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Will climate change bring benefits from reduced cold-related mortality? Insights from the latest epidemiological research

Filed under: — stefan @ 11 June 2018

Guest post by Veronika Huber

Climate skeptics sometimes like to claim that although global warming will lead to more deaths from heat, it will overall save lives due to fewer deaths from cold. But is this true? Epidemiological studies suggest the opposite.

Mortality statistics generally show a distinct seasonality. More people die in the colder winter months than in the warmer summer months. In European countries, for example, the difference between the average number of deaths in winter (December – March) and in the remaining months of the year is 10% to 30%. Only a proportion of these winter excess deaths are directly related to low ambient temperatures (rather than other seasonal factors). Yet, it is reasonable to suspect that fewer people will die from cold as winters are getting milder with climate change. On the other hand, excess mortality from heat may also be high, with, for example, up to 70,000 additional deaths attributed to the 2003 summer heat wave in Europe. So, will the expected reduction in cold-related mortality be large enough to compensate for the equally anticipated increase in heat-related mortality under climate change?

Due to the record heat wave in the summer of 2003, the morgue in Paris was overcrowded, and the city had to set up refrigerated tents on the outskirts of the city to accommodate the many coffins with victims. The city set up a hotline where people could ask where they could find missing victims of the heatwave. Photo: Wikipedia, Sebjarod, CC BY-SA 3.0.

Some earlier studies indeed concluded on significant net reductions in temperature-related mortality with global warming. Interestingly, the estimated mortality benefits from one of these studies were later integrated into major integrated assessment models (FUND and ENVISAGE), used inter alia to estimate the highly policy-relevant social costs of carbon. They were also taken up by Björn Lomborg and other authors, who have repeatedly accused mainstream climate science to be overly alarmist. Myself and others have pointed to the errors inherent in these studies, biasing the results towards finding strong net benefits of climate change. In this post, I would like to (i) present some background knowledge on the relationship between ambient temperature and mortality, and (ii) discuss the results of a recent study published in The Lancet Planetary Health (which I co-authored) in light of potential mortality benefits from climate change. This study, for the first time, comprehensively presented future projections of cold- and heat-related mortality for more than 400 cities in 23 countries under different scenarios of global warming.

Mortality risk increases as temperature moves out of an optimal range

Typically, epidemiological studies, based on daily time series, find a U- or J-shaped relationship between mean daily temperature and the relative risk of death. Outside of an optimal temperature range, the mortality risk increases, not only in temperate latitudes but also in the tropics and subtropics (Fig. 1).

Fig. 1 Exposure-response associations for daily mean temperature and the relative mortality risk (RR) in four selected cities. The lower part of each graph shows the local temperature distribution. The solid grey lines mark the ‘optimal temperature’, where the lowest mortality risk is observed. The depicted relationships take into account lagged effects over a period of up to 21 days. Source: Gasparrini et al. 2015, The Lancet.

Furthermore, the optimal temperature tends to be higher the warmer the local climate, providing evidence that humans are at least somewhat adapted to the prevailing climatic conditions. Thus, although ‘cold’ and ‘warm’ may correspond to different absolute temperatures across different locations, the straightforward conclusion from the exposure-response curves shown in Fig. 1 is that both low and high ambient temperatures represent a risk of premature death. But there are a few more aspects to consider.

Causal pathways between non-optimal temperature and death

Only a negligible proportion of the deaths typically considered in this type of studies are due to actual hypo- or hyperthermia. Most epidemiological studies on the subject consider counts of deaths for all causes or for all non-external causes (e.g., excluding accidents). The majority of deaths due to cold and heat are related to existing cardiovascular and respiratory diseases, which reach their acute stage due to prevailing weather conditions. An important causal mechanism seems to be the temperature-induced change in blood composition and blood viscosity. With regard to the cold effect, a weakening of the defense mechanisms in the airways and thus a higher susceptibility to infection has also been suggested.

Is the cold effect overestimated?

As in any correlative analysis there is always the risk of confounding, especially given the complex, indirect mechanisms underlying the relationship between non-optimal outside temperature and increased risk of death. Regarding the topic discussed here, the crucial question is whether the applied statistical models account sufficiently well for seasonal effects independent of temperature. For example, it is suspected that the lower amount of UV light in winter has a negative effect on human vitamin D production, favoring infectious diseases (including flu epidemics). There are also some studies that point to the important role of specific humidity, that, if neglected, may confound estimates of the effect of temperature on mortality rates.

Interestingly enough, there is still an ongoing scientific debate regarding this point. Specifically, it has been suggested that the cold effect on mortality risk is often overestimated because of insufficient control for season in the applied models. On the other hand, the disagreement on the magnitude of the cold effect might simply result from using different approaches for modeling the lagged association between temperature and mortality. In fact, the lag structures of the heat and cold effects are distinct. While hot days are reflected in the mortality statistics relatively immediately on the same and 1-2 consecutive days, the effect of cold is spread over a longer period of up to 2-3 weeks. Simpler methods (e.g., moving averages) compared to more sophisticated approaches for representing lagged effects (e.g., distributed lag models) have been shown to misrepresent the long-lagged association between cold and mortality risk.

Mortality projections

But what about the impact of global warming temperature-related mortality? Let’s take a look at the results of the study published in The Lancet Planetary Health, which links city-specific exposure-response functions (as shown in Fig. 1) with local temperature projections under various climate change scenarios.

Fig. 2 Relative change of cold- and heat-related excess mortality by region. Shown are relative changes per decade compared to 2010-2019 for three different climate change scenarios (RCP 2.6, RCP 4.5, RCP 8.5). The 95% confidence intervals shown for the net change take into account uncertainties in the underlying climate projections and in the exposure-response associations. It should be noted that results for single cities (> 400 cities in 23 countries) are here grouped by region. Source: Gasparrini et al. 2017. The Lancet Planetary Health

In all scenarios, we find a relative decrease in cold-related mortality and a relative increase in heat-related mortality as global mean temperature rises (Fig. 2). Yet, in most regions the net effect of these opposing trends is an increase in excess mortality, especially under unabated global warming (RCP 8.5). This is what would be expected from the exposure-response associations (Fig. 1), which generally show a much steeper increase in risk from heat than from cold. A relative decline in net excess mortality (with considerable uncertainty) is only observed for Northern Europe, East Asia, and Australia (and Central America for the more moderate scenarios RCP 2.6, and RCP 4.5).

So, contrary to the propositions of those who like to stress the potential benefits of global warming, a net reduction in mortality is the exception rather than the rule, when comparing estimates around the world. And one must not forget that there are important caveats associated with these results, which caution against jumping to firm conclusions.

Adaptation and demographic change

As mentioned already, we know that people’s vulnerability to non-optimal outdoor temperatures is highly variable and that people are adapted to their local climate. However, it remains poorly understood how fast this adaptation takes place and what factors (e.g., physiology, air conditioning, health care, urban infrastructure) are the main determinants. Therefore, the results shown (Fig. 2) rely on the counterfactual assumption that the exposure-response associations remain unchanged in the future, i.e., that no adaptation takes place. Furthermore, since older people are more vulnerable to non-optimal temperatures than younger people, the true evolution of temperature-related mortality will also be heavily dependent on demographic trends at each location, which were also neglected in this study.

Bottom line

I would like to conclude with the following thought: Let’s assume – albeit extremely unlikely – that the study discussed here does correctly predict the actual future changes of temperature-related excess mortality due to climate change, despite the mentioned caveats. Mostly rich countries in temperate latitudes would then indeed experience a decline in overall temperature-related mortality. On the other hand, the world would witness a dramatic increase in heat-related mortality rates in the most populous and often poorest parts of the globe. And the latter alone would be in my view a sufficient argument for ambitious mitigation – independently of the innumerous, well-researched climate risks beyond the health sector.

Addendum: Short-term displacement or significant life shortening?

To judge the societal importance of temperature-related mortality, a central question is whether the considered deaths are merely brought forward by a short amount of time or whether they correspond to a considerable life-shortening. If, for example, mostly elderly and sick people were affected by non-optimal temperatures, whose individual life expectancies are low, the observed mortality risks would translate into a comparatively low number of years of life lost. Importantly, short-term displacements of deaths (often termed ‘harvesting’ in the literature) are accounted for in the models presented here, as long as they occur within the lag period considered. Beyond these short-term effects, recent research investigating temperature mortality associations on an annual scale indicates that the mortality risks found in daily time-series analyses are in fact associated with a significant life shortening, exceeding at least 1 year. Only comparatively few studies so far have explicitly considered relationships between temperature and years of life lost, taking statistical life expectancies according to sex and age into account. One such studies found that, for Brisbane (Australia), the years of life lost – unlike the mortality rates – were not markedly seasonal, implying that in winter the mortality risks for the elderly were especially elevated. Accordingly, low temperatures in this study were associated with fewer years of life lost than high temperatures – but interestingly, only in men. Understanding how exactly the effects of cold and heat on mortality differ among men and women, and across different age groups, definitely merits further investigations.

166 Responses to “Will climate change bring benefits from reduced cold-related mortality? Insights from the latest epidemiological research”

  1. 151
    Hank Roberts says:

    > Dan H. … daytime highs and number of days exceeding high temperatures
    > (whether it be 90, 95, or 100F) has decreased in most major cities

    Once yet again, “Dan H.” — Please cite a source for the beliefs you post.

    We shouldn’t have to keep reminding you about that.

    If you’re ashamed of your sources, perhaps you should be. Think about it.

  2. 152
    Hank Roberts says:

    Here’s an example of a citable source:

    See on that page where they give you a link to their sources? Here it is:

  3. 153
    Al Bundy says:

    Hank Roberts quotes NPR: Those victims were generally women, living alone and over the age of 75. “Oftentimes, they won’t have their air conditioning on, or it will be malfunctioning,” says Sunenshine.

    AB: Just for information, the public library is a grand refuge. Instead of burning carbon and blowing your budget, head to where you can get social interaction, free internet, and plenty of intellectual stimulation.


    Mr KillingInaction: On a 95 degree day you can probably kill yourself by doing a hard workout in the sun and not drinking enough water – people have been demonstrating this exact feat for thousands of years.

    AB: Yes, and Muslims are at an incredible risk. When your religion says that during certain (varying) times of the year you aren’t allowed to drink water during the day, you get to either die or go to Hell.

    KIA: Trump [snip] is a friggin’ genius, best president in my 60+ years! Most entertaining in our history I’ll bet!

    AB: You are equating entertainment with intelligence. Dumb, dumb, dumb conclusion. Monkeys in the zoo are entertaining, too. Trump is destroying the USA and the planet because he’s an ignorant low-IQ psychopath who gets by because his grandfather changed his actual name “Drumpf” to “Trump” and his father was a racist thief and Drumpf himself goes bankrupt on a regular basis so as to screw hardworking folks out of their rightful pay for hard work. Drumpf can’t divulge his tax returns or his Russian connections because doing so would expose the fact that he’s scum. We’ll see if he ends up where he belongs… in prison. Seriously, look up ANY non-partisan insider’s comments about him. They ALL say he’s stupid, ignorant, and (those who “go there”) evil.

    And, by the way, I post towards you the way I do because I’m certain that it gives you a chuckle. If I offend you, just let me know.

  4. 154
    Dan H. says:


    Why would I be ashamed of the epa or noaa?

    As an example, New York City has exceeded 100 degrees F less over the past three decades, than earlier in the 20th century:
    2010s: 5
    2000s: 1
    1990s: 8
    1980s: 2
    1970s: 3
    1960s: 4
    1950s: 12
    1940s: 8
    1930s: 8 (including record high of 106F)
    1920s: 2

    A similar trend has been seen in other major cities, such as Chicago.

  5. 155
    Dan H. says:

    Yes, that link supports my contention of rising low temperatures. Hence the ratio of record highs to record lows has decreased significantly.

  6. 156
    nigelj says:

    Dan H @154

    You previously tried to deny / downplay increasing heatwave trends in the USA.

    Your own EPA source shows the opposite. It clearly states “Nationwide, unusually hot summer days (highs) have become more common over the last few decades (see Figure 2). The occurrence of unusually hot summer nights (lows) has increased at an even faster rate. This trend indicates less “cooling off” at night…” Figure one clearly depicts an increasing trend in the annual heat wave index from 1960 – 2018.

    You say: “As an example, New York City has exceeded 100 degrees F less over the past three decades, than earlier in the 20th century. Chicago is similar”

    So what? You quote just two cities. This is not representative of America as a whole, and is completely useless analysis.

    “Yes, that link supports my contention of rising low temperatures. Hence the ratio of record highs to record lows has decreased significantly.”

    So what? This doesn’t make things better.

  7. 157
    Mr. Know It All says:

    140 – nigelj

    “……. air conditioning is essential in some places.

    I dont really need air conditioning, except it gets a bit hot at night for about two weeks in summer. I have considered getting an evaporative cooler, ( a very basic air conditioner) which might use less electricity so should be less of a problem, (I must look into that). ”

    MKIA: Is AC essential in some places as you suggest? Apparently not. Thousands of years went by without it and enough people survived. Is it better with it? Many times yes, but it is also good to use less energy intensive ways to achieve comfort when possible.

    Don’t know where you live, thus can’t say if an evaporative cooler would help you much, BUT if others around you use them, then perhaps they work – ask your neighbors. In higher humidity areas they don’t do well, but if the air is dry they’re as good as refrigerated air conditioning, but they do add H2O, a GHG, to the air. Perhaps the scientists can tell us if added H2O using evap coolers is better than belching more CO2 to power a refrigerated air conditioner. Evap cooler still has to have a supply air fan, a pump, and sometimes an exhaust air fan, but the power draw should be lower than a traditional AC unit.

    In the mean time, nigelj, you can reduce the solar gain thru your windows by putting aluminum foil on them and perhaps putting some insulation behind that. Rigid foam insulation is sold here in the US that is 2′ x 4′ x 1/2″ (up to 3″ thick), and some of it has a reflective foil face. The 1/2″ sheets can be placed inside your windows, foil out, and will cut the heat gain quite a bit. Also, there are bamboo devices that hang on the outside of your windows to prevent the sun from even getting thru one layer of glass; same with awnings – best to not let the sun hit the glass at all. Good luck fighting the heat.

    I would like to see legislation that allows home owners to put up shading devices, solar heating devices, PV panels, etc for better energy efficiency and self-sufficiency even when local regulations such as HOA CC&Rs and local building codes don’t allow them. This would open up the possibility for big employment opportunities and less consumption of power and fuels.

  8. 158
    nigelj says:

    Mr KIA @157, the trouble is air conditioning has become virtually essential in some situations. High rise glass faced towers are sometimes virtually uninhabitable if the power fails in summer.

    Climates with high temperatures plus humidity can actually have lethal conditions without air conditioning, at least for the old and frail. This will become more common with climate change.

    But anyway, yes the best way to keep heat out of your house is some sort of screen on the outside of the glazing like an awning, shutter or whatever. Stop the heat before it gets inside, because not all of it will radiate back through the glass even with solar curtains. Tin foil is a cheap and cheerful version I suppose. But none of this is so easily applicable to commercial buildings.

    But these are all going to be ambulance at the bottom of the cliff solutions to global warming.

  9. 159

    KIA, #157–

    Perhaps the scientists can tell us if added H2O using evap coolers is better than belching more CO2 to power a refrigerated air conditioner.

    Ooh! I know! I know!

    Yes, it’s better because:

    1) There’s unlikely to be a net addition of water to the Earth system (though it’s possible if the water was drawn from a deep aquifer, but even then the addition is a minuscule proportion of stocks (ie., the world ocean));

    2) H2O is not long-lived in the atmosphere, typically being precipitated out in a matter of days. That’s why, despite the power of H2O as a greenhouse gas, it does not act as a climate ‘control knob.’ Put another way, a gram of water emitted to the atmosphere is going, over the lifetime of the planet, to be spending far more time in the ocean than anywhere else.

    Expanding a bit, per the USGS, Earth has about 1.4 billion km3 of water, with about 13,000 km3 in the atmosphere at any one time. (On point 2, that suggests that ‘far more time in the ocean’ really means a million times more duration.)

    Continuing to expand, at a billion tonnes per cubic kilometer, that’s ~ 13 trillion tonnes of water in the atmosphere, or 1.3 x 10e17 kg–if I’ve kept my exponents straight all the way through the calculation.

    Getting back to point #1, the mass of *CO2* in the atmosphere is about 3 x 10e15 kg.

    So adding fossil water to the atmosphere is per mass unit on the order of 100 times less effective proportionately than is adding fossil CO2–even before you try to account for the differences in atmospheric residence time. Denialati sometimes riff about how ‘tiny’ the proportion of CO2 in the atmosphere is, but in a way that’s the whole point; if the proportion weren’t relatively small, it would be much more resistant to human modification than it is.

  10. 160
    Dan H. says:

    You are correct if you confine your parameters to the short term. Over the long haul, that is not the case, as shown in the same Figure 2.

    The same is true of the heat wave index depicted in Figure 1. A short term increase, but a larger long term decrease. Are we talking weather or climate here? I would argue that the long term climatic trend has not changed with regards to high temperatures or heat waves, but has changed with regards to nightly low temperatures.

    Several other cities show the same trend. Those were just two examples to placate Hank, who has selective citation syndrome.

  11. 161
    Barton Paul Levenson says:

    KM 159,

    Good exposition overall, but you slipped a decimal point–the amount of water vapor in the air is 1.27 x 10^16 kg, not 10^17.

  12. 162
    nigelj says:

    Dan H @160, the very long term trend in figure one is not the point. It clearly shows an increasing trend since 1980 – 2018 which is pretty long term, and approximately consistent with the global warming trend, thus showing a reasonable correlation between warming and heatwaves.

    Your own source material shows an increase in high temperatures during the day. You have presented no sensible ‘argument’ to believe those are wrong. You cant ignore this and selectively focus just on night time temperatures.

    Your arguments about heatwaves are flat wrong. I have shown you research papers showing an increase in the numbers of heatwaves in America. You haven’t been able to refute these.

    What other cities? What proportion is this of all cities? (and with proof please)

    Is this all the best you have Dan? You have nothing!

  13. 163
    Carrie says:

    Dan, re who has selective citation syndrome.

    I think you’ll find that’s Universal Norm aka a Default Position. :-)

    We all do it because it’s normal. There are limits to citations refs etc. Besides the extent and the specifics of those references do not necessarily lead to a change of opinion anyway. Meaning and comprehension are more complex than a citation to data or conclusions in a paper / article etc.

    Besides that, there is never universal agreement on anyone’s Research / Science Paper – peer reviewed or not is irrelevant to this truism fact. The other universal is it’s easier to find hens teeth than it is someone to have an open-minded genuine dialogue or a discussion in good faith while maintaining an even keel. Being aware that’s even an issue worth addressing is even rarer. Cheers

    Give the world the best you have and you’ll get kicked in the teeth.
    Give the world the best you have anyway.

  14. 164
    Dan H. says:

    That is rather brazen of you to claim that my references are wrong and yours are correct. From all I can tell, our big difference is in our definition of “long-term.” I contend that long-term (with regards to AGW) refers to a period of a century or more. Your 30-year time period could be define as medium-term at best, but not long-term. The century-long epa dataset shows an increase in daily highs for the first 20 years, then a decrease over the next 40, followed by an increase over the past 40 (roughly), with no overall trend. Conversely, the increased in hot daily lows shows a smaller increase over the early years, followed by much higher increase in recent years.

    What is not evident from this dataset, but is exemplified in the heat wave index, is that most of these new highs occurred in the colder months, not in summer. Hence, heat waves decreased over the long term, and have shown no trend over the short term. Perhaps this reference may help to enlighten:

    Note in particular table 6.2, which details changes in the hottest and coldest days of the year over the past 30 years. Every region of the U.S. has experienced an increase in the coldest days, ranging from 1.1F to 4.8F, averaging 3.3F. Meanwhile, all regions but the desert southwest have witness a decrease in the hottest day, ranging from -0.2F to -2.2F, averaging -0.9F. Figure 6.3 shows the change over the past century. The coldest days increased by 6F, while the hottest days increased by 2F over the first 40 years, then decreased by 2F over the next 40, and have been relatively flat since. Heat waves are detailed in 6.4, showing a large increase from the beginning of the 20th century through the mid 1930s, followed by a larger decrease through the mid 70s, and a smaller increase to date, such that heat waves remain below the level seen at the start of the 20th century.

    From the reference, “As with warm daily temperatures, heat wave magnitude reached a maximum in the 1930s. The frequency of intense heat waves (4-day, 1-in-5 year events) has generally increased since the 1960s in most regions except the Midwest and the Great Plains. , Since the early 1980s (Figure 6.4), there is suggestive evidence of a slight increase in the intensity of heat waves nationwide.”

    Once again, the devil is in the details.

  15. 165

    #161–Thanks, Barton; that’s a type of error I’m prone to (deep layers of mathematical ‘rust’; many relatively simple calculational tasks just aren’t a regular part of my life).

  16. 166
    Mr. Know It All says:

    159 – Kevin
    Thanks for the excellent explanation!

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