Earth’s drylands could expand to cover more than 40 percent of the planet’s land area by the end of the 21st century, according to a study published in Environmental Research Letters.
Global Dryland Projections Through 2100
As the world gets hotter, many of its driest regions are expected to get even drier. When this happens, the boundaries of arid regions are expected to creep forward, transforming landscapes and killing off plant life no longer able to cope. Amidst the changes, projections suggest Earth’s drylands could grow to cover over 40 percent of the planet’s land area by the end of the 21st century, according to a new study in Environmental Research Letters.
Drylands are regions that face great water scarcity, where precipitation is low relative to the potential loss of water through evaporation and plant transpiration. While the concept is well established, and scientists broadly agree on where the world’s major drylands lie, differences in measurement techniques and climate data mean global estimates of Earth’s dryland coverage can vary substantially. As the new paper urges, there are good reasons to find common ground.
As global temperatures rise, dryland boundaries are shifting into regions that were previously more humid.
Expansion of dryland climatic conditions into formerly more humid regions may aggravate water scarcity, ecosystem degradation and desertification, posing challenges to global sustainable development,
a team led by scientists from the Chinese Academy of Forestry explains.
Given the high ecological vulnerability of drylands, updated assessments of dryland extent and future redistribution are needed to support climate adaptation and sustainable development.
In the study, the researchers used updated climate modeling to map current global dryland distribution and project how they might spread in the future.
For their evaluation, researchers applied the aridity index (AI), which equates to a region’s precipitation divided by potential evapotranspiration (PET), with areas with an AI below 0.65 classified as drylands. Based on data from a baseline period running from 1994 to 2023, these zones covered roughly 38.58 percent of the Earth’s total land surface, not counting Antarctica.
Looking ahead, the researchers project that drylands of varying intensity will increase to cover between 39.31 to 40.28 percent of Earth’s land during 2071–2100, depending on which greenhouse gas emissions scenario is used, with hyper-arid zones showing the largest increase. This top estimate of 40.28 percent corresponds to a mind-boggling increase of 2.37 million square kilometers relative to the 1994–2023 baseline, which equates to a land grab the size of Algeria, or about 30 percent of Australia.
The Carbon Dioxide Factor in Aridity Calculations
While the warming planet drives the expansion of arid conditions, rising atmospheric carbon dioxide introduces a complex counter-effect. One factor contributing to the variance we see in dryland projections is that rising atmospheric CO2 levels over time affect plant transpiration, with pores on plants called stomata partially closing when CO2 levels are high.
This means plants lose less water through transpiration overall, which in turn affects calculations using PET – but the team says it’s a factor that many previous studies haven’t accounted for.
Conventional projections often neglect CO2 effects on potential evapotranspiration, which may yield higher aridity estimates and alter aridity-change hotspot patterns
The authors
In their own analysis, the researchers used a CO2-modified PET framework that incorporated historical meteorological data for 1960–2023 and climate models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6). By incorporating this CO2-modified framework into climate models, the study team arrived at figures significantly lower than some other estimates – one study a decade ago suggested drylands could cover up to 56 percent of Earth’s land by 2100.
Regional Vulnerabilities Across Continents
Drylands currently divide into four distinct sub-types, with most of the world’s drylands being semi-arid zones (14.72 percent of terrestrial area per the team’s estimates). Arid zones, predominantly in Australia and China, account for 11.24 percent of Earth’s land.
Dry sub-humid zones are extensive in China and Russia (6.35 percent), while Algeria, Libya, and Saudi Arabia all feature hyper-arid zones, which account for 6.27 percent of terrestrial coverage.
Speaking of Australia, while the world’s driest inhabited continent might be the ultimate bolt-hole in a future calamity, it doesn’t fare well in dryland projections. The researchers predict Australia will face the most pronounced dryland changes this century, followed by Brazil, while parts of Africa are also expected to see widespread shifts.
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