Which is the Biggest Killer - Extreme Heat or Extreme Cold?

This question came about from an interesting BBC article called 'Staying Warm: What does an unheated room do to your body?' Some of the body changes that occur as room temperature drops from 21 ℃ to 10 ℃ are summarised in the pictorial below.

This BBC article is one of a number of posts offering advice on how to stay warm and safe this winter. The hike in energy prices this year means many households will be economizing on home heating and the threat of UK electricity blackouts this winter cannot be discounted.

However, it was this comment by Professor Damien Bailey that caught my eye: "The evidence clearly suggests that cold is more deadly than the heat, there are a higher number of deaths caused through cold snaps than there are through the heat snaps."

Just accepting Professor Bailey's statement as a fact would be an example of the 'argument from authority' fallacy. It doesn't mean it is wrong, just that you should be skeptical until shown the evidence. Of course, if the consensus of the scientific community, supported by the peer-reviewed literature, is that cold is the bigger killer then it is reasonable to assume that is the case based on current knowledge.

The first thing you should acknowledge is that it is probably more complicated than you think.

A report in The Lancet from 2015 clearly puts deaths from cold much higher than deaths from heat. However, this was really comparing winter deaths to summer deaths which is not the same thing. For example, deaths from influenza (flu) are well-known to peak over the winter months - one of the factors being that people congregate indoors (i.e. in the warmth) when it is cold so this should, perhaps, be collated under the deaths from heat rather than cold!

This blog post on Weather Underground, based on the peer-reviewed literature, puts the case for extreme heat being a bigger killer than extreme cold. The Lancet paper does not disagree with such a conclusion since it clearly shows that moderate cold is the biggest 'killer' due to the higher numbers of deaths from flu and other non-weather-related factors. 

You may see claims that global warming is beneficial because it means fewer deaths from extreme cold. This is likely true but is only beneficial if deaths from extreme cold are greater than from extreme heat and the latter will definitely increase with global warming.

The overall conclusion seems to be that if you correct for the seasonal cycle (the winter flu season) then, in the US, more people die from extreme heat than extreme cold. Consequently, the reduction in extreme cold deaths with global warming may be overwhelmed by the increase in extreme heat deaths. Of course, technology such as air conditioning should help reduce heat-related deaths but this is an energy-intensive  solution best avoided by mitigating global warming.