As the Earth warmsdrought is expected to intensify in arid regions, potentially leading to the expansion of deserts, changes in the environment, and the death of plants that cannot adapt to the harsh conditions.
Although carbon dioxide is the main driver of global warming, a new study in Environmental Research Letters suggests it may play an unexpected role in slowing the expansion of deserts and drylands, which could make up 40% of the land by 2100.
Drylands are defined as areas suffering from water scarcity, where rainfall is low compared to the amount of water lost by the soil and plants through evaporation and transpiration (which is the loss of water from the plant through its leaves).
Although this definition is well-known, differences in measurement methods and climate data lead to significant variations in estimates of the size of these areas around the world.
In the recent study, researchers used updated climate models to map the current distribution of drylands and predict their future spread, relying on the aridity index (AI), which is the ratio of rainfall in a region to potential transpiration (PET), classifying areas with an AI below 0.65 as drylands.
One factor contributing to the variability in drought forecasts is that rising atmospheric carbon dioxide levels over time affect plant transpiration. When carbon dioxide levels are high, the pores on plants (stomata) partially close, meaning plants lose less water through transpiration, which impacts calculations of potential evapotranspiration. The team says this is a factor that many previous studies have overlooked.
The study relied on an updated model for calculating potential evapotranspiration, taking into account the impact of carbon dioxide, and incorporated climate data spanning from 1960 to 2023, along with models from the CMIP6 project (a project of the World Climate Research Programme (WCRP) that provides climate projections to understand past, present and future climate changes).
The results showed that drylands covered about 38.58% of the land (excluding Antarctica) during the reference period 1994-2023.
Drylands are distributed into four subtypes, with semi-arid areas making up 14.72% of the land area, arid areas (mainly in Australia and China) 11.24%, semi-arid humid areas (in China and Russia) 6.35%, and extremely arid areas (in Algeria, Libya, and Saudi Arabia) 6.27%.
Researchers predicted that drylands globally would increase to cover between 39.31% and 40.28% of the Earth's land area during the period 2071-2100, depending on the greenhouse gas emissions scenario used, with extremely arid regions showing the greatest increase.
Although this trend is worrying, the carbon dioxide-adjusted potential evapotranspiration calculations used by the researchers give much lower figures than some other estimates, with a study a decade ago suggesting that drylands could cover as much as 56% of the Earth's land area by 2100.
However, the highest estimate (40.28%) corresponds to a staggering increase of 2.37 million square kilometers compared to the period 1994-2023, which is equivalent to the size of Algeria or about 30% of the area of Australia.
Researchers predict that Australia will experience the most pronounced changes in drylands this century, followed by Brazil, while parts of Africa are also expected to undergo large-scale transformations.
The results were published in the journal Environmental Research Letters.
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