The section of the Paris Agreement that sets the objective of not exceeding an increase of 1.5ºC in the global average temperature (compared to the average prior to the start of the industrial era) has become a symbol of the fight against change climate change and a reference for studies on the environmental, social and health consequences of global warming.
The latest application of the 1.5ºC indicator to human well-being is presented this week in a study published in the journal Nature Sustainability showing estimates of the millions of people who may be affected by dangerously high temperatures in the in the event that the reference indicated in the Paris Agreement is exceeded.
The first of the warnings of this study led by experts from the Institute of Global Systems of the University of Exeter (United Kingdom) is that "current climate policies will leave more than a fifth of humanity exposed to dangerously high temperatures by 2100" .
And it is that, despite the objective of the Paris Agreement to keep global warming below 1.5 ºC (compared to pre-industrial levels), it is estimated that at the current rate and with the actions planned by governments the increase real be 2.7 °C by the end of this century, recall the authors of the new study.
As will be remembered, the most recent predictions about the possible overcoming of the 1.5ºC barrier are not at all optimistic. The planet is on track to exceed the limit of 1.5ºC increase before 2027, indicated last week the report for the next five years published by the World Meteorological Organization (WMO).
To understand the effect of this global warming on human well-being, the authors use the definition of "climate niche" as the range of temperatures in which people can live without danger. Based on this definition, the percentage of inhabitants (and approximate figures) that will live outside the climatic niche is calculated depending on the increase in temperatures that is reached at each moment.
At the moment, the study indicates, it is estimated that "around 60 million people are already exposed to dangerous heat, considered as an average temperature of 29 °C or more."
But this figure is minuscule when compared to the 2,000 million people, 22% of the world population, who are estimated in this study to be subjected to or exposed to risks in the event that global warming is of the order of 2.7°C.
The now-published document highlights the "enormous potential" of decisive climate policy to limit the human costs and inequalities of climate change.
Limiting warming to 1.5°C would leave 5% of humans exposed to higher than recommended conditions, saving a sixth of humanity from dangerous heat compared to 2.7°C of warming.
In the "worst case scenarios" of global warming of 3.6°C or even 4.4°C, half of the world's population could fall outside the climatic niche, what the researchers call an "existential risk".
"The costs of global warming are often expressed in financial terms, but our study highlights the phenomenal human cost of not addressing the climate emergency," said Professor Tim Lenton, Director of the Global Systems Institute at the University of Exeter.
“For every 0.1°C of warming above current levels, around 140 million more people will be exposed to dangerous heat.
“This reveals both the scale of the problem and the importance of decisive action to reduce carbon emissions.
“Limiting global warming to 1.5°C instead of 2.7°C would mean five times fewer people in 2,100 exposed to dangerous heat.”
Historically, human population density has peaked in places with an average temperature of around 13°C, with a secondary peak of around 27°C (monsoonal climates, especially in South Asia).
Crop and livestock densities follow similar patterns, and wealth (as measured by GDP) peaks at around 13°C.
Mortality increases at both higher and lower temperatures, supporting the idea of a human "niche".
Although less than 1% of humanity currently lives in places exposed to dangerous heat, the study shows that climate change has already left 9% of the world's population (more than 600 million people) out of the niche.
“Most of these people lived near the colder 13°C peak of the niche and are now in the 'middle ground' between the two peaks. While not dangerously hot, these conditions tend to be much drier and have not historically supported dense human populations," said Professor Chi Xu of Nanjing University.
“In the meantime, the vast majority of people who will be left out of the niche due to future warming will be exposed to dangerous heat.
“Such high temperatures have been linked to problems including increased mortality, lower labor productivity, lower cognitive performance, learning disabilities, adverse pregnancy outcomes, lower crop yields, increased conflict, and the spread of infectious diseases.”
While some colder places may become more habitable due to climate change, population growth is projected to be higher in places at risk of dangerous heat, especially India and Nigeria.
Exposure to dangerous heat begins to rise dramatically at 1.2°C (just above current global warming) and increases by approximately 140 million for every additional 0.1°C of warming.
Assuming a future population of 9.5 billion people, India would have the largest population exposed to global warming of 2.7°C: more than 600 million. At 1.5°C, this number would be much lower, at about 90 million.
Nigeria would have the second largest population exposed to heat with global warming of 2.7°C, more than 300 million. With a warming of 1.5°C, this would be less than 40 million.
India and Nigeria already show "hot spots" of dangerous temperatures.
At 2.7°C, almost 100% of some countries, including Burkina Faso and Mali, will be dangerously hot for humans. Brazil would have the largest land area exposed to dangerous heat, despite almost no area being exposed to 1.5°C. Australia and India would also experience massive increases in exposed area.
The research team, which included the Potsdam Institute for Climate Impact Research, the International Institute for Applied Systems Analysis, and the Universities of Washington, North Carolina State, Aarhus, and Wageningen, emphasize that the worst of these impacts it can be avoided with quick action to reduce greenhouse gas emissions.
Speaking about the conception of his idea, Professor Marten Scheffer, from Wageningen University, said: “We were buoyed by the fact that the economic costs of carbon emissions barely reflect the impact on human well-being.
"Our calculations now help to close this gap and should stimulate new and unorthodox questions about fairness."
Ashish Ghadiali, from the Exeter Institute for Global Systems, said: "These new findings from the cutting edge of Earth systems science underscore the deeply racialized nature of projected climate impacts and should inspire a step-change in policy thinking. on the urgency of decarbonization efforts as well." as in the value of massively increasing global investment on the front lines of climate vulnerability.”
The research was funded by the Open Society Foundations and the paper is also a result of the Earth Commission, convened by Future Earth, the Earth Commission is the scientific cornerstone of the Global Commons Alliance.
Wendy Broadgate, Executive Director of the Land Commission at Future Earth, said: “We are already seeing the effects of dangerous levels of heat on people in different parts of the world today. This will only accelerate unless we take immediate and decisive action to reduce greenhouse gas emissions."
Work on climate solutions from the University of Exeter's Institute for Global Systems identified "positive tipping points" to accelerate action, including a recent report that highlighted three "super leverage points" that could trigger a decarbonisation cascade.