Researchers are a step closer to answering an important question about the health risks of climate change: Are people acclimatizing to higher global temperatures?

A study published in Environmental Health Perspectives investigated whether the temperature at which mortality is lowest in a particular location, or minimum mortality temperature (MMT), changed as global mean surface temperature increased with climate change. Researchers looked at daily mortality and temperature data from Stockholm, Sweden, for the period 1901-2009.

“Researchers hypothesized that, if people are acclimatizing to higher temperatures, then MMT would increase over time,” says study co-author Kristie Ebi. “We found that this was the case for Stockholm over the twentieth century.” Ebi is a professor of global health and environmental and occupational health sciences at the University of Washington School of Public Health and a visiting professor at Umeå’s department of public health and clinical medicine, where the study was conducted.

Evidence shows that there is a shift to more days with warmer temperatures and fewer days with colder temperatures. Extreme ambient temperatures, be they hot or cold, are known to cause negative effects on human health, the study noted.

Temperatures in Stockholm increased over the study period and they are projected to increase further. Researchers suggest that MMT could continue to rise with increasing temperatures. The increase in MMT suggests people can adapt to gradual increases in average temperatures to some degree. Whether the rate of increase will be sustained with climate change is an open question.

Over the study period, housing standards improved with the introduction of central heating and better sanitary conditions. People died at older ages and life expectancy at birth increased. Virtually all who were able and willing to work became employed, which led to greater economic security. Also, the GDP per capita increased substantially. These changes yielded large benefits to public health.

Acclimatization within the context of these epidemiological, demographical and societal changes offset some of the mortality from higher temperatures by shifting the MMT to higher values, the study found.

MMT varies greatly across countries and regions, according to the study. MMT can range from a daily mean temperature of 50-54 degrees Fahrenheit in Scandinavian countries to 80 degrees Fahrenheit in Miami, Florida.

This study was the first of its kind to use daily mortality and temperature data spanning more than a century, augmenting the ability to test for trends.