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If global warming continues unchecked, the heat that is coming later this century in some parts of the world will bring “nearly unlivable” conditions for up to 3 billion people, a study released Monday said.

Climate change could push 1.5 billion people to migrate to escape unlivable heat in the next 50 years even if global targets are reached, says the study published in the peer-reviewed journal Proceedings of the National Academy of Sciences (PNAS). The countries most at risk include India, Nigeria and Pakistan.

The scientists conducting the study predict that by 2070, much of the world’s population is likely to live in climate conditions that are “warmer than conditions deemed suitable for human life to flourish.”

The study report – “Future of the human climate niche” – by Chi Xu, Timothy A. Kohler, Timothy M. Lenton, Jens-Christian Svenning, and Marten Scheffer, (PNAS first published on May 4, 2020 https://doi.org/10.1073/pnas.1910114117), was prepared by an international research team of archaeologists, ecologists and climate scientists.

The report warned that unless greenhouse gas emissions (GHG) are curtailed, average annual temperatures will rise beyond the climate “niche” in which humans have thrived for 6,000 years in a “climate niche”. A “climate niche” is an area in which average annual temperatures stay below 25 degrees Celsius. Most people live in areas with temperatures less than 15 C.

That range of temperatures is likely to represent fundamental constraints on what people need to survive, such as the ability to farm, according to the study by scientists from China, the U.S and Europe.

The scientists found that people, despite all forms of innovations and migrations, have mostly lived in these climate conditions for several thousand years.

“We show that in a business-as-usual climate change scenario, the geographical position of this temperature niche is projected to shift more over the coming 50 years than it has moved (in the past 6,000 years),” the study warned.

These brutally hot climate conditions are currently experienced by just 0.8% of the global land surface, mostly in the hottest parts of the Sahara Desert, but by 2070, the conditions could spread to 19% of the Earth’s land area.

This includes large portions of northern Africa, the Middle East, northern South America, South Asia, and parts of Australia.

The future scenario used in the paper is one in which atmospheric concentrations of GHG are high. The burning of fossil fuels such as coal, oil and gas releases GHG such as carbon dioxide (CO2) and methane into Earth’s atmosphere and oceans. The emissions have caused the planet’s temperatures to rise to levels that cannot be explained by natural factors, scientists report.

Temperatures over the next few decades are projected to increase rapidly as a result of human GHG emissions.

“Large areas of the planet would heat to barely survivable levels and they wouldn’t cool down again,” said study co-author Marten Scheffer of Wageningen University in the Netherlands. “Not only would this have devastating direct effects, it leaves societies less able to cope with future crises like new pandemics. The only thing that can stop this happening is a rapid cut in carbon emissions.”

Rapid reductions in GHG emissions could halve the number of people exposed to such hot conditions. “The good news is that these impacts can be greatly reduced if humanity succeeds in curbing global warming,” said study co-author Professor Tim Lenton, climate specialist and Director of the Global Systems Institute at the University of Exeter in the U.K.

“Our computations show that each degree warming (Celsius) above present levels corresponds to roughly 1 billion people falling outside of the climate niche,” Lenton said. “It is important that we can now express the benefits of curbing greenhouse gas emissions in something more human than just monetary terms.”

In our current climate, the most extreme heat is restricted to the small black areas in the Sahara Desert region. But by 2070, that area will expand to the shaded areas across portions of Africa, Asia, Australia and South America, according to the study.

That figure corresponds to a scenario in which global temperature rise is limited to below 2 C, the target that has been agreed by global governments, and it rises by 1bn people with every degree of additional warming. Current emissions trajectories put us on course for a rise of more than 3 C.

Not all those who end up living in these temperatures are likely to migrate, the study notes, given all of the other factors that discourage mass movement.

But, Prof Lenton noted, many of the areas that will be worst hit may struggle to fund expensive mitigation measures.

“Our concern is not so much rich people who can insulate themselves in air conditioned buildings but with people with less means to do that,” he said. “There’s also the question of who and where are we going to be producing food?”

The study report said:

All species have an environmental niche, and despite technological advances, humans are unlikely to be an exception. Here, we demonstrate that for millennia, human populations have resided in the same narrow part of the climatic envelope available on the globe, characterized by a major mode around ∼11 °C to 15 °C mean annual temperature (MAT). Supporting the fundamental nature of this temperature niche, current production of crops and livestock is largely limited to the same conditions, and the same optimum has been found for agricultural and nonagricultural economic output of countries through analyses of year-to-year variation. We show that in a business-as-usual climate change scenario, the geographical position of this temperature niche is projected to shift more over the coming 50 y than it has moved since 6000 BP. Populations will not simply track the shifting climate, as adaptation in situ may address some of the challenges, and many other factors affect decisions to migrate. Nevertheless, in the absence of migration, one third of the global population is projected to experience a MAT >29 °C currently found in only 0.8% of the Earth’s land surface, mostly concentrated in the Sahara. As the potentially most affected regions are among the poorest in the world, where adaptive capacity is low, enhancing human development in those areas should be a priority alongside climate mitigation.

Global warming will affect ecosystems as well as human health, livelihoods, food security, water supply, and economic growth in many ways. The impacts are projected to increase steeply with the degree of warming. For instance, warming to 2 °C, compared with 1.5 °C, is estimated to increase the number of people exposed to climate-related risks and poverty by up to several hundred million by 2050. It remains difficult, however, to foresee the human impacts of the complex interplay of mechanisms driven by warming. Much of the impact on human well-being will depend on societal responses. There are often options for local adaptations that could ameliorate effects, given enough resources. At the same time, while some regions may face declining conditions for human thriving, conditions in other places will improve. Therefore, despite the formidable psychological, social, and political barriers to migration, a change in the geographical distribution of human populations and agricultural production is another likely part of the spontaneous or managed adaptive response of humanity to a changing climate. Clearly, there is a need to understand the climatic conditions needed for human thriving. Despite a long and turbulent history of studies on the role of climate, and environment at large, on society in geography and beyond, causal links have remained difficult to establish, and deterministic claims largely refuted, given the complexities of the relationships in question. Rather than reentering the murky waters of environmental determinism, here we take a fresh look at this complex and contentious issue. We mine the massive sets of demographic, land use, and climate information that have become available in recent years to ask what the climatic conditions for human life have been across the past millennia, and then examine where those conditions are projected to occur in the future.

It said:

The transparency of our approach is appealing, but inevitably implies some loss of nuance. For instance, temperature captures only part of the relevant climate, and potentially important drivers of human thriving are linked in complex ways to climate. Importantly, while our projection of the geographical shift of the temperature niche is illustrative, it cannot be interpreted as a prediction of migration, as many factors other than climate affect decisions to migrate, and much of the migration demand may potentially be addressed through climate adaptation. Those complexities invite reflections on two key questions: First, how could the narrow realized temperature niche be explained? Second, what are the implications in terms of potential future migration in response to geographical displacement of the temperature niche?

In the study report’s The Question of Causality section, the scientists said:

Why have humans remained concentrated so consistently in the same small part of the potential climate space? The full complex of mechanisms responsible for the patterns is obviously hard to unravel. The constancy of the core distribution of humans over millennia in the face of accumulating innovations is suggestive of a fundamental link to temperature. However, one could argue that the realized niche may merely reflect the ancient needs of agrarian production. Perhaps, people stayed and populations kept expanding in those places, even if the corresponding climate conditions had become irrelevant? Three lines of evidence suggest that this is unlikely, and that instead human thriving remains largely constrained to the observed realized temperature niche for causal reasons.

First, an estimated 50% of the global population depends on smallholder farming, and much of the energy input in such systems comes from physical work carried out by farmers, which can be strongly affected by extreme temperatures. Second, high temperatures have strong impacts, affecting not only physical labor capacity but also mood, behavior, and mental health through heat exhaustion and effects on cognitive and psychological performance. The third, and perhaps most striking, indication for causality behind the temperature optimum we find is that it coincides with the optimum for economic productivity found in a study of climate-related dynamics in 166 countries. To eliminate confounding effects of historical, cultural, and political differences, that study focused on the relation within countries between year-to-year differences in economic productivity and temperature anomalies. The ∼13 °C optimum in MAT they find holds globally across agricultural and nonagricultural activity in rich and poor countries. Thus, based on an entirely different set of data, that economic study independently points to the same temperature optimum we infer.

Altogether, it seems plausible that the historically stable association between human distribution and temperature reflects a causal link rather than a legacy, contingent on ancient patterns reflecting agrarian needs or still-more-ancient hunter-gatherer preferences. This supports the view that the historically stable and tight relationship of human distribution to MAT represents a human temperature niche reflecting fundamental constraints on human populations.

In the Migration as a Possible Response to Climate Change, the study report said:

Obviously, our hypothetical redistribution calculations cannot be interpreted in terms of expected migration. First of all, detailed regional studies suggest that migration responds nonlinearly to temperature. Thus, migration may speed up only when a critical climate threshold is reached. More generally, migration decisions tend to be avoided and depend on a complex array of factors including adaptation options. This implies that realized migration numbers will likely be much lower than suggested by the discrepancy between the expected location of the temperature niche and actual distributions of population, even though we have not considered several drivers that could exacerbate movements, such as extreme weather events or projected sea-level rise, which may by themselves lead to substantial population displacements worldwide.

Clearly, projections of the magnitude of climate-driven future migration (including asylum seeking) will remain highly uncertain. Even seemingly straightforward links between climate and recent conflicts and migration waves are contentious. For instance, in the years leading up to the current Syrian exodus, the fertile crescent has likely been experiencing the worst drought in 900 y, making subsistence farming in the countryside extremely hard and driving millions in Syria to the cities, where tensions increased. However, as many factors play a role, assessing the relative role of climate in such specific conflict or mass migration events always remains challenging. This is not to say that there is no evidence for a causal relationship between conflicts and climate events such as prolonged droughts, both now and in the past. In fact, the literature is replete with evidence for ancient episodes of climate-triggered human migration and upheaval. For instance, the coldest phase of the Little Ice Age in Europe (1560 to 1660 AD) has been causally linked to a peak of migration (1580 to 1650 AD) and a European population collapse to a minimum in 1650 AD. Earlier, the Late Antique Little Ice Age from 536 to about 660 AD affected most of the Northern Hemisphere, likely contributing to the transformation of the Roman Empire, movements out of the Asian steppe and Arabian Peninsula, spread of Slavic-speaking peoples, and upheavals in China. Clearly, lessons from such ancient dynamics cannot be directly extrapolated to modern times. However, while outcomes are context dependent, and confounding social, cultural, and political considerations are always present, a range of analyses suggests that changes of climatic conditions can exert enough stress to trigger migration, part of which can take the form of asylum-seeking waves in response to climate-driven conflicts.

It thus seems reasonable to assume that at least part of the discrepancy caused by the projected geographical shift in the human temperature niche could be reduced through different forms of migration. However, it remains impossible at this point to foresee the extent of climate-driven redistribution of the human population. Technoeconomic scenarios, political developments, institutional changes, and socioeconomic conditions that affect adaptation options may profoundly affect outcomes in ways that will be worth exploring in further scenario analyses utilizing the different assumptions underlying the SSPs. Also, rising mortality impacts of heat waves on dense populations in already-hot places such as India invite further scrutiny. Follow-up work is needed to search for integrative avenues for effective adaptation, as well as defining fundamental limitations to what is possible given available resources.

The study report said:

In summary, our results suggest a strong tension between expected future population distributions and the future locations of climate conditions that have served humanity well over the past millennia. So far, the scope for local adaptation has been the dominant focus for analyses of possible responses to a changing climate, despite a striking lack of realized adaptation in most regions. It is not too late to mitigate climate change and to improve adaptive capacity, especially when it comes to boosting human development in the Global South. However, our approach naturally raises the question of what role redistribution of populations may come to play. Migration can have beneficial effects to societies, including a boost to research and innovation. However, on larger scales, migration inevitably causes tension, even now, when a relatively modest number of ∼250 million people live outside their countries of birth. Looking at the benefits of climate mitigation in terms of avoided potential displacements may be a useful complement to estimates in terms of economic gains and losses.


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