A new modeling study projects that Earth’s drylands could expand to cover more than 40 percent of the planet’s land area by the closing decades of this century, as a warming climate pushes rainfall and evaporation further out of balance across already dry regions. The analysis, led by researchers at the Chinese Academy of Forestry, revisits how scientists calculate dryland extent by accounting for a detail many earlier climate models left out: the way rising carbon dioxide levels change how much water plants actually lose to the atmosphere. That correction produces a lower, though still substantial, forecast than some previous estimates of a hotter, drier planet.
Measuring Dryness With the Aridity Index
The researchers define drylands using the aridity index, a measure equal to a region’s precipitation divided by its potential evapotranspiration, or the amount of water that could evaporate from soil and plants under given conditions. Areas with an aridity index below 0.65 count as drylands under the definition used by the United Nations Environment Programme, which splits the category into four sub-types: dry sub-humid, semi-arid, arid, and hyper-arid lands, the last of which some agencies classify as true deserts and exclude from the broader dryland tally altogether. Scientists broadly agree on the rough location of the world’s major dry zones, but the study’s authors note that differences in measurement techniques and underlying climate datasets have long produced global dryland estimates that vary substantially from one analysis to the next.
Where the World’s Dry Zones Sit Today
Using a baseline period of 1994 to 2023, the team calculated that drylands already cover about 38.58 percent of Earth’s land surface, not counting Antarctica. Semi-arid zones make up the largest share at 14.72 percent, while arid zones, concentrated in Australia and China, account for another 11.24 percent. Dry sub-humid land stretches across parts of China and Russia at 6.35 percent, and hyper-arid zones in countries including Algeria, Libya, and Saudi Arabia cover 6.27 percent of Earth’s terrestrial surface. Other assessments, including a widely cited United Nations figure, put current dryland coverage closer to 41 percent, a gap the researchers attribute to those same differences in methodology rather than to any disagreement about the underlying trend.
The CO2 Twist Most Models Miss
A key part of the new projection involves a physiological quirk in plants. When atmospheric carbon dioxide concentrations rise, the tiny pores on leaves called stomata partially close, reducing how much water plants transpire into the air even as temperatures climb. That change affects potential evapotranspiration calculations, but the research team says many prior dryland projections never factored it in, effectively assuming warming alone would drive evaporation higher without accounting for how plants themselves respond. To correct for the gap, the researchers built a CO2-modified potential evapotranspiration framework, combining historical meteorological records from 1960 to 2023 with climate models from the sixth phase of the Coupled Model Intercomparison Project, a standard set of simulations used across the climate science field.
Two Emissions Scenarios, One Growing Trend
Depending on which greenhouse gas emissions pathway plays out, the researchers project drylands will cover between 39.31 and 40.28 percent of Earth’s land during the 2071-2100 period, with hyper-arid zones showing the largest proportional increase of any sub-type under both scenarios. The higher-end figure translates to roughly 2.37 million square kilometers of new dryland relative to the 1994-2023 baseline, an area comparable to the size of Algeria or about 30 percent of Australia’s entire landmass. That expansion would not happen evenly; the researchers found the changes cluster heavily in regions that are already on the dry side of the aridity threshold today.
A Softer Number Than a Decade-Old Warning
The new estimate lands well below a widely cited projection from a decade earlier, which suggested drylands could eventually cover as much as 56 percent of Earth’s land by 2100. Researchers involved in the current study, published this month in Environmental Research Letters, say the difference traces largely to that earlier work’s failure to account for how elevated carbon dioxide throttles plant water loss. The finding does not reverse the broader trend toward a drier planet, nor does it suggest drylands will stop spreading. It suggests instead that some previous forecasts likely overstated how fast, and how far, that expansion would run by century’s end.
Australia and Brazil in the Crosshairs
Even with the more moderate projection, the researchers found that Australia is set to experience the most pronounced dryland changes of any country this century, followed by Brazil, with parts of Africa also facing widespread shifts in climate zones. The study’s authors warn that as dryland conditions push into regions that were formerly more humid, the change can aggravate water scarcity, degrade ecosystems, and accelerate desertification, adding new strain to global efforts around sustainable development and land management. The team argues that updated, methodologically consistent assessments of dryland extent, like the one it produced, are needed to support climate adaptation planning in the decades ahead rather than leaving governments to reconcile wildly different estimates of how much of their territory will turn arid.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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