The probability that summers between 2061 and 2080 will be warmer than the hottest on record stands at 80 percent 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 percent.

"Extremely hot summers always pose a challenge to society," said NCAR scientist Flavio Lehner, lead author of the study.

"They can increase the risk for health issues, and can also damage crops and deepen droughts. Such summers are a true test of our adaptability to rising temperatures," Lehner said.

The researchers used two existing sets of model simulations to study what future summers might look like.

They created both by running the NCAR-based Community Earth System Model 15 times, with one simulation assuming that greenhouse gas emissions remain unabated and the other assuming that society reduces emissions.

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.

"This is the first time the risk of record summer heat and its dependence on the rate of greenhouse gas emissions have been so comprehensively evaluated from a large set of simulations with a single state-of-the-art climate model," said NCAR scientist Clara Deser.

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.

In some regions, the likelihood of summers being warmer than any in the historical record remained less than 50 percent, but in those places - including Alaska, the central US, Scandinavia, Siberia and continental Australia – summer temperatures naturally vary greatly, making it more difficult to detect effects of climate change.

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 percent, even if emissions were reduced.
    
However, parts of Brazil, central Europe, and eastern China would see a reduction of more than 50 percent in the chance that future summers would be hotter than the historic range.

The study appears in the journal Climatic Change.

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