The risk of hot and cold weather

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:

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.


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I agree with the technical points, but disagree with part of the conclusions about the relative consequences.

It's true we can easily adapt to colder weather on an individual basis, given sufficient cultural and technological support, but colder weather hurts agriculture more than hotter weather.

I'm sure we could eventually adapt an agricultural system to the climate of the Cretaceous - it was very productive, right?

But Ice Ages make most temperate areas un-farmable - restoring the Sahara bread-basket won't be enough to compensate.

Fortunately (sort of), we do not have to worry about an Ice Age for a very long time.

But if a cloud of dark matter swooped in and cut the amount of sunlight by 10%, causing an immediate descent toward an Ice Age, do you think we would hear any talk about how "CO2 has no effect on temperature", "it's just natural climate change - nothing we can do"?

Hell no! We'd be burning that coal like crazy to compensate.

Ironically, by burning up all the fossil fuels as fast as possible now, we are reducing our options to prevent future Ice Ages.

By wehappyfew (not verified) on 21 May 2015 #permalink

Colder weather can definately hurt agriculture more, but moving to warmer conditions from the present relatively warm period (relative to the time frame for the invention and use of agriculture) won't necessarily help it. It may do more to help pathogens than desirable plants. So far that has been the case.

Current climate change is also changing the hydro cycle enough to have a huge net negative impact on agriculture.

We're not talking about comparing the ice ages to a future 2-3 degree C planet. We are comparing the present to that warmer future.

In any event, the paper in question here is not about agriculture. No one really thinks that agriculture will be improved by inundating huge amounts of crop land and setting up numerous long term severe droughts, etc. But people haven't though too much about just plain old heat and cold, other than heat waves, which this paper explicitly does not address.

We have already put enough extra CO2 into the atmosphere, and then a heap load more, to forestall any significant future ice age. That train left the climatic station around 1980.

No doubt, warmer is worse than current conditions. Colder is worse than current conditions.

Our agriculture is optimized for current climate - meaning temperature, variability, and rainfall patterns. Change any of those to our detriment. People live and farming occurs where those are acceptable now.

And even if Siberian tundra eventually warms to the perfect temperature for some agricultural practice, there are no people, soil or infrastructure there.

I just think colder is worser than warmer. It's a moot point, as we won't be getting colder for a very long time.

By wehappyfew (not verified) on 21 May 2015 #permalink

You picked up on all the points I noticed, Greg. In particular I would have been interested in an analysis over time. For example, the European heat waves took a terrible toll.

All of us should be concerned by the charts in Figure 1. The death rate shot up very sharply at only 30C. What happens when many places get heat waves of several days at 45C plus? That won't be too far into the future in the mid-latitudes. (Melbourne Feb 2009 was just a warning shot.)

wehappyfew: Siberia may well warm up and the permafrost metl, but much of Siberia is low altitude, so sea level rise will then come long and eat up all that potential crop or forest land!

Humans have lived (and evolved) in colder environments, though not agriculture so much. We've not existed in warmer environments....

And as Sou points out, we are actually already seeing those effects starting and even if we clean up our acts really fast, as the earth's surface temperature reaches balance, that will become more of a problem in many regions.

Wow, the graphs in Sou's article really make your point. A small increase above "optimum" temp leads to a non-linear runaway death risk, while it takes a much larger decrease to get to the non-linear part of the too-cold curve.

You should add one of those graphs to this article to illustrate the asymmetric risk. If you just go by the numbers in the summary you quoted, as I did, one might get the impression that more deaths for colder temps means higher risk in the cold direction compared to hotter (for a given temperature change). The opposite is true, as the graph shows.

We are apparently optimized for a range of temps, and we are already at the upper end of that range - a point I did not get until I saw the graph.

So my point about agriculture is incomplete. We could, in theory farm in the Cretaceous climate, but it would have to be with robots, and without warm-blooded livestock, as no warm-blooded animals currently evolved could survive in the tropics.

By wehappyfew (not verified) on 21 May 2015 #permalink

@wehappyfew: It isn't just the tropics that run into trouble. Some areas of the tropics (if they are high enough to avoid inundation and don't suffer from failure of seasonal rains) don't fare as badly as some middle latitude areas do. Among the areas that could become uninhabitable are the midwestern US, Pakistan/northern India, and a big chunk of interior China. That would be some of the world's most productive agricultural land, under current climate conditions. These areas can get as hot, or even hotter, than the tropics during the summer months. For instance, temperatures above 40 C often occur in the Midwest during heat waves, while Miami International Airport has never reported a temperature higher than 38 C.

According to Jeff Masters' blog, the highest temperature recorded in the Northern Hemisphere in April 2015 was 48 C, at a site in Pakistan. I remind you that Pakistan is well north of the tropics. And while the hottest days of the year tend to come early there due to monsoon effects, April is rather early to be seeing such temperatures--June would be more normal.

By Eric Lund (not verified) on 21 May 2015 #permalink

Please correct me if I'm wrong, but doesn't the article examine only short-term/immediate effects of temp on mortality? Chronic impacts would therefore be missed.

Warming temps can make pollution worse, particularly ozone, and I wonder if that's missed from the study. I suppose cold could expose people to more indoor air pollution but I doubt that's as important, especially in our urbanizing world.

By Brian Schmidt (not verified) on 21 May 2015 #permalink

Brian, pollution is addressed in the original paper but I think not in the way you are asking, and certainly chronic effects are not addresses at all.

It's always fun to point out to those who say Earth is cooling, not warming, that this is an even more compelling reason to keep fossil fuels in the ground. We can then burn them to stay warm, and increase CO2 when needed to warm the planet.

Likewise, "putting solar panels on your roof isn't going to stop warming because China and India". Well, maybe, but they will power the air conditioner to deal with the increased temperatures.

Mitigation and adaptation are remarkably congruent in many areas. We should emphasize this more in public discussions.

This analysis does not seem to deal with the issue of the danger of higher nighttime minima, especially in hot climates. During waking hours adaptation to or toleration of extremes of heat are possible, but cause stress, for which relief and recovery requires night-time maxima below the level of stress. Daytime extremes vary more than night time minima, the average level of which is critical to human survival in hot climates. So also is dew point - wet bulb temperatures over 30C are barely tolerable - 36C is fatal. Wetbulb temperatures reached 32C over the China sea last summer, very close to the limit. Nighttime minima can be as high as 28C (wetbulb 26C) in my former home Darwin during the West season, offering little relief from daytime heat.
In summary, persistently higher night-time minima due to climate change are more significant to habitability than daytime temperature extremes, and wet-bulb temperatures in tropical climates, in both day and night time, are the critical parameters of mortality due to global worming.

By Des Carne (not verified) on 22 May 2015 #permalink

There's an old proverb:

When Christmas is white
The graveyard is lean;
But fat is the graveyard
When Christmas is green.

I always interpreted it to mean that a good, cold winter kills the germs, and I have wondered whether a warmer, moister climate means a lot of newer, warm-weather germs and insects to deliver disease. Add to that the US reluctance to keep up with infrastructure, and willingness to shut off people's water service and you have the makings of pandemics that aren't as scary as Ebola, but that could still kill a lot of unlucky people.

As pandemics go, Ebola wasn't that bad. It was very difficult for Ebola to go very far beyond the initially affected areas. An airborne pathogen with a high infection rate would have been much much worse.

"An airborne pathogen with a high infection rate would have been much much worse."

How does the length of time from exposure to the onset of symptoms play into this? It would seem that the "lag time" needs to be large enough so that even when the infected travel by air they are able to get out and mingle in a new population before they go down.

A couple of problems with this study. First, it sounds like the researchers seem to have simply declared as a premise that whatever season at which deaths were lowest had the "optimum temperature." Or rather, even worse, this "21-day lag period" sounds like it could mean that if deaths are lowest on March 1 in a given country, the temperature may be assumed to have been at its best on Feb. 7. Hard to see why the deadly stress of environmental variation should take that long to kill you.

Well, whatever. The huge problem with this presumption is that many other things fluctuate seasonally: availability of fresh food, access to physical activity or need for manual labor, parasite and pathogen loads, social activities including warfare and reproduction, sunlight exposure and vitamin D levels (a big one), etc. Attributing all excess deaths above a seasonal minimum to the harmful effects of imperfect temperature is utterly unjustified.

Second, I would bet that if they did look at each nation separately, they found different optimal temperatures in different countries, despite the limited and nonrepresentative range of countries included. It is a well-known fact that what temperatures you perceive as "too hot" or "too cold" are affected by your physical and cultural adaptations. People today regularly live in places where it gets hot enough that some doomers claim humans will be unable to survive in lands that become that hot due to climate change (and indeed, fat white AC-habituated people in T-shirts would die pretty fast out of doors there). Very possibly those people's ideal temperature from a health perspective, if it could be identified, would be higher than that of the Lapplanders.

There are unfortunate practical and philosophical implications of the opinion they're promoting. First, the attribution of all deaths above a yearly minimum to temperature variation teaches believers that we are awfully fragile flowers who can't bear more than tiny environmental fluctuations before we start dropping over. This belief can only encourage the global wealthy to view high heating in the winter and aggressive air-conditioning as Needs (like my mother-in-law who "needs" to heat to 78 and then, blech, A/C down to 68). "You suggest setting the A/C higher and using a fan? Obviously you want to kill Granny! Death Panels!" It's been argued that attempting to scare people into taking action on climate change is an ineffective tactic; if so, it's especially futile to scare thanatophobic Americans in ways that encourage them to see health and life as dependent upon burning more fossil fuels.

Finally, a minor semantic or philosophical issue. One may suspect that any "excess" deaths that are genuinely attributable to non-extreme seasonal temperature variations are of frail individuals who would likely have died in the relatively near future anyway. This raises the question of whether we should view death rates as being "tragically increased" year-round except for a brief moment of "normalcy" in late spring, say, or "happily reduced" from normal during that specially salubrious season of late spring. I find the latter more congenial because (a) it does not encourage the common American delusion that mortality is optional, and (b) it does not encourage us to view the real world outside the glass windows as an inimical environment lying in wait to kill us. (This is good for environmentalism; you protect what you care about or perceive as beneficial, not what you perceive as dangerous to you, so people who just hide indoors in the nice safe heating and A/C do not end up being people who will sacrifice much to save the biosphere.)

Jane at #15.


Humans evolved adaptations to heat when proto-humans learned not to drag their knuckles across the savanah:…

but being heat-adapted is different to being able to deal with increased mean global temperature:

The problem is that in relative terms we're already operating near the physiological ceiling for temperature when ≥2 σ annual extreme events are considered. Greg's already indicated this - when it comes to heat mammals and especially humans are already bouncing around near the ceiling, and the ability to adapt to warm or cool mean-temperature cliamtes does not confer the ability to crash through the metabolic physics of heat dispersal.

Nice try to distract from the point but burying your head in the desert sand won't change the direction in which the wind is blowing.

By Bernard J. (not verified) on 28 May 2015 #permalink