Evolution of Minimum Mortality Temperature in Stockholm, Sweden, 1901-2009

Abstract

Background: The mortality impacts of hot and cold temperatures have been thoroughly documented, with most locations reporting a U-shaped relationship with a minimum mortality temperature (MMT) at which mortality is lowest. How MMT may have evolved over previous decades as the global mean surface temperature has increased has not been thoroughly explored.

Objective: We used observations of daily mean temperatures to investigate whether MMT changed in Stockholm, Sweden, from the beginning of the 20th century until 2009.

Methods: Daily mortality and temperature data for the period 1901-2009 in Stockholm, Sweden, were used to model the temperature-mortality relationship. We estimated MMT using distributed lag nonlinear Poisson regression models considering lags up to 21 days of daily mean temperature as the exposure variable. To avoid large influences on the MMT from intra- and interannual climatic variability, we estimated MMT based on 30-year periods. Furthermore, we investigated whether there were trends in the absolute value of the MMT and in the relative value of the MMT (the corresponding percentile of the same-day temperature distribution) over the study period.

Results: Our findings suggest that both the absolute MMT and the relative MMT increased in Stockholm, Sweden, over the course of the 20th century.

Conclusions: The increase in the MMT over the course of the 20th century suggests autonomous adaptation within the context of the large epidemiological, demographical, and societal changes that occurred. Whether the rate of increase will be sustained with climate change is an open question.

Citation: Oudin Åström D, Tornevi A, Ebi KL, Rocklöv J, Forsberg B. 2016. Evolution of minimum mortality temperature in Stockholm, Sweden, 1901-2009. Environ Health Perspect 124:740-744; http://dx.doi.org/10.1289/ehp.1509692.

Figure 1

Estimates of minimum mortality temperature (MMT) during 1901–2009 in Stockholm, Sweden. Filled circles indicate the absolute value of the estimated MMT (°C), and open triangles indicate the corresponding estimate of the MMT as a percentage of the temperature distribution. Estimates were derived using distributed lag non-linear models of 30-year moving averages with a 21-day lag, adjusted for weekdays, holidays, and smoothed time trends (8 degrees of freedom per year). The smoothed evolution of the MMT was examined by repeating the model stepwise across the century while including 30 years of data in each model and changing the time period of study by 1 year at a time. Thus, the first data point created for calculating the MMT corresponds to observations for 1901–1930, which was centered around the year 1915; the second corresponds to observations for 1902–1931 (centered around the year 1916); and so forth until the last data point, which corresponds to 1980–2009.

Figure 2

Independently estimated minimum mortality temperatures (MMTs) during 1901–2009 in Stockholm, Sweden. Filled circles indicate the absolute value of the estimated MMT (°C), and open triangles indicate the corresponding estimate of the MMT as a percentage of the temperature distribution. Estimates were derived using distributed lag non-linear models of independent 10- and 20-year periods with a 21-day lag, adjusted for weekdays, holidays, and smoothed time trends (8 degrees of freedom per year).