The climate crisis is on track to displace one-third of humanity.

Climate change is redrawing the map of where humans can live on Earth. As optimal conditions migrate away from the equator and toward the poles, more than 600 million people have already been stranded outside of a crucial environmental niche, according to scientists. According to a study published last month in the journal Nature Sustainability, 3 to 6 billion people, or between one-third and one-half of humanity, could be trapped outside of that zone by the end of this century, facing extreme heat, food scarcity, and higher mortality rates, unless emissions are drastically reduced or mass migration is accommodated.

The research, which adds novel information about who will be most affected and where, implies that climate-driven migration could easily surpass even the largest estimates as vast portions of the global population seek refuge. It also makes a moral case for immediate and aggressive policies to prevent such a change from occurring, in part by illustrating how unequally the suffering will be distributed and how great the gains could be with even modest successes in slowing the rate of warming.

In an interview, Timothy Lenton, one of the study’s main authors and director of the Global Systems Institute at the University of Exeter in the United Kingdom, stated, “The numbers have clear and profound ethical implications.” “International action on this issue will never advance if we cannot face this injustice head-on and be truthful about it.”

Based on research published in the Proceedings of the National Academy of Sciences in 2020, the concept of a climate niche asserts that for the past 6,000 years, humans have gravitated toward a narrow range of temperatures and precipitation levels that supported agriculture and, later, economic growth. This study warned that global warming would make these conditions elusive for a growing portion of the human population and found that while only 1% of the earth’s surface is currently intolerably heated, by 2070 that number could increase to nearly 20%.

This map is based on data from the Proceedings of the National Academy of Sciences-published 2020 study on the climatic niche conducted by researchers. Al Shaw and ProPublica

The new study reexamines population growth and policy options and investigates scenarios that substantially increase earlier estimates, demonstrating that the global environment has already undergone significant change. It concentrates more on temperature than precipitation and concludes that the majority of people have thrived in annual mean temperatures of 55 degrees Fahrenheit.

If the world continues on its current path — making gestures toward moderate reductions in emissions but not meaningfully reducing global carbon levels (a scenario similar to what the United Nations refers to as SSP2-4.5) — the planet is likely to exceed the Paris Agreement’s goal of limiting average warming to 1.5 degrees Celsius and instead warm by about 2.7 degrees. This scenario, which factors for population growth in hot places, could result in 2 billion people falling outside the climate niche within the next eight years, and 3.7 billion by the year 2090. However, the authors of the study, who have argued in other publications that the most extreme warming scenarios are well within the scope of possibility, caution that the worst-case scenarios must also be considered. With 3.6 degrees of warming and a pessimistic climate scenario that includes continued fossil fuel use, resistance to international migration, and much more rapid population growth (a scenario referred to by the U.N. as SSP3-7), the shifting climate niche could pose what the authors refer to as “an existential risk,” affecting as many as 6.5 billion people directly.

The data indicate that the world is rapidly approaching a tipping point, after which even modest increases in global average temperature will have dramatic effects. Already, the world has warmed by approximately 1.2 degrees Celsius, forcing 9 percent of the global population out of its climate niche. At 1.3 degrees, the study estimates that the rate will accelerate significantly, and for every tenth of a degree of additional warming, Lenton estimates that an additional 140 million people will be forced out of the niche. “There’s a real nonlinearity lurking in there that we hadn’t seen before,” he said.

Reducing global emissions would significantly reduce the number of people displaced or coping with conditions outside their niche. According to a calculation that isolates the effect of warming, half as many individuals would be left outside of the optimal zone if warming were limited to the 1.5 degrees Celsius target set by the Paris accords. The number of individuals suffering from extreme heat would decrease from 22% to 5% of the global population.

Climate research frequently frames the implications of warming in terms of its economic effects, couching damages in monetary terms that are sometimes used to imply that minor average temperature increases can be managed. The study rejects this traditional economic framework, which Lenton deems “unethical” due to its prioritization of currently-living wealthy individuals, and instead frames the climate crisis in moral terms. The findings indicate that climate change will disproportionately impact poorer regions of the globe, effectively condemning the inhabitants of developing nations and small island states to extreme temperatures, crop failure, conflict, water and food scarcity, and rising mortality. Migration is the final option for many individuals. The anticipated size of the affected populations, whether 2 billion or 6 billion, indicates a period of global disruption.

India will have by far the largest population outside of the climate niche, according to the study. At the present rate of warming, the researchers estimate that over 600 million Indians will be impacted, which is six times more than if the Paris goals are met. More than 300 million Nigerians will be exposed, which is seven times more than if emissions were drastically reduced. Indonesia could lose 100 million people from a secure and predictable environment, the Philippines and Pakistan could lose 80 million each, etc. In Brazil, Australia, and India, the most land would become uninhabitable. In contrast, the land in a number of smaller nations, including Burkina Faso, Mali, Qatar, the United Arab Emirates, and Niger, would be rendered virtually uninhabitable by conventional standards. Even though the United States will experience much more moderate impacts, the South and Southwest will move toward the warmest end of the niche, resulting in increased mortality and internal migration to the north.

The researchers estimate that the average individual who will be exposed to unprecedented heat originates from a region that emitted roughly half as much carbon dioxide per capita as wealthy nations. The authors note out that American emissions per capita are more than twice those of Europeans, who continue to live a prosperous and modern lifestyle, so there is ample room for comfortable change without substantial sacrifice. “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,” said Lenton.

The study found that each American emits nearly enough emissions over the course of their lifespan to force one Indian or Nigerian of the future out of their climate niche (1.2 Americans to 1 future person, to be exact), demonstrating the extent of the damage that individual American actions can cause. The implications for lifestyle and public policy are evident: Reducing consumption today reduces the number of people who will endure the consequences tomorrow and prevents a significant portion of the instability that would otherwise ensue. “As a citizen of a planet with this level of emerging risk, I must also have a human and moral response to these numbers,” Lenton said. “Each of us must deal with that in our own way,” he added.

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