Scientists Warn ‘Drylands’ Could Spread to Over 40% of Earth by 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.

But in a surprise twist, the most prominent contributor to climate change – rising carbon dioxide (CO₂) levels – may actually be slowing the expansion of Earth’s drylands, researchers say.

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.

“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 the analysis, drylands were defined using a measure called 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.

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 write.

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).

Their results suggest that, based on a baseline period of data from 1994–2023, drylands covered about 38.58 percent of Earth’s total land, not counting Antarctica.

Drylands come in four sub-types, with most of the world’s drylands being semi-arid zones (14.72 percent of terrestrial area per the team’s estimates), with arid zones, predominantly in Australia and China, accounting 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.

In the future, the researchers project that drylands globally 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.

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While it’s a concerning trend, it’s worth noting the CO₂-modified PET calculations used by the researchers put their 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.

Still, the 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.

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.

The findings are reported in Environmental Research Letters.