This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Drylands are defined as regions where rainfall is low relative to the atmosphere's ability to draw water from soil and plants. Expanding boundaries of these regions are frequent concerns in climate change considerations.
While they can support large populations and ecosystems, dryland expansion can worsen water scarcity, land degradation and desertification. Many projections of future dryland expansion assume warming simply raises evaporation, but a new study suggests the process is more complex. The study, published in Environmental Research Letters, incorporates the reduced evapotranspiration that plants exhibit under increased carbon dioxide (CO2) levels and finds that previous estimates of dryland expansion may be off.
Previous estimates of Earth's total dryland area have varied, ranging from roughly 37% to 47% of land area, depending on the data and methods used. These estimates typically use the aridity index, which is defined as the ratio of precipitation to potential evapotranspiration (PET), where PET is the maximum amount of water that would evaporate from the soil and transpire from plants if an unlimited water supply were available. The data used to calculate rainfall and PET vary among studies, however.
The authors of the new study write, "Many of these assessments, however, relied on climate data prior to 2010, whereas global climate conditions have changed substantially over the past decade. Recent drought intensification and land-atmosphere feedbacks further suggest that some regions may already have shifted toward drier conditions. Because dryland boundaries are defined by aridity index (AI), which integrates precipitation and PET, changes in water supply or atmospheric evaporative demand can alter the spatial extent of drylands.
Therefore, the current spatial distribution of global drylands needs to be reassessed using updated climate data." The issue extends to future assessments as well. The authors note that projected dryland expansion in conventional aridity-index studies has varied from about 4% to 10% and 11% to 23% of land area under different greenhouse gas scenarios. However, these models usually leave out changes in plant evapotranspiration under high CO2 levels.
The researchers sought to better quantify the current and future distribution of global drylands and identify hotspots of increasing drying and wetting under three different emissions scenarios by incorporating plant responses to CO2. They combined historical climate observations with projections from 19 major climate models. They used multiple classes of dryland, from semi-arid to hyper-arid, and compared a standard evaporation calculation with one adjusted for the effects of rising CO2 on plant water use.
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