A study by researchers at Columbia University's Mailman School of Public Health debunks the assumption that global warming will lead to a decline in the number of deaths in winter. Findings by Professor Patrick Kinney, ScD, professor of Environmental Health Sciences and director of the School's Climate and Health Program, showed that a warming climate trend led to much smaller reductions in cold-related mortality than some experts have anticipated.

Among 39 cities in the U.S. and France, there was no evidence that cities having warming temperatures experienced any less winter mortality than did cooler cities.

"Some have claimed that warmer winters due to climate change will lead to big reductions in winter deaths. Our work suggests that this is unlikely to be the case," said Dr. Kinney, who was a lead author on the recent report from the Intergovernmental Panel on Climate Change and also serves on the New York City Panel on Climate Change.

If cold temperatures were directly responsible for winter mortality rates, then we would expect future warming to lead to substantial reductions in winter mortality, according to Dr. Kinney. On the other hand, "climate warming would have little benefit if seasonal factors other than temperature are mainly responsible for winter excess mortality," he noted.

To determine whether and to what extent cold temperatures affect excess winter mortality, Dr. Kinney and colleagues analyzed temperature and mortality data from 36 U.S. cities and Paris, Lyon, and Marseille in France. Mortality rates were obtained from the U.S. National Center for Health Statistics and the French National Institute for Statistics and Economics Studies for the period 1971-2007.

Findings showed that cities with warmer winters have similar rates of winter deaths compared to their colder winter-counterparts and that there was little relationship evident between mortality and cold temperatures.

"These cities vary widely in demography, urban design, and socio-cultural background, all of which might influence exposure to outdoor temperature and related mortality risks," said Dr. Kinney.

The lack of correlation between seasonal temperature differences and winter season excess mortality suggests that other seasonal factors are driving winter excess mortality including lack of exercise, low humidity and time spent indoors which may lead to increased risk of flu and other respiratory infections and its complications.