Climate Change Could Become Leading Global Risk Factor for Health
Population exposure to heat is increasing due to climate change. Globally, extreme temperature events are observed to be increasing in their frequency, duration, and magnitude.

Rising global temperatures caused by greenhouse gas emissions could lead to 83 million excess temperature-related deaths by 2100, projects a new study conducted by a researcher at Columbia University’s Earth Institute. 

The study, published last week in Nature Communications, is one of the first to calculate the mortality impacts of climate change in the kinds of integrated assessment models (IAMs) that are being used by climate economists and policymakers to calculate the social cost of carbon (SCC). 

Integrated assessment models are increasingly being used by governments to make decisions about different climate mitigation policy choices, such as shifting investments from coal to renewable energy, based on social costs and benefits that can be obtained. But health impacts relied on outdated studies and comprised a small portion of the overall impacts of climate change in these models – leaving health as the ultimate outlier in the climate debate.  

The study created an extension to the influential Dynamic Integrated Climate-Economy model (DICE) created by Nobel prize-winning economist William Nordhaus, adding an Endogenous Mortality Response (EMR). 

The DICE model “is currently one of the models used by the US government to estimate the social cost of carbon, which informs trillions of dollars of regulations in the US, and the US social cost of carbon number is also used by other countries and states,” Daniel Bressler, lead author of the study and a PhD candidate in the Sustainable Development program at Columbia University, told Health Policy Watch. 

DICE-EMR was used to create a new metric, the “Mortality Cost of Carbon (MCC),” which can be considered as social cost calculations.

“In the DICE model, optimal climate policy is an emissions plateau and gradual reductions starting in 2050. This results in 3.5°C warming by 2100. Thus, the DICE model suggests that the UN Paris target of 2°C is too expensive relative to the benefits of limiting warming, and instead the world should aim for 3.5°C,” said Bressler

“However, once I go through the exercise of updating the temperature-related mortality impacts to the latest science, while keeping all other parts of the DICE model the same, the optimal climate policy now involves large immediate emissions reductions and full decarbonization by 2050, resulting in 2.4°C warming by 2100,” Bressler added.

Pursuing a more stringent climate policy that includes full decarbonization and keeps temperature rise to 2.4°C, would save 74 million lives over the course of the 21st century, the paper concludes – and that is in terms of temperature-related excess deaths alone. Such deaths would fall from 83 million to 9 million if temperature rise is kept in check.

The research comes ahead of the next major climate conference – scheduled to take place in November in Glasgow – which will be a pivotal moment in the fight against climate change.

Mortality from climate change is in fact occurring as a result of a much wider range of factors than temperature alone. These include food insecurity, increased infectious diseases, air and water pollution, and deforestation, as well as deaths from extreme weather events. Such l health impacts have been assessed by the World Health Organization and other global bodies.

But those estimates rarely find their way into the kinds of mainstream models being used for climate policy decisions at national or global level – leaving large and critical human health impacts of policy choices as “outliers” in key climate decisions – even though it is people, first and foremost, who are impacted by climate change.  

“Although substantial advances in climate impact research have been made in recent years, IAMs are still omitting a significant portion of likely damages (13,14)” the paper states.

“Another major line of criticism is that a wide variety of climate damages—sea level rise, extreme weather, the direct effects of heat on productivity, agricultural impacts, and many more—must be monetized and summarized into a single number, and the relative contribution of these damages is often unclear (11,13,15).”

The Mortality Cost of Carbon (MCC) metric – does just that, connecting the dots between the broad climate and human health impacts.   

Real health impacts are still under-assessed  

Even so, the assessment remains limited to just assessing the impact of climate change on temperature-related mortality: the net effect of more hot days and fewer cold days. Many studies have shown that greater exposure to heat increases mortality through pathways such as dehydration, heart attack and stroke. The higher mortality from heat is expected to outpace the lower mortality from cold in most parts of the world.

As a result, the projections made are likely underestimates of total climate change mortality, due to the numerous other negative environmental impacts and secondary effects on health, livelihood, and wellbeing, Bressler notes. 

“Because I only project temperature-related mortality, you’d probably expect the mortality projections from the study to go up if we were able to capture other mortality pathways in the model,” said Bressler. 

“Climate change is likely to increase future mortality rates through a number of channels including the direct effects of ambient heat, interactions between higher temperatures and surface ozone formation, changes in disease patterns, flooding, and the effects on food supply,” said the study.

Climate change could rank sixth on Global Burden of Disease Risk Factors   

The model builds out projections for two heating scenarios – a baseline scenario in which the average temperature rise is 4.1°C by the end of the century, and the optimal DICE-EMR scenario in which temperatures would only rise by 2.4°C. 

The projected cumulative number of excess deaths from climate change in the DICE baseline scenario and the DICE-EMR optimal scenario.

In the case of 4.1°C, excess deaths would cumulatively reach 83 million, compared to nine million deaths associated with a temperature rise of 2.4°C.

“In total, we find that there are 83 million projected cumulative excess deaths between 2020 and 2100,” said the study, based on the baseline scenario. 

The business-as-usual scenario would leave climate change to rank sixth in terms of global burden of disease risk factors – even ahead of air pollution. 

“By the end of the century, the projected 4.6 million excess yearly deaths would put climate change 6th on the 2017 Global Burden of Disease risk factor list, ahead of outdoor air pollution (3.4 million yearly excess deaths) and just below obesity (4.7 million yearly excess deaths),” said the study. 

Mitigation costs and benefits compared from the health angle 

Based on those aggregate projections, the paper quantifies reductions in excess mortality that could be obtained by reducing one million metric tons (Mt) of carbon emissions per year.  

That is equivalent to the average annual emissions of 35 commercial airliners, 216,000 passenger vehicles, or 115,000 homes in the US. 

Each one Mt increase in CO2 emissions over 2020 levels is estimated to cause 226 deaths globally by the end of the 21st century – a decrease will save the same number of lives. 

The number of excess deaths from a marginal increase in temperatures is initially relatively modest but increases substantially with increasing temperatures.

Based on the model and its estimates, policy makers can then calculate the lives saved by climate mitigation measures – in terms of extreme heat exposures.  

For instance, replacing a coal-fired power plant with a zero-emissions alternative for one year could save 904 lives over the course of a century, the study projects.

Big disparities in deaths caused by emissions from rich and poor countries 

The study also points to the huge disparities between the huge carbon emissions of high-income and those in low-income countries. The lifetime emissions of 3.5 Americans – 4,434 metric tons of carbon dioxide – added to 2020 levels will lead to one excess temperature-related death by 2100.

It calculates that while the lifetime emissions of 3.5 Americans will result in one death, it would take 146.2 Nigerians to cause a single death. Globally, the lifetime emission levels of 12 people cause one death. 


The excess deaths per average citizen’s lifetime emissions, calculated as 2017 carbon dioxide emissions production per capita multiplied by 2017 life expectancy at birth.

“[These findings] could well have a significant impact on climate change policies,” Richard L. Revesz, Professor at New York University School of Law and one of the US’ leading experts on environmental law and policy, told the New York Times

The rise in global temperature could be tempered by aggressive climate policies

The study predicts that global temperatures will rise by 4.1°C above pre-industrial temperatures by the turn of the century if trends in emissions continue on the current trajectory. This will cause the mortality rate to increase by 3.8%.

Mortality rises at an increasing rate as the global temperature escalates.  

“When global average temperatures exceed 2°C, the first derivative is quite steep and increasingly so as the world continues to warm,” said the study. “This gives societies a strong incentive to avoid scenarios where global average temperatures are especially damaging.”

According to Bressler, the study’s model can be used to assess the effects of policy changes, such as the pursuit of different emissions targets, on mortality. 

If the world undertakes far-reaching efforts to reduce emissions, the rate of global warming could be slowed, found the study.

The study compared the baseline scenario with the initial DICE model – consisting of an emissions plateau and then gradual reductions starting in 2050 – and the revised DICE-EMR model – involving large emissions reductions and full decarbonization by 2050.

The first is projected to result in 3.5°C warming by 2100, while the second would result in 2.4°C warming by 2100. 

Integrated assessment models (IAMs) assess the cost of reducing emissions and the damages from climate change. They can be used normatively to determine optimal climate policy.

“Optimal climate policy changes from gradual emissions reductions starting in 2050 to full decarbonization by 2050 when mortality is considered,” said the study. 

“My model shows that a significant number of lives can be saved from pursuing a more aggressive global climate policy,” said Bressler. 

“If the world undertakes the optimal emissions path in DICE-EMR and restrains global average temperatures to 2.4°C, we largely avoid the temperatures where marginal increases in temperature resulting from a marginal emission today are most damaging,” said the study.

Image Credits: Oxfam East Africa, World Meteorological Organizations, Nature Communications.

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