A new paper is just out in The Lancet that examines the mortality risk of high and low ambient temperatures. The basic idea is that if it is either to hot or too cold, mortality may increase, possibly with the weather being a factor to augment the effects of other health problems, or as a direct result. The paper is methodologically reasonably well done but leads to conclusions that I think will be misinterpreted and misused. The paper implies that a shift to a warmer world would have lower mortality effects than a shift to a colder world might. Or, more significantly, that a shift to a warmer world will reduce ambient temperature related mortality by reducing the effects of cold. This is incorrect for a number of reasons.

Having said that, this paper does make a valuable contribution to public health, though here I’ll note that only in passing (see below).

First, I’ll give you the author’s viewpoint directly by quoting from the abstract, then I’ll tell you what I think about it.

The paper is “Mortality risk attributable to high and low ambient temperature: a multicountry observational study,” by Antonio Gasparrini and a host of other authors. It says:

Summary
Background Although studies have provided estimates of premature deaths attributable to either heat or cold in selected countries, none has so far offered a systematic assessment across the whole temperature range in populations exposed to different climates. We aimed to quantify the total mortality burden attributable to non-optimum ambient temperature, and the relative contributions from heat and cold and from moderate and extreme temperatures.

Methods We collected data for 384 locations in Australia, Brazil, Canada, China, Italy, Japan, South Korea, Spain, Sweden, Taiwan, Thailand, UK, and USA. We fitted a standard time-series Poisson model for each location, controlling for trends and day of the week. We estimated temperature–mortality associations with a distributed lag non-linear model with 21 days of lag, and then pooled them in a multivariate metaregression that included country indicators and temperature average and range. We calculated attributable deaths for heat and cold, defined as temperatures above and below the optimum temperature, which corresponded to the point of minimum mortality, and for moderate and extreme temperatures, defined using cutoffs at the 2·5th and 97·5th temperature percentiles.

Findings We analysed 74225 200 deaths in various periods between 1985 and 2012. In total, 7·71% (95% empirical CI 7·43–7·91) of mortality was attributable to non-optimum temperature in the selected countries within the study period, with substantial differences between countries, ranging from 3·37% (3·06 to 3·63) in Thailand to 11·00% (9·29 to 12·47) in China. The temperature percentile of minimum mortality varied from roughly the 60th percentile in tropical areas to about the 80–90th percentile in temperate regions. More temperature-attributable deaths were caused by cold (7·29%, 7·02–7·49) than by heat (0·42%, 0·39–0·44). Extreme cold and hot temperatures were responsible for 0·86% (0·84–0·87) of total mortality.

Interpretation Most of the temperature-related mortality burden was attributable to the contribution of cold. The effect of days of extreme temperature was substantially less than that attributable to milder but non-optimum weather. This evidence has important implications for the planning of public-health interventions to minimise the health consequences of adverse temperatures, and for predictions of future effect in climate-change scenarios.

What could possibly go wrong?

Obviously, the main way this paper could derail is when climate change denialists make the claim that global warming is good because cold weather causes more mortality than warm weather, so we’ll have less mortality. This, however, is incorrect for several reasons.

Assume the paper has correctly characterized mortality effects of weather. From this we assume that there is a certain mortality profile with temperature as the cause. Medium temperatures don’t exacerbate population level mortality but above and below that, mortality increases. This is expected for any species where temperature matters. Key life history traits including maintainance (and thus, basic survival) may be linked to temperature, and if temperature is too high or too low, things go badly. The problem with the assumption that increasing temperatures will decrease mortality is that the more logical interpretation, which fits with what we know about the biology of warm blooded animals, is that any change, up or down, in the range and average of temperatures will have negative effects. In other words it is incorrect to assume that since there is less mortality at the upper end of the range that heading for that direction is good. There is increased mortality at both ends, movement in either direction should be assumed to increase mortality in that direction.

One result of the paper, only briefly touched on but clear from the data, is that the lower end of ambient temperature mortality effects has a long distribution, while the upper end (warmer temperatures) have a lower end. This means that the two phenomena, too cold and too warm, have different statistical characteristics. For this reason, comparing the two is a rather dicy affair. It is like comparing the negative effects of driving too slow and driving too fast. Either one can mess you up on the highway, but there is a lot more room at the lower end. Where the speed limit is 60, a very very fast speed is 20 miles an hour faster, but a very very slow speed might be 50 miles an hour slower. Not only are the statistics very different (and thus not comparable between the two ends) but the mechanisms are different. With respect to temperature, colder conditions probably exacerbate mortality by requiring that the body spend more energy on maintenance, thus taking away energy from immune response. At the upper end, added heat does something entirely different, requiring an entirely different mechanism (cooling) to kick in, and in more extreme cases, causing a direct pathological outcome, heat stress.

Human Evolution

I think it is helpful to put this paper in evolutionary context. Humans are primates, and as such, evolved in the tropics. We evolved, in other words, at the higher end of the temperature range discussed in this study, and should be adapted to warm conditions. Humans are hominoids (apes) which, among primates, evolved in the warmer end of that range, with extra humidity. We are hominins, a special kind of ape, that extended its habitat to include somewhat (but not too much) drier conditions. When our genus arose, we began to spread into novel habitats, including some that were cooler, but for nearly two million years various human ancestors were limited to tropical or sub tropical conditions. It was not until our species evolved that we fully adapted to the full range of conditions from very cold to very warm, from very dry to very wet. Indeed, technologically modern humans who rely on agriculutre have been excluded from the most extreme environments our foragering ancestors occupied thousands of years ago, and adapted to both physically and culturally.

In other words, from our evolutionary history we would predict that humans would suffer relatively low levels of heat related mortality and high levels of cold related mortality, and that many of our more recently developed, and limited, adaptations are to cold while our deeper and longer-term adaptations are to heat. We may even walk upright because of heat (that is one theory that has never been tossed out to explain why a chimp-like ancestor was selected to become more upright). Modern humans mostly have the physical form of a tropical African because we mostly come fairly recently from tropical Africa. Our heat related adaptations tend to be physical, long-evolved, built in. Our cold adaptations (with a few exceptions) tend to be cultural, technological, added-on. The range of temperatures that actually occur on Earth that heat stress us is limited, the range of temperatures that actually occur on Earth that cold-stress us is very large.

Our evolutionary biology predicts that we would have a higher mortality rate under cold than under heat, and this paper confirms that.

This isn’t your great great great great grand daddy’s planet

The problem arises when we leave the Earth in which we evolved and arrive on a hot new planet. Most of the physical evolution (our exact ratio of body parts, our respiratory adaptations, skin and hair related adaptations, fat distribution, etc.) and our cultural adaptations (fire, clothing, shelter, mobility, diet) that related to temperature, cold or hot, arose over the last two million years, which is coincident with the Pleistocene. During this period atmospheric CO2 levels ranged around an average of about 250ppm, rarely going above 300ppm, and never approaching 400ppm. CO2 levels correlated well with the surface temperatures in which we live, and the current atmospheric CO2 level is 400ppm and rising. It will take time (a few more decades) for the new temperature regime, the one human greenhouse gas pollution is causing, to be realized. But when that happens we’ll be living on a planet with temperature characteristics not seen during the entire course of human evolution. Some regions will have temperature ranges that go well beyond the ranges explored in this paper. Extrapolating the effects of high ambient temperatures from the last several decades to the middle of the 21st century is difficult at best.

Normal human body temperature is 98.6 degrees F. That is higher than the average daily temperature for most places humans have lived over the last 2 million years, but within the range of the highest daily temperatures. The problem is, when ambient temperatures are higher than this amount, our brains are at risk, and cooling adaptations have to kick in. If average ambient temperatures in a warm region go too high, these adaptations will not be adequate, and heat spells may become routinely fatal. If, on the other hand, in other regions of the world, average ambient temperatures went way down (like, if Florida became like Minnesota) we would adapt by changing the geographical distribution of the use of existing and well established technologies.

Putting this another way, humans can adapt, and in the past have adapted, to cooling. We can not adapt, in the warmest regions, to heating. There is not a mechanism that allows this, and there is no practical technology other than air conditioners, and it is not really practical, ore even possible to create air conditioners that would allow survival in tropical regions for anyone other than the elite and ex patriots.

Another factor, already implied above but I’ll underscore it, is the fact that moving towards warmer conditions does not remove colder conditions. The mortality induced by cooler ambient temperatures discussed in this paper is not from people freezing to death on ice flows. It is from ambient temperatures mostly above freezing, which in the absence of a technological fix, cause added stress. Even with increased surface temperatures caused by global warming, these conditions will still persist. The range of temperatures over which this cold induced stress occurs is, as stated, very large and even if overnight temperature minima rise with global warming in a given region, most of those temperatures will still be regularly represented.

From the paper:

Despite the attention given to extreme weather events, most of the effect happened on moderately hot and moderately cold days, especially moderately cold days. This evidence is important for improvements to public health policies aimed at prevention of temperature-related health consequences, and provides a platform to extend predictions on future effects in climate-change scenarios.

This is an important and valid point. This cold related mortality can be addressed with some simple technological fixes, including even a modest amount of insulation or other improvements in constructions in homes in tropical or subtropical areas where it is warm enough that many people at the lower end of the socioeconomic spectrum (i.e., almost everybody) does not typically bother with such things. This is an important result of the paper that has nothing to do with global warming but should be paid attention to.

A potential bias

The paper relies on mortality data for cold vs. hot ambient temperature periods. However I question the ability to obtain data on heat related deaths that are comparable to cold related deaths. In the more extreme cool areas, the northern countries, there are good data on mortality. In the more extreme warm areas, along the equator, the mortality data one would ideally use are virtually non existent. Of the seven billion people who live on the earth, the one billion that are most unlikely to show up in any systematic data of any kind live along the equator, and very few (some reindeer herders in Siberia, for example) live in the cooler climates. I don’t think the authors address this kind of bias adequately.

It’s about time

The study period ranges from the 1980s to recent. This is the period of time during which about half of the surface warming experienced during the 20th century has occurred, and at a high rate. The authors do not examine change over time in mortality at either end of the temperature range. Since the key variable, ambient surface temperature, changes dramatically over the study period, this should have been addressed. It may be that due to the nature of the data this could not be done, but the paper explicitly makes assertion about change over time in the future without addressing change over time during the study period. This makes me sad.

In summary, shifting to a warmer world will have more negative effects than shifting to a cooler world, when it comes to human health related response to ambient conditions. The paper implies that warming is not as bad as cooling might have been, and this will be used by those who deny the very existence of, or human fingerprint on, or importance of, or ability to address, climate change. The paper lacks contextualization of the problem in terms of well known human adaptations. There may be significant biases or problems with respect to reported mortality an change through time during the study period in the key variable, ambient surface temperature.