Hong Kong(CNN) If the planet continues to warm at current levels over the next 50 years, up to 3 billion people could be living in areas that are too hot for humans, a new study has found.
For thousands of years, humans have lived within a narrow "climate niche" where average temperatures are ideal for society to flourish, and conditions favorable to grow food and keep livestock.
In findings published in the Proceedings of the National Academy of Sciences on Monday, an international team of archaeologists, climate scientists and ecologists said that if heat-trapping greenhouse gas emissions continue at the current pace, by 2070 billions of people will be living in conditions hotter than those that have allowed life to thrive for the past 6,000 years.
For every 1°C (1.8°F) of warming, 1 billion people will either have to migrate to cooler regions or adapt to extreme heat conditions, the study found.
Tim Kohler, an archaeologist at the University of Washington and co-author of the study said that these findings can be viewed as a worst case or "business-as-usual" scenario of "what could happen if we don't change our ways."
This is what climate change looks like
An iceberg floats in a fjord near the town of Tasiilaq, Greenland, in June 2018. Greenland is often considered by scientists to be
ground zero of the Earth's climate change. The massive island is mostly in the Arctic, which is warming twice as fast as the rest of the planet. Melting ice from Greenland's ice sheet is the largest contributor of all land sources to the rising sea levels that could become catastrophic for coastal cities around the world. "Seeing the size of these icebergs in the water was like looking at entire city blocks floating around," Reuters photographer
Lucas Jackson said.
A neighborhood is flooded in Beaumont, Texas, a day after
Hurricane Harvey came ashore in August 2017. The Category 4 storm caused historic flooding. It set a record for the most rainfall from a tropical cyclone in the continental United States, with 51 inches of rain recorded in areas of Texas. An estimated 27 trillion gallons of water fell over Texas and Louisiana during a six-day period. "Warmer sea water from our changing climate is causing tropical storms to be more wet and powerful," photographer
George Steinmetz said.
Peia Kararaua, 16, swims in a flooded area of Kiribati's Aberao village. Kiribati is one of the countries most affected by sea-level rise, photographer
Vlad Sokhin said. During high tides many villages become inundated, making large parts of them uninhabitable. This photo was taken in an area that, when dry, is a soccer field. "Prior to this, a man moved his vehicle from the lower part of the field to the higher point, and the vehicle ended up being parked on an 'island' when the water came," Sokhin said. "Young people started swimming there and playing when I took this shot. It was strange to see such a scene: happy kids swimming along the remains of the dead palm trees."
A woman walks through a cactus field in a drought-stricken area of western Somaliland, a breakaway state from Somalia. "In 2016 I came across a group of women washing their clothes in a roadside puddle — the only water they could find," photographer
Nichole Sobecki said. "We spoke for a while of the challenges they faced, of the animals they'd lost in the drought, and the wells that had dried up. Somalia has long been a place of extremes, but climate and environmental changes are compounding those problems and leading to the end of a way of life."
Jorgen Umaq and his dogs traverse an icy area near Qaanaaq in northern Greenland. It is one of the northernmost towns in the world. Because ice thickness there has been declining, hunters like Umaq can't travel as far as they could before, said photographer
Anna Filipova. "Navigating this terrain was dangerous and difficult," she said. "We needed to manually move the sledge and twice needed to rescue the dogs who had fallen into the cracks in the sea. ... Each year, people lose their lives on the sea ice because of fast-changing conditions."
Bangladesh was recently ranked by research firm Maplecroft as the country
most vulnerable to climate change, due to its exposure to threats such as flooding, rising sea levels, cyclones and landslides as well as its susceptible population and weak institutional capacity to address the problem. This aerial photo, taken by
Ignacio Marin, shows where some homes used to be before the river washed them away. "From where I was standing, at the riverbank, it was hard to imagine that there were nine houses where I could only see water," Marin said. "So I decided to fly the drone. Only then, watching the area from above, I realized the scale of the disaster."
Sheep graze in the dry, dusty fields of Farmersville, California. "This image was made in 2014 while working on a short film about the ongoing drought in California," photographer
Ed Kashi said. "Tens of thousands of acres of arable land was turning to dust, massive orchards were being ripped out due to a lack of irrigation water, and farmers and ranchers who for generations had worked this land were wondering if their way of life was sustainable." Intense droughts like the one that plagued California this decade are
becoming more likely due to global warming.
Oil refineries are seen in Carson, California, in this 2017 photo taken by
Edward Burtynsky for The Anthropocene Project, which explores how humans have contributed to climate change and the state the planet is in today. Part of
the project includes a film, "Anthropocene: The Human Epoch," that opens September 25 in 100 theaters across the United States.
Two people are seen at an ice cave entrance on the Rhone Glacier in the Swiss Alps. Every summer, the glacier is covered with huge sheets of white fleece blankets to slow down its melting, according to photographer
Orjan F. Ellingvag. "The fleece-covered cave attracts more and more tourists worried about global warming and wanting to see the remnants of a dying glacier," Ellingvag said.
A wildfire burns in Tocantínia, Brazil, in September 2018. In the Cerrado region, wildfires are common for two reasons, said photographer
Marcio Pimenta. One is extreme heat. The other is farmers clearing space for soybeans and livestock.
This aerial photo shows Ejit, an islet in the Marshall Islands, in 2015. The islands are threatened by rising seas. "I flew a drone above the island showing just how precarious its location is: Homes clinging to the edge of an eroding coastline as unrelenting waves chisel away at what remains," said
Josh Haner, a photographer with The New York Times. "After I saw what was happening on Ejit, I realized that climate change is not something nebulous that will only start affecting us in the future, but rather something happening right now. Residents are being forced to make the most difficult decision: Do they stay and build sea walls to buy some more time, or do they relocate?"
Temperatures changing more than in past 6,000 years
Using data on historical global temperatures and the distribution of human populations, the researchers found that just like other species of animals, humans thrive best within a narrow "climatic envelope" around the world.
Most of the world's population live in areas with a mean annual temperature of between 11 and 15 degrees Celsius (51.8 to 59 degrees Fahrenheit). A smaller band of between 20 to 25 °C (68 to 77°F) encompasses areas in South Asia that are affected by the Indian monsoon -- the annual rains that irrigates large swathes of cropland vital for food production.
Surprisingly, the scientists said, humans have favored living in these conditions for the past 6,000 years -- that's despite recent technological advances such as air conditioning that have allowed us to push this boundary.
"As an archaeologist I always tell my students that our technology, our minds, and our cumulative culture have enabled us to live anywhere," Kohler said. "That's true of course. But it turns out that there is a distinct climate zone in which our numbers are greatest, and within which we have also been most economically productive."
But change could soon be forced upon us.
The Earth is currently on track for 3°C of warming by 2100. The study suggests that because land areas are warming faster than the oceans, temperatures experienced by humans are likely to rise by about 7.5°C by 2070.
As our planet rapidly heats up due to rising emissions, the temperature experienced by an average person is projected to change more in the coming decades than it has over the past 6,000 years, the study found.
And it could have severe consequences for food production, access to water sources, conflict and disruption caused by migration.
"It's reasonable to conclude that if something has been reasonably stable for 6000 years, we're not going to change it painlessly or quickly," Kohler said.
Extreme heat areas will expand
Among the hottest places on Earth is the Sahara region of Africa, which experiences annual mean temperatures above 29°C (84.2°F). These extreme conditions cover 0.8% of the earth's land area.
However, researchers said that those extreme heat areas are expected to spread to 19% of the Earth's surface, affecting 3.5 billion people by 2070.
Regions that stand to be affected include parts of Sub-Saharan Africa, South America, India, Southeast Asia, Arabian Peninsula and Australia -- regions with fast-growing populations, said Chi Xu, from Nanjing University's School of Life Sciences, and another co-author of the report.
"Those countries are mostly in global south, with the fastest population growing rate, such as India and Nigeria. These two countries are projected to accommodate the largest populations under extreme temperature conditions," Chi said.
The study's projection of 3.5 billion possible climate migrants goes far beyond the World Bank estimates, which suggested 143 million people across South Asia, Sub-Saharan Africa and Latin America were at risk of being displaced.
It's a dire warning of what could happen if the climate crisis is left unchecked.
But there is hope. The scientists said that by rapidly and substantially reducing global carbon emissions, the number of people exposed to brutally hot conditions could be halved.
The authors explain that there are a number of uncertainties with how the climate crisis will fuel displacement and to what extent, and said the study can't be used as a prediction of migration.
The figures are also based on worst case projections and there are questions over how actions to mitigate against climate change, including "political developments, institutional changes, and socioeconomic conditions" may affect these outcomes.
"The worst case scenario can be largely avoided if effective cut of greenhouse gas emissions is achieved," Chi said. "Many effective measures of climate mitigation and local adaptation would help to alleviate the negative influence of climate change on human societies."