Published as billions of people are locked down by the corona crisis, the findings are a stark warning that continued carbon emissions would put the world at increasing risk of further unprecedented crises, the international research team of archaeologists, ecologists, and climate scientists concludes.

”We have discovered, that humans by and large have lived in areas with moderate temperatures during the last 6000 years, despite all innovations and migrations," says professor Jens-Christian Svenning, head of Center for Biodiversity Dynamics in a Changing World at Aarhus University and a coauthor of the study.

Human populations are largely concentrated in narrow climate bands, with most people living in places where the average annual temperature is about 11-15°C and a smaller number of people living where the average temperature is about 20-25°C.

“This strikingly constant climate niche likely represents fundamental constraints on what humans need to survive and thrive,” says Professor Marten Scheffer of Wageningen University, who coordinated the research with his Chinese colleague Xu Chi, of Nanjing University.

From 15° to unbearable in 50 years

By combining that knowledge with the projected developments in global temperatures and populations, the scientists have reached the conclusion, that by year 2070 more than three billion people may be living in areas with mean annual temperatures above 29 °C - like you only find in small areas in the Sahara region in our time.

If the human greenhouse gas emissions continue unabated, a person in 2070 on average will experience an increase in temperature twice as high as the projected global average increase. That is because land will warm much faster than the ocean and also because population growth is biased towards already hot places.

These climate conditions are currently experienced by just 0.8% of the global land surface, mostly in the hottest parts of the Sahara desert, but by 2070 the conditions could spread to 19% of the planet’s land area.

”This would bring 3.5 billion people into near-unliveable conditions,” says Jens-Christian Svenning.

On billion people per degree

However, he sees a possible light at the end of the tunnel. A small one.

”A starting point of our study is a pessimistic but realistic scenario for what may happen, if we do not manage to reduce the carbon emissions. Rapid reductions in greenhouse gas emissions could halve the number of people exposed to such hot conditions. Each degree warming above present levels corresponds to roughly one billion people falling outside of the climate niche. On the other hand that means, that for every degree that we manage to curb the global warming we can save a billion people from the extreme heat," says Jens-Christian Svenning.

So why is the light at the end of the tunnel small? Because global warming is already happening.

”Even with the most positive projections, at least one billion people are going to experience a mean annual temperature over 29°C where they live by 2070,” he says.

Mass migration or human development

The authors note that part of the 3.5 billion people exposed to extreme heat if climate change goes unabated may seek to migrate, but stress that many factors other than climate affect decisions to migrate and part of the pressure to move could potentially be addressed through climate adaptation.

“Foreseeing the actual magnitude of climate driven migration remains challenging. People prefer not to migrate. Also there is scope for local adaptation in part of the world within limits, but in the Global South this will require boosting human development rapidly. This study underscores why a holistic approach to tackling climate change that includes adapting to its impacts, addressing social issues, building governance, and empowering development as well as compassionate legal pathways for those whose homes are affected, is crucial to ensuring a world in which all humans can live with dignity,” says professor Marten Scheffer.

And Jens-Christian Svenning adds:

"It will take a strong society to cope with the stress, that such high temperatures will induce. With the prospect of at least a billion people expecting such extreme temperatures in the future, it is important to facilitate a strong human development in the developing countries in the affected areas."

Climate vs. corona

The corona virus has changed the world in ways that were hard to imagine a few months ago.

"And our results show how climate change could do something similar. Change would unfold less rapidly, but unlike with the pandemic, there would be no relief to look forward to: large areas of the planet would heat to barely survivable levels and they wouldn’t cool down again. Not only would this have devastating direct effects, it leaves societies less able to cope with future crises like new pandemics. The only thing that can stop this happening is a rapid cut in carbon emissions,” says Marten Scheffer.

Double-checking the results

According to professor Xu Chi of the Nanjing Universitety, the researchers were blown away by their own initial results.

“As our findings were so striking, we took an extra year to carefully check all assumptions and computations. We also decided to publish all data and computer codes for transparency and to facilitate follow-up work by others. The results are as important to China as they are to any other nation. Clearly we will need a global approach to safeguard our children against the potentially enormous social tensions the projected change could invoke,” says Xu Chi.

Contact

Professor Jens-Christian Svenning,

Department of Biology 
Center for Biodiversity Dynamics in a Changing World
Aarhus University
Mail: svenning@bios.au.dk
Mobile: +45 2899 2304

Further information

Please find the scientific article Future of the human climate niche in PNAS here

and watch professor Marten Scheffer at Wageningen University explaining the study in the video below.