Climate change is unlikely to significantly reduce winter mortality rates

Assessments of the health risks of climate change often conclude that the general warming associated with climate change and the corresponding reduction in the severity and length of the winter season could considerably decrease deaths attributable to cold temperatures. Determining the degree to which warming temperatures could affect cold-related mortality requires understanding the role of temperature in the etiology of specific cold-related health outcomes and in cold season mortality patterns, and the mechanisms whereby climate change could alter temperature-mortality relationships. This information is critical for assessing how the balance of hot and cold temperature-related mortality could alter in a changing climate. This information also is needed by public health officials to support their efforts to reduce cold season mortality.

In addressing this issue, it is critical to distinguish between seasonal mortality patterns and mortality due to extreme temperature events. That more deaths occur in temperate regions in the winter, generally defined as the three coldest months of the year, is well documented. The question is how much of this seasonal variation is dependent on temperature versus other factors that vary with the seasons.

The evidence suggests a warming climate will likely reduce the frequency and severity of extreme cold spells compared with current conditions, with a corresponding reduction in associated mortality. However, it should be recognized that cold spells are a marginal contributor to overall winter mortality. As a result, changes in mortality attributable to cold spells have little potential to shape overall winter mortality. Because colder temperatures during the winter season will continue to occur in a warmer climate, it is reasonable to expect that relatively cold days will continue to increase mortality in susceptible individuals, particularly if temperature variability in winter is a more important risk factor than absolute temperature.

Cardiovascular disease (including hypertension, ischemic heart disease, myocardial infarction, and cerebrovascular disease) accounts for the majority of excess deaths during the winter season. A wide range of biologically important processes that can lead to cardiovascular stress have different patterns between winter and summer. The largest proportional increases in winter mortality are in countries with less severe, milder climates. The association of cardiovascular disease with temperature is weak, with temperature contributing to at most a small proportion of deaths. The limited number of studies comparing whether seasonality mortality patterns are due to temperature or whether cardiovascular diseases are seasonal for other reasons, generally conclude that winter mortality patterns are due to factors other than temperature. An additional consideration is that winter mortality rates have been decreasing because of infrastructure, health care, and other factors; it is unclear the rate at which this trend will continue, but it would be expected to contribute to a further decline in winter deaths.

Respiratory disease mortality is a relatively small proportion of winter deaths. Colder winter temperatures can be linked to increased respiratory infections as colder air induces broncho-constriction and suppresses mucociliary defenses, resulting in local inflammation. Many winter respiratory deaths are due to influenza, with studies not consistently reporting an association between ambient temperature and the incidence of influenza and other respiratory mortality. Cold air alone does not appear to be a significant factor in respiratory infections.

Therefore, assuming no changes in acclimatization and the degree to which temperature-related deaths are prevented, climate change may alter the balance of deaths between winters and summers, but is unlikely to dramatically reduce overall winter mortality rates.