The potential impact of climate change on annual and seasonal mortality for three cities in Québec, Canada

Abstract

Background: The impact of climate change and particularly increasing temperature on mortality has been examined for three cities in the province of Québec, Canada.

Methods: Generalized linear Poisson regression has been fitted to the total daily mortality for each city. Smooth parametric cubic splines of temperature and humidity have been used to do nonlinear modeling of these parameters. The model, to control for day of the week and for non-temperature seasonal factors, used a smooth function of time, including delayed effects. The model was then used to assess variation in mortality for simulated future temperatures obtained from an atmospheric General Circulation Model coupled with downscaling regression techniques. Two CO2 emission scenarios are considered (scenarios A2 and B2). Projections are made for future periods around year 2020 (2010-2039), 2050 (2040-2069) and 2080 (2070-2099).

Results: A significant association between mortality and current temperature has been found for the three cities. Under CO2 emission scenarios A2 and B2, the mortality model predicts a significant increase in mortality in the summertime, and a smaller, but significant decrease in the fall season. The slight variations in projected mortality for future winter and spring seasons were found to be not statistically significant. The variations in projected annual mortality are dominated by an increase in mortality in the summer, which is not balanced by the decrease in mortality in the fall and winter seasons. The summer increase and the annual mortality range respectively from about 2% and 0.5% for the 2020 period, to 10% and 3% for the years around 2080. The difference between the mortality variations projected with the A2 or B2 scenarios was not statistically significant.

Conclusion: For the three cities, the two CO2 emission scenarios considered led to an increase in annual mortality, which contrasts with most European countries, where the projected increase in summer mortality with respect to climate change is overbalanced by the decrease in winter mortality. This highlights the importance of place in such analyses. The method proposed here to establish these estimates is general and can also be applied to small cities, where mortality rates are relatively low (ex. two deaths/day).

Figure 1

Mortality-temperature relationship for Montréal, Québec City and Saguenay. These graphs illustrate the relationship obtained between mortality and average daily temperature for the cities of Montréal, Québec City and Saguenay. The top graphs come from the model in which the confounding factors are disregarded (it is assumed that C = 0 in equation 2). The bottom graphs correspond to the case in which the confounding factors are taken into account (term C is added to the model with λ_j = 7 d.f./year).

Figure 2

Mortality projections under scenarios A2 and B2 for Montréal, Québec City and Saguenay. This figure presents the variations in summer and annual mortality predicted for scenarios A2 and B2 in the cities of Montréal, Québec City and Saguenay. The variations are expressed as a percentage (%) of the historical mortality for the 1981–1999 period. The confidence interval (95%) is also given. The difference between the projections of scenarios A2 and B2 is seen to be not significant.

Figure 3

Seasonal mortality projections under scenario A2 for Montréal using monthly and yearly future simulated temperature anomalies. Seasonal mortality variations are presented for the city of Montréal. These variations are estimated using future temperature data obtained by adding monthly or yearly simulated temperature anomalies (A2 scenario) to historical data. Monthly or yearly temperature anomalies lead to different variations since the mortality-temperature relationship is nonlinear. Mortality variations are expressed as a fraction of the average historical mortality from 1981 to 1999. Confidence bands (95%) are illustrated with the vertical lines.

Figure 4

Québec with a few geographic references.