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

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 past decades as global mean surface temperature 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 non-linear 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 inter annual climatic variability, we estimated MMT based on 30-year periods. Further, we investigated whether there were trends in the absolute value of the MMT and 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 last century. CONCLUSIONS: The increase in MMT over the course of the last 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