The climate crisis is not a future problem – it is already here and affecting our health. In the biggest study of its kind, a team of epidemiologists and climate scientists has determined that 37% of global heat-related deaths over the past 30 years can be attributed to human-induced climate change.
The study, published in Nature Climate Change, used data from 1991 to 2018, taken from 732 locations in 43 countries around the world. The results are grim: an increase in mortality since the preindustrial period is evident on every single inhabited continent.
This is another stark warning that as temperatures rise and heatwaves become more severe and frequent, climate change is directly affecting human health.
Ana M. Vicedo-Cabrera, lead author from the University of Bern, Switzerland, says: “We expect the proportion of heat-related deaths to continue to grow if we don’t do something about climate change or adapt. So far, the average global temperature has only increased by about 1°C, which is a fraction of what we could face if emissions continue to grow unchecked.”
In an accompanying News & Views article, climate expert Dann Mitchell from the University of Bristol, UK, explains: “Mortality associated with rising temperatures is one of the clearest and impactful fingerprints of a changing climate.
“Increased air pollution, wildfires, agricultural stresses and direct physiological failures – these are just some of the consequences of a heatwave that impact health.”
Teasing out the complex relationship between heat and health is not a trivial undertaking. The study draws on the largest available dataset of heat mortality to date, with particularly detailed coverage in the Americas and Europe. The team combined epidemiological methods and climate models to create alternate history scenarios – examining past weather conditions with and without human-induced climate change.
This allowed them to separate anthropogenic-warming-related health impacts from natural trends.
The team found that more heat-related deaths occurred in the world with human-induced climate change, and these deaths increased exponentially during the extreme temperatures of the hot season.
“The real danger, however, occurs when these heat-mortality-sensitive countries coincide with rapid temperature increases from climate change,” Mitchell says.
“Paraguay and North Vietnam were highlighted as acute examples of where this has already happened. In these countries, total mortality – that is, mortality from any cause – was modelled to have increased by up to 5% since pre-industrial times due solely to more heat-related deaths from climate change, although uncertainties in these numbers are large.”
A large percentage of climate change-related deaths were reported from warmer countries in southern Europe, as well as Central and South America (for example, up to 76% in Ecuador or Colombia) as well as southern and western Asia (48–61%). This makes sense because the effects of rising temperatures are uneven – some regions experience more warming than others, putting people at more risk of heat-related health impacts.
The study did not, however, include data from most of Africa and the tropics, and for some countries data could only be taken from a few cities.
Mitchell explains that multi-continent studies of the relationship between heat and mortality are fairly new, requiring “sophisticated statistical methods combined with daily mortality observations for individual cities or provinces, and many countries lack these data”.
“Indeed, for many lower- and middle-income countries, these data do not appear to exist at all, and certainly not in accessible formats. Therefore, a truly global heat-health analysis is not yet possible,” he writes.
In the future, collecting this data and understanding regional differences will be crucial – because countries without such data are often among the poorest and most vulnerable to climate change, as well as the places that will likely experience the most population growth.
Hanigan, who was not involved in this new work, says it adds “new information that suggests that whilst Australia might initially experience a net reduction in temperature-related deaths because the winters become more mild, the increasing heat-related impacts in summer will be a substantial public health concern”.
“In fact, it’s almost certain that future summer deaths will come to account for the majority of temperature-related deaths in Australia,” he says.
Hanigan stresses that if we continue our emissions on a business-as-usual pathway, this switch will happen by the middle of this century.
“This study shows that this dire prediction is in fact supported by the recent historical data.”
Another independent researcher, climate scientist Sarah Perkins-Kirkpatrick from UNSW Canberra, says the results are important but unsurprising.
“It is good to actually see some studies actually starting to address this issue and be able to give us numbers and more hardcore scientific evidence that yes, more people will be adversely affected health-wise,” she says.
Perkins-Kirkpatrick also notes that a multidisciplinary approach, involving epidemiologists and climate scientists, will be increasingly important as we continue to grapple with the impacts of climate change.
“We have to be able to speak the same language to get this work done,” she says.
The problem is, she explains, “sometimes these estimates of health burdens can’t be estimated from the climate data because the models simply aren’t in a format or in the right condition to be able to do that” – but now, with collaborations across disciplines, there is likely to be a lot more research in this space over the coming years.
The researchers hope that their findings will trigger not only further studies, but also action on climate change.
In their paper, they write that their results “support the urgent need for more ambitious mitigation and adaptation strategies to minimise the public health impacts of climate change”.
Perkins-Kirkpatrick agrees that we need to think seriously about adaptation.
“Moving forward we cannot consider relying on the same health resources we have, before we need more injections into those to cope with heatwaves better because it’s only going to get worse,” she says.
Senior author Antonio Gasparrini from the London School of Hygiene & Tropical Medicine concludes: “The message is clear: climate change will not just have devastating impacts in the future, but every continent is already experiencing the dire consequences of human activities on our planet. We must act now.”