Their conclusion follows from a new methodology for finding climate analogues – climates already in existence that will be experienced in other places when the world is warmer. The methodology shows that even with 2°C of global warming – the limit set by the Paris Agreement – more than one-fifth of the land on Earth will experience climates that have no analogue today. For 1.5°C of warming that figure is some 15%.

The prospect of such novel climates is a problem for science communication, says Erich Fischer of ETH Zurich, and it could also pose a greater risk to the flora and fauna of those regions. "Even 20% of the land facing a situation that we don’t experience anywhere today could mean that those ecosystems find it particularly hard to adapt," he explained.

Climate analogues have long been used to communicate the possible effects of climate change. Their aim is to explain that when a certain region’s climate changes in the future under global warming, it will feel like the climate of some other region today.

There are various tools to identify climate analogues, but according to Fischer they are not as useful as they could be. Often the analogues are presented with no indication of their accuracy, he says, and often they fit a region only for a particular season, not all year round, and for one particular variable, such as temperature.

So Fischer and colleagues came up with a new methodology for identifying climate analogues that excludes those that are simply too dissimilar. The thresholds for an analogue’s inclusion were mean temperature and precipitation that were no more than twice the natural internal variability of the non-analogue’s. "We still accept an analogue if [for example] it doesn’t perfectly fit the winter, so long as it fits the other three seasons," said Fischer.

Generally, the researchers found that regions with more variable temperature and precipitation – typically inland places – had analogues. In a world warmer by 2°C, for instance, Chicago, might come to feel like Louisville, Kentucky; Helsinki’s rain and temperature might become more like that of Danzig in Poland; and Manchester might take on a climate like that of Rennes in France.

Depending on the degree of warming, however, up to one-third of regions – typically tropical and subtropical places with little day-to-day variation, and especially on the coasts – had no analogues.

The researchers also found that the climates of similar fractions of land would disappear, potentially leaving their native flora and fauna with no place on Earth they would feel at home, climatically speaking. "Of course, you are not necessarily safe even if an analogue exists," noted Fischer. "A species can’t just jump over the Atlantic, for example, to find somewhere with a suitable climate."

In future work, Fischer and colleagues want to improve their methodology to include more climate variables, so that analogues become even more accurate.

The study is published in Environmental Research Letters (ERL).

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