Widespread "increases in extreme heat” due to climate change could bring unprecedented risks to the US in coming decades, a new study has warned.

By 2050, hundreds of American cities could experience an entire month each year with US "heat index" temperatures above 100F (38C) if nothing is done to tackle emissions and the resultant climate crisis, scientists said.

In the US, the National Weather Service heat index scale starts topping out above temperatures of 127F (52C), depending on the combination of temperature and humidity.

Few places would be unaffected by extreme heat conditions by 2050 and only a few mountainous regions would remain extreme heat refuges by the century’s end, the team from the Union of Concerned Scientists said.

They said this failure to reduce emissions could set the country on a path to soaring temperatures including conditions so far in excess of current climate trends they will surpass the heat index.

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This is a measure of how hot it feels when humidity is factored in with the air temperature, providing a number on a coloured scale, starting with 80F (26C) which is yellow – caution, and rising through dark yellow beginning at 91F (33C) – extreme caution, orange at 103F (39.5C) – danger, and up to red beginning at 126F (52C) – extreme danger.

The average number of days per year nationwide with a heat index above 105 degrees Fahrenheit would more than quadruple to 24 by mid-century and increase eight-fold to 40 by late century, the analysis, published in the journal Environmental Research Communications, revealed.

Such “off-the-charts” conditions could pose unprecedented health risks, the Union of Concerned Scientists said.

“Our analysis shows a hotter future that’s hard to imagine today,” the study's co-author Kristina Dahl, a climate scientist at the Union of Concerned Scientists, told the USA Today newspaper.

“Nearly everywhere, people will experience more days of dangerous heat in the next few decades.”

The work is the first study to examine the impacts of climate change on the heat index – instead of just temperature – when calculating the impacts of warming, Dr Dahl said.

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“We have little to no experience with ‘off-the-charts’ heat in the US,” said Erika Spanger-Siegfried, co-author of the report and lead climate analyst at the Union of Concerned Scientists.