A new study by the Chinese Academy of Forestry, published in Environmental Research Letters, suggests that rising levels of carbon dioxide (CO2)—a gas we exhale and release through various human activities—are reshaping the world's climate zones, potentially increasing the amount of dry land on Earth. Currently, 38.58 percent of the planet’s land is classified as arid, and this could rise to 40.28 percent by the year 2100, according to climate models. This change is not due to less rainfall, but rather to the way CO2 affects plant behavior, which in turn influences how much water is lost to the atmosphere. Drylands, which include deserts and semi-arid regions, are categorized based on their level of dryness. These categories are defined using the aridity index (AI), a measure that compares the amount of precipitation a region receives to the amount of water that could potentially be lost through evaporation and plant transpiration. If the AI drops below 0.65, the area is classified as a dryland. The study highlights that this index has been used inconsistently, leading to conflicting global maps of dryland areas. To better understand how drylands might expand in the future, the researchers combined historical weather data from 1960 to 2023 with the latest climate models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6). These models help scientists simulate future climate conditions and predict how dry areas might grow by the end of the century. The study found that higher CO2 levels cause plants to partially close their stomata—tiny pores on leaves that control water loss. This reduces transpiration, which is the process by which plants release water vapor into the air, and thus limits how much water is lost to the atmosphere. This discovery is important because it shows that previous climate models may have underestimated the role of plants in regulating dryland expansion. By adjusting how potential evapotranspiration is calculated based on CO2 levels, the researchers improved the accuracy of their projections. However, the study also points out that current methods for mapping drylands vary widely, leading to conflicting estimates. The researchers emphasize the need for a more unified approach to measuring drylands, as these inconsistencies affect how governments and organizations plan for water scarcity, ecosystem loss, and desertification. They call for regular updates to dryland assessments to better understand and address the risks associated with these changes.