Oases are important habitats and water sources for dryland regions, sustaining 10% of the world's population despite taking up about 1.5% of land area. But in many places, climate change and anthropogenic activities threaten oases' fragile existence. New research shows how the world's oases have grown and shrunk over the past 25 years as water availability patterns have changed and desertification encroaches on these wet refuges.

"Although the scientific community has always emphasized the importance of oases, there has not been a clear map of the global distribution of oases," said Dongwei Gui, a geoscientist at the Chinese Academy of Science, who led the study. "Oasis research has both theoretical and practical significance for achieving United Nations Sustainable Development Goals and promoting sustainable development in arid regions."

The study found that oases around the world grew by more than 220,149 square kilometers (85,000 square miles) from 1995 to 2020, mostly due to intentional oasis expansion projects in Asia. But desertification drove the loss of 134,300 square kilometers (51,854 square miles) of oasis over the same period, also mostly in Asia, leading to a net growth of 86,500 square kilometers (about 33,400 square miles) over the study period.

The findings highlight the risk that climate change and anthropogenic stressors pose to these wet sanctuaries, and can inform water resource management and sustainable development in arid regions. The study was published in Earth's Future.

The birth and death of an oasis

Oases are important sources of water for humans, plants and animals in the world's drylands, supporting a majority of productivity and life in deserts. They form when groundwater flows and settles into low-lying areas, or when surface meltwater flows downslope from adjacent mountain ranges and pools. The existence of an oasis depends primarily on having a reliable source of water that is not rainfall. Today, oases are found in 37 countries; 77% of oases are located in Asia, and 13% are found in Australia.

Gui and his co-investigators wanted to understand the global distribution and dynamic changes of oases and investigate their responses to a changing environment, such as variations in climate, water resources and human activities. Using data from the European Space Agency's Climate Change Initiative Land Cover Product, the team categorized the land surface into seven categories: forest, grassland, shrub, cropland, water, urban and desert.

The researchers used satellite data to look for green, vegetated areas within dryland areas, indicating an oasis, and tracked changes over 25 years. Changes in the greenness of vegetation indicated changes in land use and oasis health, the latter of which can be influenced by both human activity and climate change. They also looked at changes in land surface type to find conversions of land use.

The researchers found that global oasis area increased by 220,800 square kilometers (85,251 square miles) over the 25-year timeframe. Most of that increase was from humans intentionally converting desert land into oases using runoff water and groundwater pumping, creating grasslands and croplands.

The increase was concentrated in China, where management efforts have contributed more than 60% of the growth, Gui said. For example, more than 95% of the population in China's Xinjiang Uygur Autonomous Region lives within an oasis, motivating conservation and a 16,700 square kilometers (6,448 square miles) expansion of the oasis, Gui said.

Countering human efforts to expand oases, desertification contributed to oasis loss. Worldwide, the researchers found that there was a loss of more than 134,000 square kilometers (51,738 square miles) of oasis land over the past 25 years. The researchers estimate that changes to oases have directly affected about 34 million people around the world.

Overall, between gains and losses, oases had a net growth of 86,500 square kilometers (33,397 square miles) from 1995 to 2020—but most gains were from the artificial expansion of oases, which may not be sustainable in the future.

Long-term oasis sustainability

The study highlighted ways to sustain healthy oases, including suggestions for improving water resource management, promoting sustainable land use and management and encouraging water conservation and efficient use. These efforts are especially important as the climate continues to change, Gui said.

Humans' over-exploitation of dwindling groundwater can limit oasis sustainability, as well as long-term glacier loss. While higher temperatures increase glacier melt, temporarily boosting oases' water supplies, "as glaciers gradually disappear, the yield of meltwater will eventually decrease, leading to the shrinkage of oases once again," Gui said.

International cooperation plays a crucial role in oasis sustainability, Gui said.

"Due to the unique mechanism of oasis formation, a river basin often nurtures multiple oases across several countries, making transboundary cooperation key to addressing water scarcity and promoting sustainable development," he said.