The latest update of the five-year moving average of the Actuaries Climate Index (ACI), which tracks changes in extreme weather conditions and sea levels, shows an increase in spring 2025 compared to the previous winter.
The average now stands at 1.43, compared to a baseline average of zero for the reference period spanning 1961 to 1990.
In the last update released in August 2025 for the winter season, the ACI was at 1.37. “The increase is due to shifts in nearly all metrics, including continuous dry days, precipitation, sea level, and frequency of temperatures above the 90th percentile and below the 10th percentile,” notes the press release issued on December 2.
Comparison with spring 2020
The five-year moving average of the ACI has been revised following the addition of spring 2025 seasonal data. This data reached 1.68 for Canada and the United States.
Since the moving average is calculated over five years and updated quarterly, the spring 2025 data replaces the spring 2020 data in the average. In spring 2020, the seasonal data stood at 0.61 and the five-year index was 1.20.
When ACI averages rise above the baseline of zero, it indicates that the frequency of extreme weather events is increasing and that sea levels are continuing to rise.
Combination
Seasonal quarterly data are collected to establish the index. Variables with the greatest impact on the population and the economy are used: high and low temperatures, heavy rainfall, drought, high winds, and sea level.
For the sea level sub-index, the five-year moving average for Canada and the United States reached 2.75 in spring 2024, compared to 3.07 in the previous quarter. In spring 2020, the same sub-index stood at 2.07.
Reactions
Actuary Yves Guérard served for several years on the joint committee that publishes the ACI. For several months, he has been in discussions with the Canadian Institute of Actuaries to make ACI-related publications more transparent.
“The ACI is a statistical measure that gauges the probability of climate change compared to the average observed during the 1961–1990 reference period. A value of 1.43 represents the deviation from that average. Given that the standard deviation is 0.45, the probability of rejecting the hypothesis of climate change is 0.0742 per cent,” he wrote to the Insurance Portal.
Guérard also adds that the statistical data confirm that global warming “is not a hoax” and that climate risks will continue to increase due to the cumulative impact of greenhouse gas (GHG) emissions in the atmosphere.
For his part, Alain Bourque, executive director of the Ouranos Consortium, tells the Insurance Portal that “it would be helpful to see scientists take a closer look to evaluate whether the weighting of the various components makes sense and reflects real impacts. Unfortunately, this is not the kind of work the scientific community tends to undertake.”
Bourque adds that universal indicators such as heat, cold, and sea level “are evolving in predictable ways.” However, he notes that more complex indicators, such as extreme precipitation and wind, deserve closer scrutiny. These “can be much more sensitive to the location of measurement stations” and to how representativeness is weighted across a territory.
Large-scale interpretations of the ACI “appear consistent with observed climate change, but more nuanced interpretations are possible for someone who understands the strengths and limitations of the various indicators,” he adds.
