Posted on 3 July 2023 by dana1981

This is a re-post from Yale Climate Connections

Tom Toro is a cartoonist and writer who has published over 200 cartoons in The New Yorker since 2010.

Like Baby Bear’s porridge to Goldilocks, for thousands of years, the climate over much of the planet was neither too hot nor too cold but “just right.” Those ideal conditions enabled people to develop advanced agriculture, build cities, and invent industry and advanced technology.

But after relatively stable surface temperatures during the past 7,000 years, global warming is now rapidly disrupting the reliable climate that allowed humanity to flourish.

Two dangerous consequences of the fast changes were described in a new study published in the journal Nature Sustainability. First, large populations find their home regions shifting outside of the Goldilocks zone that scientists call the “climate niche.”

Second, risks are growing for more dangerous extreme heat events like the 2021 heat wave that caused over 1,000 deaths in Oregon, Washington, and British Columbia. But the study also shows that making the right moves now will help immeasurably. For every tenth of a degree of global warming that society can avoid, more than 100 million people will remain in a more favorable climate. That could go a long way toward preventing the problems the study authors foresee, such as mounting risks of disease, less productive agriculture and labor, and rising risks of violence, unrest, and mass migration.

What is the Goldilocks zone?

The study’s authors selected the climate of 1980 as adequately representative of humans’ “climate niche.” At that time, global temperatures had only risen about 0.3°C (0.5°F) above preindustrial levels. The study found that most people in 1980 lived in areas where year-round temperatures averaged between about 7°C and 17°C (45–63°F), with a peak population density at 13°C (55°F). That’s consistent with a 2015 study finding that societies’ optimal economic productivity happens in regions with an average temperature of about 13°C. That’s the climate in which conditions appear to be ideal for worker and agricultural productivity — humanity’s “just right” Goldilocks zone.

There’s also a smaller second temperature regime with mean annual temperatures around 23–27°C (73–80°F) in which many people in places like India and Brazil reside. In those regions, rainfall from weather events like monsoons enables agricultural productivity despite hot temperatures. People living in these hotter regimes are particularly vulnerable to the shifting climate. The study authors found that as mean annual temperatures surpass 27°C (80°F), the number of dangerous extreme heat days each year exceeding 40°C (104°F) begins to increase markedly. And almost no humans in 1980 lived in places with average temperatures above 29°C (84°F).

Higher temperatures will actually raise temperatures closer to humanity’s ideal in some cold regions indicated in green in the figure below from the study, such as the northern United States, Canada, and northern and eastern Europe. But the climate in large swathes of hotter areas closer to the equator indicated in orange and red like the southern United States, India, Brazil, Africa, and Southeast Asia will grow less suitable and often dangerously hot. 

Displacement of the ‘climate niche’ or ‘Goldilocks zone’ for 2.7°C (4.9°F) global warming (top frame) and for 1.5°C (2.7°F) warming (bottom frame). Red indicates a decrease in suitability and green an increase. From Lenton et al. (2023), Nature Sustainability. Republished under Creative Commons License 4.0.

That change would create tremendous challenges, as the paper documented.

“Exposure outside the niche could result in increased morbidity, mortality, adaptation in place or displacement (migration elsewhere). High temperatures have been linked to increased mortality, decreased labour productivity, decreased cognitive performance, impaired learning, adverse pregnancy outcomes, decreased crop yield potential, increased conflict, hate speech, migration and infectious disease spread,” the authors wrote.

The study’s lead author, Timothy Lenton of the University of Exeter, noted that these impacts could result in significant climate-forced migration. “If the people stayed where they are projected to be it would pose serious challenges for agriculture (and many other activities),” Lenton said via email. “I’d be surprised if a lot of the people didn’t also opt to migrate to cooler climates.” Co-author Chi Xu from Nanjing University added, “Many (or a majority of) interconnected components of human societies would be seriously challenged as inferred from existing studies. Mass migration and social instabilities in general, which potentially cascade to amplify existential risk of human civilizations, is the major concern.”

A huge number of people could be affected

In the figure below from the paper, the area under the purple line represents the Goldilocks zone humanity still enjoyed in 1980. The other colored lines indicate the projected future distribution of the human population for different levels of global warming. Populations living in places with average temperatures exceeding 29°C would be in unprecedented and dangerously hot territory. And portions of the curves above the purple line indicate populations living outside the range that the Earth has supported at those temperatures.

For example, about 60% of people in 1980 lived in areas with an average temperature of 7–17°C (45–63°F). But 2.7°C global warming would shift the climate such that about 60% of people would instead live in areas with an average temperature of 17–28°C (63–82°F).

Changes in relative human population density with respect to mean annual temperature (MAT). From Lenton et al. (2023), Nature Sustainability. Republished under Creative Commons License 4.0.

Each fraction of a degree lessens dangerous, inequitable heat exposure

If governments can meet the ambitious Paris target of limiting the warming to 1.5°C (2.7°F), the consequences will be much less severe than if current policies persist. Those policies are projected to cause 2.7°C (4.9°F) of global warming above preindustrial temperatures by 2100.

As with many climate consequences, countries that are least responsible for causing the climate crisis are the most vulnerable to these impacts. The study concluded that at 2.7°C of global warming, over 2 billion people would be exposed to the unprecedented dangerous heat associated with mean annual temperatures exceeding 29°C. About half those would reside in just three countries: India (over 600 million), Nigeria (over 300 million), and Indonesia (100 million). Per-person emissions of heat-trapping greenhouse gases in each of these countries are less than half of the global average.

But limiting global warming to 1.5°C would reduce the number of people exposed to unprecedented dangerous heat in these countries to about 130 million, a stark contrast to the over 1 billion exposed at 2.7°C. And the study authors found that for every 0.1°C of avoided global warming, over 100 million fewer people will be shifted outside the Goldilocks zone and pushed into unprecedented heat exposure.

As the study concluded, “Our results show the huge potential for more decisive climate policy to limit the human costs and inequities of climate change.” Each tenth of a degree of avoided global warming can make a big difference.