the extension of arid zones questioned

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According to new work, the impact of global warming on arid areas is more nuanced than one might fear. The results were published in the journal Nature’s climate change this March 11.

With a climate becoming increasingly hot, one can intuitively expect an extension of arid zones. In other words, to an increase in the area covered by desert, savanna, grassland or even subtropical steppe type landscapes. A set of ecological areas which currently covers 40% of continental areas. This sentiment has been supported by many works published over the past decades.

Are arid zones set to expand?

However, recent results question this paradigm. Indeed, they show that the way in which the extension of arid zones was so far calculated suffers from a significant bias. “Historically, we had relatively good rainfall and temperature readings but really poor measurements of the land surface, like soil moisture and vegetation,” explains Kaighin McColl, lead author of the study. “Consequently, the definitions of arid zones relied solely on the state of the atmosphere as an approximation of the earth’s surface ‘.

Arid zones could change very little despite global warming. Credits: Pixabay.

However, with the continuous improvement of climate models, it becomes obsolete to focus only on atmospheric variables. The simulations now include a sufficiently realistic representation of the soil and vegetation to be able to directly assess their reaction to global warming. In fact, thearidity index usually used has been replaced by a metric newly developed by researchers. Called hydroecological index, it takes into account the physiology of plants.

Looking directly at the earth’s surface in climate models, we find that these do not show a clear increase in drylands over time and that there is great uncertainty as to the future state of these», Explains the main author. The resulting image therefore becomes much more nuanced.

Atmosphere and vegetation: a differentiated response

In summary, the atmospheric variables transcribe an extension of arid conditions but not the variables linked to the soil. How to explain this paradox ? The researchers actually show that there is a divergence between the behavior of the atmosphere and that of the earth’s surface as the climate warms. Indeed, if the air bears an increasingly arid signature, soil and vegetation do not passively undergo evolution. With a higher concentration of CO2, for example, plants store water better. Also, exposure to water stress is found to be reduced. In addition, reduced plant transpiration helps to dry out the surrounding air. A phenomenon which partly explains the observed divergence.

“If you want to know if the soil is going to get drier, if the crops are going to fail, or if a forest is going to dry out, you have to look at the soil itself”, reports Alexis Berg, co-author of the paper. “Our research shows that while some drylands may expand, climate models do not predict a global and rapid expansion of drylands.”, adds Kaighin McColl. However, it should come as no surprise that the matter is resolved.” There is still a lot of uncertainty on how vegetation and the water cycle will change in a warming world ”, he specifies. Enough to give grain to grind for future work.

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