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Chest X-ray of pulmonary tuberculosis
© Puwadol Jaturawutthichai/Shutterstock
PUBLIC HEALTH
19 June 2015 0:01

Explainer: Will rising temperatures mean more lives are saved than lost?

Roz Pidcock

Roz Pidcock

06.19.15
Roz Pidcock

Roz Pidcock

19.06.2015 | 12:01am
Public healthExplainer: Will rising temperatures mean more lives are saved than lost?

Higher temperatures and longer heatwaves will push more of us beyond our tolerance limits, leading to a rise in the number of deaths from heat-related illnesses, scientists say.

But you’ll  sometimes see it argued in  parts of the  media that the number of lives saved each year as winters get warmer will outweigh these extra deaths caused by heat-exposure.

new study strongly counters this view, however.

The research, published today in the journal Environmental Research Letters, says we shouldn’t expect a substantial drop in winter deaths after all, and that rising temperatures will have an overwhelmingly negative impact on death rates.

While that last part is almost certainly true, delving into the literature suggests there’s far more to this topic than headlines like this suggest. Carbon Brief weighs up the evidence.

As temperatures rise, so do the risks

Global temperature has risen by 0.85C over the industrial period and we’re already seeing more hot days, as well as more frequent and more intense heatwaves in Europe, Asia and Australia.  Scientists expect that with further warming, heat waves will become more common.

Maximum summer temperatures in the UK are expected to rise 2.8-5.4 degrees Celsius by the 2080s, relative to the 1961-1990 average, for example.

Maximum UK summer temperatures in the 2080s under a medium emissions scenario

Maximum UK summer temperatures in the 2080s under a medium emissions scenario, relative to a 1961-1990 average. The middle panel is the central estimate. Source: UK Climate Projections 2009.

There’s a limit to how much heat even healthy adults can tolerate. Rising temperatures in the coming decades will put more people at risk of excessive exposure, the Intergovernmental Panel on Climate Change (IPCC) says in its latest report:

“Throughout the 21st century, climate change is expected to lead to increases in ill-health…Examples include greater likelihood of injury, disease, and death due to more intense heat waves”.

Climate change will cause death mostly by exacerbating existing health problems, such as heart and respiratory disease, the report notes. Older people and those working outside are most at risk.

How many deaths will rising temperatures cause?

A 2014 study by Dr Shakoor Hajat from the London School of Hygiene and Tropical Medicine (LSHTM) looks at some figures for temperature-related death rates in the UK.

The paper estimates that around 2,000 people die each year in the UK from illnesses linked to hot weather. By 2050, the authors expect the death toll in the UK to rise by 257% as temperatures climb higher.

The change is equivalent to more than 5,000 extra heat-related deaths per year, taking the total up to more than 7,000 per year by mid-century. You can see this see by comparing the first and third red columns in the left-hand graph below.

Heat-related and cold-related deaths in the UK per year for all age groups

Heat-related (left) and cold-related (right) deaths in the UK per year for all age groups. Average estimates across nine models are shown, with the arrows representing the range of uncertainty. Source: Hajat et al., (2014).

These figures for future death rates assume we do nothing to increase resilience. But they are also likely to be on the low side because they assume emissions follow the IPCC’s medium emissions scenario, whereas the world is currently tracking much higher than that.

A UK government report covered by the Independent recently found that extreme heat in summer is likely to cause up to 1,700 more deaths in the UK over the next five years.

A large part of the increase in hot deaths in the coming decades is down to a growing population. The UK’s population is expected to rise from 63.7m in 2012 to 77m by 2050. That means on top of individual human tolerance levels being exceeded more often, there will be more people around to be exposed to the high temperatures. The population is also ageing, which means the proportion of the people most vulnerable to heat is also growing.

We’ll come back to the issue of population rise in a minute. For the moment, let’s turn our attention to deaths due to cold weather – and how they compare in the UK.

At the moment, cold kills more people than heat in the UK

In the UK, the cold is currently a bigger problem than heat. That is to say, more people in England and Wales currently die from cold in winter than from heat in summer.

The 2014 LSHTM study  estimated that about 41,000 people in the UK die each year in winter from exposure to the cold. That’s about 20 times as many as the 2,000 or so deaths each year from illnesses linked to hot weather.

You can see this difference by comparing the first blue column in the right hand graph above with the first red column on the left hand graph.

A recent study in The Lancet suggests a similar pattern exists across Europe. Collecting data from 13 countries, the authors found the cold kills 20 times as many people as heat.

And in the United States, twice as many people each year between 2006 and 2010 died from excessive cold than excessive heat, according to a recent health statistics report .

But what if climate change is altering those risks?

On balance, more lives will be lost than saved as temperatures rise

It’s been suggested that as temperatures rise, we’ll see a reduction in some parts of the world in the number of people dying from cold in winter. The IPCC says:

“Positive effects [of climate change] are expected to include modest reductions in cold-related mortality and morbidity in some areas due to fewer cold extremes.”

This is largely based on the idea that colder weather exacerbates conditions, such as bronchitis, pneumonia and heart disease, especially in older people.

The question this often leads to is, does the expected decrease in cold-related deaths outweigh the increase in heat deaths?

Let’s run the numbers for the UK.

Hajat et al.  predict 2% fewer cold deaths in the UK by the 2050s. Based on their estimate of around 41,000 fatalities in the UK each year now, that would suggest about 800 lives saved.

But with around 5,000 extra heat-related deaths by 2050 (see discussion above), that’s more than 4,000 extra fatalities each year than there are now, all other things being equal.

In other words, as UK temperatures rise, the increase in heat-related deaths is expected to rise faster than the decline in cold-related deaths. You can see this by comparing the slope of the bars in the red and blue graphs above.

But there are a few things to note about this conclusion. Here’s where it gets complicated.

Village covered in snow

Scientists disagree on whether or not warmer temperatures will save lives in the UK. Source: Village covered in snow via Shutterstock.

Are scientists sure that warmer winters will save lives?

The first thing is that not all scientists agree that rising temperatures will save lives. That’s where today’s research fits in.

The new paper examines the causes of death in older people during winter in 36 US cities from 1985-2006 and in three French cities from 1971-2007. The authors conclude that cities with warmer winters have a similar number of deaths in winter as those with colder winters, suggesting that cold-related deaths in winter may be much less than some have assumed.

Lead author Prof Patrick Kinney from Columbia University says in a press release:

“Most older people who die over the winter don’t die from cold, they die from complications related to flu and other respiratory diseases.”

A 2014 study by Philip Staddon from the University of Exeter came to a similar conclusion. It argued that while warmer winters were linked to fewer deaths per year up to the 1970s, we’re no longer reaping the benefits. Almost all winter deaths in England and Wales are now due to influenza and there’s no evidence milder temperatures will prevent them, it said.

But Hajat doesn’t agree. He and Dr Sari Kovatz, an expert on climate change and health at LSHTM, published a letter in Nature Climate Change last year, criticising the Staddon method for being too simplistic. It looked at how many extra people die in winter compared to other seasons and not all deaths due to cold weather are restricted to winter, they said.

The Hajat et al. approach is more comprehensive as it examines the direct relationship between temperature and mortality over a number of years (1993-2006) and uses that relationship, together with scientists’ projections of warming, to estimate future mortality.

The influence of a growing population

There’s a second point to make about the conclusion that the number of deaths from heat is increasing faster than the number of deaths from the cold is falling, which is that the picture looks quite different if you take population rise out of the equation.

In their 2014 paper, Hajat et al. repeat their calculations holding population steady – as though today’s population were transported 40 years into the future.

In this scenario, fewer people are exposed to high temperatures in the future. The result is that the number of lives lost as the climate warms goes down and the number of lives saved goes up. The balance shifts so dramatically that the drop in cold deaths by 2050 outweighs the rise in hot deaths, suggesting several thousand lives are saved each year.

But a future where the population doesn’t rise is an unrealistic one when thinking about changing risk. It’s a very different world from the one that’s predicted.

Nevertheless, you’ll see the static-population scenario  used from time to time in parts of the media, sometimes as part of a wider argument that climate change will “is good for us.”

The same point can be argued a different way.

Hajat et al. also show that for every different age group in the UK, the increase in heat-related deaths per 1,000 people by 2050 is outweighed by a greater reduction in cold-related deaths. This could be – and often is – used to argue that at a given point in time, any single individual has a greater risk of death by cold than by heat. That’s not incorrect – but it doesn’t tell you anything about the global risk or how it’s changing.

A too-simple lens

This topic is almost boundlessly complicated.

The research that exists suggests the burden of cold weather in the UK is currently greater than for heat, but that the increase in heat-related fatalities expected from climate change outweighs any potential benefit in terms of lives saved, if indeed there is a benefit at all.

That means that unless measures are taken to protect vulnerable groups, the UK faces a significant new public health risk from heat-related illnesses as temperatures rise – adding to the already considerable burden of deaths from the cold.

Of course, arguments like this about how many lives will be lost or saved as temperatures rise are a oddly callous lens through which to view changing risk, as it assumes a life saved in one place compensates for one lost somewhere else.

Moreover, the impact of climate change on human health is infinitely more complicated than this. A new Lancet report expected on Tuesday will look at how climate change affects the distribution of vector-borne diseases, reduces water and food security, and increases extreme weather. Today’s paper also highlights how food poisoning increases with warmer summer temperatures, while rising pollution and pollen worsen respiratory problems.

Finally, comparing mortality rates in the UK or any single country in the northern hemisphere – though it may be where most of the evidence exists – is an extremely narrow way to view a global issue.  Climate change will shift the balance of risk over different timescales, in different regions and in different ways and we should be prepared to intervene to protect those most at risk, today’s paper concludes.

Main image: Chest X-ray of pulmonary tuberculosis.
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