Habitability is in danger in many regions around the equator as the world is heating up.
Even if we can limit global warming to 2 C above pre-industrial levels, the tropics and subtropics will experience dangerously hot temperatures most days of the year by the year 2200.
Each year the mid-latitudes of the world will experience heat waves. Researchers predict a 16-fold increase in heat waves in Chicago by the end of the century.
Is there a chance that we avoid that fate? Warming to below 1.5 C above pre-industrial temperatures is projected to be about 0.1 percent. According to researchers, the world will have exceeded 2 C of warming by the end of the century.
Researchers say that heat stress will be a feature of the climate in sub-Saharan Africa, parts of the Arabian peninsula, and much of the Indian subcontinent.
There will be many deaths unless the world is able to implement rapid and widespread adaptation measures. Every tenth of a degree of less heat will save lives.
Estimates show that global warming is responsible for one in three heat related deaths.
Climate change is predicted to cause a record number of deaths in the coming decades, based on these rates.
Humidity is one of the factors that make it difficult for humans to deal with heat stress. The current estimates are based on the heat index, which only takes into account relative humidity up to certain temperatures
Recent studies have shown that the human body might not be able to cope with as much heat and humidity as this index indicates.
The ceiling of what is survivable is 93 C (200 F) on the heat index.
Young and healthy people may not live past 31 C.
The traditional heat index considers temperatures to be dangerous when they exceed 40 C and dangerous when they exceed 51 C.
There is a chance that these thresholds are an underestimation of what will happen in the future.
Humans' prospects look dire even by this measure.
In the tropics and subtropics, the dangerous heat index threshold was exceeded 15 percent of the time.
It was rare for temperatures to become very dangerous as per the heat index.
The problem is only getting worse and the same can't be said of today.
Fifty percent of the days each year could be dangerous due to the heat index. It can be exceeded on most days by the end of the century.
About 25 percent of those days could be so hot that they could be dangerous.
By the end of the century, it is likely that large portions of the global tropics and subtropics will experience higher than considered dangerous heat index levels.
This would increase the incidence of heat-related illnesses and reduce outdoor working capacity in many regions where subsistence farming is important.
The consequences of the health and society would be profound.
The study was published in an environmental journal.
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