Cold Kills More than Heat. But is it True?

Figure from a recent paper by Clarke et al.

You probably heard the line, “Global Warming is a good thing because cold kills more than heat”. As for all things in the debate, it is partly true and partly wrong. You may die of excess heat or excess cold; it depends on many things.

The figure above is from a recent paper by Clarke et al., who examined the consequences of the recent heat wave in Western Europe. The figure is not specific for the current heat wave; it reports previously available data on the “relative mortality rate” for a number of European cities. That is, how likely are you to die when the place where you live reaches a certain temperature? Note that this is a relative probability of death. It doesn’t say anything about your overall probability of dying in a certain year. Nevertheless, it is an interesting set of data.

The first thing to note is that your probability of dying from a heat wave is much larger than for a cold spell in all cities, except London. Even in London, though, you have three times more chances to die in a heat wave than on an ordinary day. Paris, then, is especially bad for the mortality associated with heat waves: four times higher than in an average day

The differences between cities are marginal; they may be attributed to factors such as the diffusion of air conditioners and efficient heating systems. You often read that old people in Britain tend to die of cold because they can’t afford to turn on the heating; the data seem to confirm the story. Note also that measurements stop at 30 °C. It is easy to extrapolate that for the levels reached during the latest heat wave, nearly 40 °C, the mortality may have been much higher.

The authors of the paper on the recent heat wave start from these data, match them with the temperatures, and calculate the excess mortality. Whereas the data in the figure are real mortality data, their results are an extrapolation, not real data about deaths. But it is an extrapolation that makes sense. We know that heat kills. It does.

It still remains true that, if you look at the overall mortality data, your probability of dying in Winter remains higher than in Summer. It is because of increased cardiovascular risks and the diffusion of respiratory illnesses (flu and the like). So, in this sense, it is true that cold kills more than heat. But note that there is no evidence that global warming is reducing cold-related deaths.

Heat waves are becoming more and more frequent and harsher. And the more heat waves arrive, the more likely it is that you’ll die of excess heat. There is an upper limit of temperature and humidity that human beings can survive: it can be defined in terms of the “Wet Bulb” temperature. The concept is explained in a post by Kevin Hester. Here, let’s just say that we haven’t yet reached that condition anywhere in the world, but we are moving in that direction. And if that point is reached where you live, if you don’t have air conditioning or an underground refuge, you die, full stop. Old people will die even before that point is reached. We are getting what we knew we would get. So is life.

A specific point about the Imperial College paper is that it comes from an institution that had been at the forefront of modeling the infection progress during the COVID-19 Pandemic. But their epidemiological models were based on questionable data, and sometimes on purely hypothetical data. That gave the Imperial College a bad reputation (in many respects not undeserved) in many circles. So, the reaction to the paper on heat-based mortality has often been to dismiss it as coming from an untrustworthy source. It would be a mistake. This paper is an extrapolation, but it is based on solid data. As with every extrapolation, it is subject to uncertainty, but it is still a serious warning. Do not fall into the typical reaction that goes as “since scientists are often corrupt (true), then everything that science produces is wrong (false).” There is still good science being produced, and if you ignore it, you do that at your risk and of all of us