Environmental advocates claim heat results in more deaths than any other weather-related event but I recently read a conflicting claim about weather-related deaths. The New York City Environmental Justice Alliance released a new report, NYC Climate Justice Agenda 2018 – Midway to 2030: Building Resiliency and Equity for a Just Transition, that claims “Extreme heat results in more deaths than any other weather-related event”. On the other hand, a study in Lancet, “Mortality risk attributable to high and low ambient temperature: a multicountry observational study”, notes that “most of the temperature-related mortality burden was attributable to the contribution of cold”. I did some research and now I think I know what is going on for these two differing claims.
The NYC Climate Justice Agenda bases their claim that extreme heat causes more deaths than cold based on an EPA reference. The EPA extreme heat webpage uses data from the National Oceanic and Atmospheric Administration Natural Hazard Statistics: Weather Fatalities website. The home page for that site lists 508 fatalities from all weather events in 2017, including 107 from extreme heat, 26 from extreme cold, 10 from winter storms, 1 from ice, and 3 from avalanches. Those data that show that the more people died due to extreme heat than other cause, narrowly beating out flash floods, and that more people die from heat than cold-related events. The data for the website are compiled from information in the National Weather Service (NWS) storm events database.
The Global Warming Policy Foundation April 9 2018 newsletter reported that 48,000 Britons died this winter due to cold weather. Those numbers are obviously far different than the NWS data. The Lancet paper by Gasparrini et al. notes that:
Although consensus exists among researchers that both extremely cold and extremely hot temperatures affect health, their relative importance is a matter of current debate and other details of the association remain unexplored. For example, little is known about the optimum temperatures that correspond to minimum effects for various health outcomes. Furthermore, most research has focused on extreme events and no studies have comparatively assessed the contribution of moderately high and low temperatures. The underlying physiopathological mechanisms that link exposure to non-optimum temperature and mortality risk have not been completely elucidated. Heat stroke on hot days and hypothermia on cold days only account for small proportions of excess deaths. High and low temperatures have been associated with increased risk for a wide range of cardiovascular, respiratory, and other causes, suggesting the existence of multiple biological pathways.
I believe that the reason for the difference in the two conclusions is explained by this statement by Gasparrini et al.: “The dose-response association, which is inherently non-linear, is also characterised by different lag periods for heat and cold—i.e., excess risk caused by heat is typically immediate and occurs within a few days, while the effects of cold have been reported to last up to 3 or 4 weeks.”
According to the NWS instructions for storm data preparation the storm data report documents:
- The occurrence of storms and other significant weather phenomena having sufficient intensity to cause loss of life, injuries, significant property damage, and/or disruption to commerce;
- Rare, unusual, weather phenomena that generate media attention, such as snow flurries in South Florida or the San Diego coastal area; and
- Other significant meteorological events, such as record maximum or minimum temperatures or precipitation that occur in connection with another event.
The key point is that the storm data report makes a distinction between direct and indirect deaths. Only direct deaths are tabulated when a local weather office prepares the storm report. For example, in winter storms deaths from heart attacks from shoveling snow are indirect. If a person wanders outside and freezes to death that’s a direct death. Furthermore, while indirect deaths are included in the storm narratives the numbers are not included in the tabulated data and storm reports are prepared within days of the event so any indirect deaths due to excessive cold caused by weeks-old impacts would not be included. Details on the difference between direct and indirect deaths are found in the instruction document on pages 9 to 12.
In their study of Gasparrini et al. found that temperature is responsible for 7.7% of mortality. Cold was responsible for “most of the burden”. Although in the study over 90% was attributed to cold the paper noted that “This difference was mainly caused by the high minimum-mortality percentile, with most of the mean daily temperatures being lower than the optimum value”. I interpret that to mean that some of the difference was due their classification methodology. In line with the indirect death distinction it is notable that over 85% of the mortality attributable to temperature was related to moderate cold. Offhand I think there must be more causes of death associated with freezing weather than hot weather. For example, auto accidents on icy roads has to cause more deaths than any hot weather impact on travel.
In conclusion, there is a data base that does show that extreme heat results in more deaths than any other weather-related event. However, the database used to justify that claim only includes direct deaths. An epidemiological study that does include indirect deaths concludes the majority of deaths are associated with moderate cold weather.
Relative to climate change policy the distinction between heat and cold is important. If the argument is that we must mitigate human impacts on climate to reduce mortality due to temperature than because a warming climate will result in less moderate cold then that means warming will have a beneficial effect. An unintended consequence of climate change mitigation through the implementation of renewable energy is the universal increase in cost. Given the impacts on indirect deaths I believe that increased heating cost will adversely affect mortality if low income people cannot afford to keep their homes warm enough to prevent potential health impacts of cold weather. Finally, the fact is that climate is a reason many more people move to Phoenix AR than move to the “ice box of the nation”, International Falls, MN, suggests we are better able to adapt to warm than cold.