Climate change is remapping where humans can exist on the planet. As optimal conditions move away from the equator and toward the poles, more than 600 million people have already been stranded outside a crucial environmental niche that scientists say is best sustaining life. By the end of this century, according to a study published last month in the journal Nature Sustainability, 3 to 6 billion people, or between a third and a half of humanity, could be trapped outside that zone, facing the extreme heat, food shortages and rising death rates, unless emissions are drastically reduced or mass migration is accommodated.
The research, which adds new details about who will be most affected where, suggests climate-driven migration could easily eclipse even the broadest estimates, as huge segments of the Earth’s population seek safe havens. It also makes a moral case for immediate and aggressive policies to prevent such a change from happening, in part by showing how unequal the distribution of pain will be and how big the improvements could be with even small results in slowing the pace of warming.
There are clear and profound ethical consequences to the numbers, Timothy Lenton, a lead author of the studies and director of the Global Systems Institute at the University of Exeter in the UK, said in an interview. If we can’t level up with that injustice and be honest about it, then we will never make headway in international action on this issue.
The notion of a climate niche builds on work researchers first published in the Proceedings of the National Academy of Sciences in 2020, which determined that over the past 6,000 years, humans have gravitated toward a narrow range of temperatures and climate levels. rainfall that supported agriculture and, later, economic growth. That study warned that warming would make those conditions elusive for growing segments of humanity, and found that while just 1% of the earth’s surface is now intolerably hot, nearly 20% could be doing so by 2070.
The new study reconsiders population growth and policy options, and explores scenarios that greatly increase previous estimates, showing that the world’s environment has already changed significantly. He focuses more on temperature than precipitation, finding that most people have thrived on average annual temperatures of 55 degrees Fahrenheit.
If the world were to continue on its current path by gesturing towards moderate emissions reductions but without significantly reducing global carbon levels (a scenario close to what the United Nations calls SSP2-4.5) the planet will likely exceed the target of the Paris Accords to limit average warming to 1.5 degrees Celsius and instead warming to around 2.7 degrees. This pathway, which explains population growth in hot places, could push 2 billion people out of the climate niche within the next eight years, and 3.7 billion by 2090. But the study authors, who have argued in other articles that the most extreme warming scenarios are well within the realm of possibility, warn that even the worst cases should be considered. With 3.6 degrees of warming and a pessimistic climate scenario including ongoing use of fossil fuels, resistance to international migration and much faster population growth (a scenario referred to by the United Nations as SSP3-7), the niche A changing climate could pose what the authors call an existential risk, directly affecting half of the projected total population, or in this case, up to 6.5 billion people.
The data suggests that the world is rapidly approaching a tipping point, after which even small increases in global average temperature will start to have dramatic effects. The world has already warmed by about 1.2 degrees Celsius, pushing 9% of the earth’s population out of the climate niche. At 1.3 degrees, the study estimates the pace would pick up considerably, and for every tenth of a degree of additional warming, Lenton said, 140 million more people would be pushed out of the niche. There’s a real non-linearity lurking in there that we’ve never seen before, he said.
Slowing global emissions would dramatically reduce the number of people displaced or struggling with conditions outside the niche. If warming were limited to the 1.5 degrees Celsius under the Paris Accords, half of people would be outside the optimal zone, according to a calculation that isolates the warming effect. The population suffering from extreme heat would be reduced five-fold, from 22% to just 5% of the people on the planet.
Climate research often frames the implications of warming in terms of economic impacts, expressing the damages in monetary terms that are sometimes used to suggest that small increases in average temperature can be managed. The study disavows this traditional economic framework, which Lenton says is unethical because it prioritizes the wealthy who are alive today, and instead puts the climate crisis in moral terms. Findings show that climate change will disproportionately affect the poorest parts of the world, effectively dooming people living in developing nations and small island states to extreme temperatures, poor crops, conflict, water and food shortages and increased mortality. The last option for many people will be migration. The estimated size of the populations affected, whether 2 billion or 6 billion, suggests an era of global upheaval.
According to the study, India will have by far the largest population outside the climate niche. At current rates of warming, researchers estimate that more than 600 million Indians will be affected, six times as many as if the Paris goals had been met. In Nigeria, more than 300 million citizens will be exposed, seven times as many as if emissions were drastically reduced. Indonesia could see 100 million people leave a safe and predictable environment, the Philippines and Pakistan 80 million people each, and so on. Brazil, Australia and India would see the largest land area become less habitable. But in many smaller countries, all or most of the territory would become almost unlivable with traditional measures: Burkina Faso, Mali, Qatar, United Arab Emirates, Niger. While facing much more modest impacts, the United States will also see its South and Southwest fall toward the warmer end of the niche, leading to higher mortality and prompting northward internal migration.
Worldwide, the researchers estimate, the average person who will be exposed to unprecedented heat comes from a place that emits about half as many emissions per person as those in rich countries. American per capita emissions are more than double those of Europeans, who still live a prosperous and modern existence, the authors point out, so there is ample room for comfortable change short of substantial sacrifices. The notion that you need the level of wasteful consumption … that occurs on average in the United States to be part of a happy, thriving, wealthy, democratic society is obviously nonsense, Lenton said.
Every American today emits nearly enough emissions in their lifetime to push a future Indian or Nigerian out of their climate niche, the study found, showing exactly how much damage individual Americans’ actions can cause (1.2 Americans to 1 future person, to be exact). The lifestyle and policy implications are obvious: cutting consumption today reduces the number of people elsewhere who will suffer the consequences tomorrow and can prevent much of the instability that would otherwise result. I cannot as a citizen of a planet with this level of risk that opens up not also have some sort of human and moral response to the figures, Lenton said. We all have to face it, he added, in our own way.
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