential for more extreme weather events could result in more frequent and severe floods. This situation poses significant risks to communities, infrastructure, and ecosystems across the country, underlining the need for adaptation and mitigation strategies to address the growing threat of climate-induced flooding.

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For this event, we have  medium-low confidence in the robustness of our approach given the available climate data, as the event is similar to other past events in the data record.

ClimaMeter Analysis

We analyze here (see Methodology for more details) how events similar to the low pressure system leading to the Canada Floods changed in the present (2001–2023) compared to what they would have looked like if they had occurred in the past (1979–2001) in the region [84°W 74°W 40°N 45°N]. The Surface Pressure Changes show no significant changes. The Temperature Changes show up to +2°C warmer conditions in the present than in the past. Warmer temperatures typically lead to heavier precipitation, as a warmer atmosphere can hold more moisture.  Precipitation Changes  show that significant increases (ranging from 0 to 6 mm/day) are observed only in limited areas. In most of the regions analyzed, changes in precipitation patterns remain statistically non-significant, suggesting that the impact of climate change on precipitation may vary widely across different parts of Canada. Windspeed Changes indicate no significant changes. We also note that Similar Past Events occur with about the same seasonality in the past and present periods. Changes in Urban Areas reveal that Toronto, Mississauga and Barrie are 0 to 3 mm/day wetter in the present compared to the past.  The changes are however non statistically significant.

Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal Oscillation may have influenced the changes in this event. This suggests that the changes we see in the event compared to the past may be due to human driven climate change, with a  minor contribution from natural variability.

Conclusion

Based on the above, we conclude that depressions similar to those producing Canada Floods show  locally increased  precipitation (0-6 mm/day) while most of the region analysed does not show significant changes in precipitation.  Although natural climate variability likely played a modest role, for this event, we have  medium-low confidence in the robustness of our approach given the available climate data and that changes detected are local and highly dependent on the domain chosen.

Contact Authors

-Gianmarco Mengaldo NUS, Singapore 📨mpegim@nus.edu.sg  🗣️Italian, English, 

-Davide Faranda, IPSL-CNRS, France 📨davide.faranda@lsce.ipsl.fr 🗣️French, Italian, English