The simulations, which looked for the "emergence" of climate parameters significantly more extreme than their historical levels, also suggest that extremes of rainfall, driven by anthropogenic carbon emissions, are likely to appear in coming decades.

"Given that there is such high year-to-year variability in climate extremes, I was surprised that we were already seeing emergence in [extremes of temperature]," said Andrew King of the University of New South Wales, Australia.

Although a lot of policymaking is determined by changes in climate averages, it is climate extremes that are thought to be most dangerous – extremes of rainfall can cause floods, for instance, or extremes of temperature cause heat-related deaths.

Statistically, any period of history will exhibit extremes of temperature, rainfall and other climate parameters, but King and his colleagues – based at the University of New South Wales and other institutions in Australia, Switzerland and the UK – are interested in when these extremes become significantly more extreme than before. The "time of emergence" is when these more pronounced extremes become so common as to be the norm.

The researchers began their study by merging climate simulations covering the period 1860 to 2099. From that they calculated a baseline distribution of temperature and rainfall for 1860 to 1910, which is thought to be before much anthropogenic change in the climate had taken place.

King and colleagues then split the remaining time into two-decade windows. For each window, they performed a statistical test to determine whether its extremes were significantly different from the window preceding it, and whether they could be said to be anthropogenic in origin. If the new extremes were significantly different, and if they did not disappear in subsequent windows, the researchers denoted that point as being a time of emergence of a new climate extreme.

Using this method, King and colleagues found that hot and cold extremes have already emerged across many areas of the globe, including Latin America, Africa and northeast Asia. "It varies … [but] the temperature extremes tend to emerge a few decades – 20 to 40 years, typically – later than the mean temperature," said King.

Although the results implied that only new extremes of temperature have emerged so far, they did also indicate that extremes of rainfall are forthcoming. Such high rainfall is likely to appear in the next few decades in the northern hemisphere over winter.

King plans to broaden the scope of his group’s study. "I’m looking at applying these methods to more observational datasets, and potentially doing more regionally-based analyses looking at the time of emergence in more detail," he said.

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