In biology, it is quite simple: living beings always settle where they find good conditions, where there is enough food and pleasant temperatures. Humans are no exception, at least to a large extent: This is why most people live in regions with an average annual temperature of between six and 28 degrees Celsius – in their climate niche. But with climate change, these so-called niches will become fewer. People will have to live where it is far too hot and too dry, in zones that are considered "almost uninhabitable". Exactly: outside the climate niche.

Pia Heinemann

Editor Nature and Science

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Scientists now want to use this concept of the climate niche to make the general public aware of the consequences of global warming more vividly than is the case with previous representations that focus on the economic costs.

In a study now published in the journal "Nature Sustainability", an international team of researchers led by Timothy M. Lenton from the University of Exeter and Marten Scheffer from the University of Wageningen has calculated how many people will fall out of the climate niche by the end of the century. "So far, nine percent, and thus more than 600 million people, live outside this niche," the authors write. By the end of the century, it could be a third of humanity. At least if the current climate policy would not change drastically – and thus a global warming of 2.7 degrees Celsius would be achieved. By contrast, if it were possible to limit warming to just 1.5 degrees Celsius, only 14 percent of all people would have to live outside the climate niche by the end of the century.

The scientists assume a global demographic increase to up to 9.5 billion people in 2070 with a subsequent decline. This puts you in a medium scenario. In Nigeria, India and Indonesia, many people will slip out of the climate niche, according to the calculation. In Burkina Faso, Mali and Qatar, particularly large areas would become almost uninhabitable.

In the study, Lenton's team also emphasizes that for every 0.3 degrees Celsius avoided temperature increase, 350 million fewer people would have to live in regions beyond the climate niche.

Lisa Schipper, who researches developmental geography at the University of Bonn and was not involved in the study, tells the Science Media Center that the concept of the human climate niche is "extremely useful." It is true that no metric, not even that of the current study, will accurately reflect reality. "The idea that fewer and fewer people will be able to live a decent life is directly related to the IPCC's warning that the window of opportunity to ensure sustainable and livable lives for all people is closing," she says. "Even with 1.5 degrees of warming, this is probably not possible, as the authors of the current study show."

Christian Franzke from the IBS Center for Climate Physics at Pusan National University in South Korea also thinks the concept of the climate niche makes sense. "The 'human climate niche' is, of course, a simplification of the complexity of the problem, but it is illustrative and shows the implications for poor countries, which will have less damage in economic terms because they are poor. That's the advantage of this approach."

How much migration will be needed?

Previous studies have focused either on the economic costs of climate change or on mortality. The current study uses demographic projections that a growing population assumes, especially in already warm areas. These regions in particular are severely affected, but have fewer opportunities to adapt. "Adaptation is possible, but it will cost money that poor countries and poor people don't have." For example, in developed countries such as Germany, most work takes place in buildings that can be cooled, but in many developing countries this is not the case. There would be a lot of work outside. "You could change the working hours and introduce a siesta at noon like in Spain, but you probably won't be able to maintain all the labor productivity."

Richard J. T. Klein of the International Climate Risk and Adaptation team at the Stockholm Environment Institute (SEI) in Sweden complains that the study does not take into account the fact that habitats could also become uninhabitable, even though temperatures are still tolerable. "For example, drought and desertification can already occur within the climate niche and make agriculture almost impossible and thus the region uninhabitable. The same applies to low-lying areas threatened by flooding and sea level rise. In other words, there are regions within the human climate niche that could become uninhabitable for other reasons."

However, he also emphasizes positively that the concept of the climate niche illustrates how differently global warming affects rich and poor countries. People can adapt to many things, but only if there are enough resources. "However, for many of the people and countries affected, this is not an option," Klein says. "So the question is what these people are going to do. Moving to cooler places? What are these places, and what opportunities will they have there? Could this lead to conflicts over scarce resources?

However, the study is not a cause for panic, Klein said. Rising temperatures increase the risk of an increasing number of climate refugees. "However, we should not forget that the vast majority of climate refugees have moved within their own country or to neighbouring countries." It is becoming increasingly important "to support the local people and to urgently limit further warming".