The probability that summers between 2061 and 2080 will be warmer than the hottest on record stands at 80 per cent across the world's land areas, excluding Antarctica, which was not studied, according to scientists at the US National Centre for Atmospheric Research (NCAR).
If greenhouse gas emissions are reduced, however, that probability drops to 41 per cent.
"Extremely hot summers always pose a challenge to society," said NCAR scientist Flavio Lehner, lead author of the study.
The researchers used two existing sets of model simulations to study what future summers might look like.
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By using simulations created by running the same model multiple times, with only tiny differences in the initial starting conditions, the scientists could examine the range of expected summertime temperatures for future scenarios.
The researchers compared results to summertime temperatures recorded between 1920 and 2014 and to 15 sets of simulated summertime temperatures for the same period.
The results show that between 2061 and 2080, summers in large parts of North and South America, central Europe, Asia, and Africa have over 90 per cent chance of being warmer than any summer recorded if emissions continue unabated.
Reducing emissions would lower the global probability of future summers that are hotter than any in the past, but would not result in uniformly spread benefits.
In some regions, including the US East Coast and large parts of the tropics, the probability would remain above 90 per cent, even if emissions were reduced.
The study appears in the journal Climatic Change.