The latest update to the five-year moving average of the Actuarial Climate Index (ACI), which measures changes in extreme weather conditions and sea levels, shows a slight decline in summer 2025 compared with the previous spring.
The five-year average now stands at 1.41, compared with a baseline average of zero during the reference period from 1961 to 1990.
In the previous update released in December 2025 for the spring period, the ACI stood at 1.43.
“Although the five-year average temperature anomalies increased, the five-year wind and sea-level anomalies decreased, the net effect being a slight drop in the composite index,” the organization responsible for calculating the ACI says in a statement published on March 5.
Comparison with summer 2020
The five-year moving average of the ACI was revised following the addition of the seasonal data for summer 2025. That seasonal reading reached 1.53 for Canada and the United States.
Because the moving average is calculated over five years and updated quarterly, the summer 2025 data replaces the summer 2020 reading in the calculation of the five-year average. In 2020, the summer value stood at 1.92, while the five-year index was 1.24.
When ACI averages rise above the zero baseline, it indicates that the frequency of extreme climate events is increasing.
Combination of variables
Quarterly seasonal data are collected to determine the index. The calculation combines the variables that have the greatest impact on populations and the economy: high and low temperatures, heavy precipitation, drought, strong winds and sea level.
For the sea-level sub-index, the five-year moving average for Canada and the United States stood at 2.93 in summer 2025, compared with 2.97 in the previous quarter. In summer 2020, the same sub-index averaged 2.64.
