The Actuaries Climate Index® (ACI) is intended to provide a useful monitoring tool—an objective indicator of the frequency of extreme weather and the extent of sea level change. This website provides graphics and data for download for those who wish to explore the Index. The ACI is available for the United States and Canada and 12 subregions thereof, and will be released when analysis of data for each meteorological season is complete, on both a monthly and a seasonal basis (months ending February, May, August, and November).
In April of 2019, a change was made to the ACI calculation methodology. The methodology used to produce releases prior to April 2019 is referred to as “ACI 1.0”, while the methodology currently in use is referred to as “ACI 1.1”. A description of the change of methodology, and the rationale for the change, can be found in the data disclosure.
The six components of the Actuaries Climate Index are:
- High temperatures;
- Low temperatures;
- Heavy rainfall;
- Drought (consecutive dry days);
- High wind; and
- Sea level.
The temperature components are defined as the change in frequency of warmer temperatures above the 90thpercentile (T90) and of colder temperatures below the 10th percentile (T10), relative to the reference period of 1961 to 1990. As temperatures are warming over the United States and Canada in recent decades, T10 is generally less than it was during the reference period; i.e., the change is a negative number, while the change in T90 is generally a positive number. To properly reflect this change in the temperature distribution, the sign of T10 is reversed in the Actuaries Climate Index to properly reflect its contribution to this shift. An increased value of the Index due to the reduction in cold extremes is consistent with an increased risk of perils due to melting permafrost, the propagation of diseases, and the population of pests and insects that were previously less likely to survive in lower temperatures.
The precipitation components are the maximum 5-day rainfall (P) in the month, which measures flood risk, and the maximum number of consecutive days in a year with less than 1mm of daily precipitation, which measures drought (D). As with each of the other components, differences between the 5-day rainfall maxima and the consecutive dry days and their respective average values in the reference period are calculated for each month, with the latter being approximated by interpolating the annual values.
Daily wind speed measurements are converted to wind power (W), which is proportional to the cube of the wind speed. Wind Power is used because impacts from high winds (i.e., damages) have been shown to be more closely related to the cube of wind speed. The procedure used for temperatures is followed, by finding the 90th percentile of wind power for each month or season and subtracting the 90th percentile of wind power for that month or season over the reference period.
Sea level measurements are available on a monthly basis via tide gauges located at permanent coastal stations in Canada and the United States. The tide gauges measure sea level relative to the land below, but because the land is moving in many places, the ACI sea level component measures the combined effect on coastal shorelines of the generally rising seas and the rising or falling land.
For the purpose of combining the six components, the monthly differences versus the reference period are divided by the reference period standard deviation. This ratio is a dimensionless quantity known as the standardized anomaly. The approach allows such inherently different quantities to be combined in a single index while preserving the accuracy of the components. For any individual indicator, the standardized anomaly corresponds to how unusual that month’s/season’s value is, compared to the reference period mean and standard deviation for that month/season. Hence, each component is in units of the standard deviation of that quantity.
The final expression for the ACI, then, is:
ACI = mean(T90std – T10std + Pstd + Dstd + Wstd + Sstd)