The first world map of oases shows their uncertain future, between expansion and depletion of their sources

They barely represent 1.35% of the planet’s land surface, but some of the milestones of human civilization were only possible thanks to oases. Without them, the first humans to leave Africa would have had a much harder time doing so. They were the scaffolding on which the Silk Road that connected Europe and Asia in ancient times was supported. These water islands were props of key historical processes, such as the slave trade between sub-Saharan Africa and imperial Rome, the invasions of the West from East Asia or the spread of Islam. Now, a group of scientists has created their world map by observing two parallel processes of which, in the near future fueled by climate change, only one can remain: while some of these orchards are expanding due to human intervention, others are retreating. before the advance of the desert. In the medium term, experts are only clear about one thing: its future is uncertain.

Researchers from the Chinese Academy of Sciences and the National Research Institute in Cairo (Egypt) have created what they consider the first world map of oases. Mediated by literature and films, the Western view of these topographic formations is made up of palm trees, a water well and caravanners. The missing Palmyra (whose ruins were destroyed again by the Islamic State terrorists) is the prototype of that image. The city of date trees was the connecting place between the west and the east for millennia and between the Persian and Roman empires for centuries. But the reality is much more complex.

According to this new map, there are oases in 37 countries and the majority of their surface area is not in Africa and its two main deserts, the Sahara and the Namib. 77% of these green havens are located in Asia. And, although a good part of them are in the Arabian Peninsula and the rest of the Middle East, the main oases are located so far north that they do not have palm trees and the deserts that surround them are so cold that, during their winters, the temperature drops to as low as -20° or -30°. Specifically, more than half of these concentrations of life are found in central Asia and northwest China. The interior of Australia (with 13.36%) and portions of the western fringe of the two Americas (5.02%) complete the list. African oases barely represent 4.21% of the total. These percentages allow us to better define what an oasis is: an area within an arid region, not necessarily hot, surrounded by desert, not necessarily sand, that has a stable source of water that is not rain and is not always underground.

The world map of oases drawn now is not a still photo. Its authors have based themselves on the images of the Climate Change Initiative of the European Space Agency (ESA). Supported by its satellites, it has been mapping the Earth’s surface and its different types of vegetation cover, from frozen wastelands to the most lush jungles, for decades. This file is what has now allowed us to animate that map of the oases and see how they have evolved since 1995. In fact, the dynamics of these orchards is the central part of this new research, published in the scientific journal Earth’s Future.

The history of images from satellites shows two parallel processes. On the one hand, the study found that oases, in general, grew by more than 220,149 square kilometers (km²) between 1995 and 2020. But, at the same time, other oases lost 134,300 km² during the same period. So there was a net growth of 86,500 km². In total, on the entire planet, there are about 1.9 million km² of oases.

“Water is the only limiting factor that determines the existence, development and extinction of oases; when there is not enough, the desert recovers its territory”

Dongwei Gui, researcher at the Xingiang Institute of Ecology and Geography (China)

“Oases are distributed throughout different desert regions of the world, so different oases have different situations,” says Dongwei Gui, a researcher at the State Laboratory of Desert and Oasis Ecology at the Xingiang Institute of Ecology and Geography (China), in an email. ) and co-author of the study. “While some in China have been able to expand due to the human factor (extracting groundwater, for example), others in Egypt have been able to contract due to lack of sufficient water supply,” he adds. In fact, the authors maintain that practically all of the expansion has had anthropogenic causes, while the losses would be due to the desertification process experienced by large areas of the planet. “Water is the only limiting factor that determines the existence, development and extinction of oases; when there is not enough, the desert recovers its territory occupied by the oases,” adds Gui.

Xingiang reflects well what is happening. It is the northwesternmost province of China. With an area three times larger than that of Spain and almost the same size as Mexico, it is home to some of the largest oases on the planet, such as Turfán, a city of more than half a million inhabitants. Since the Chinese regime liberalized the ownership and uses of the land, the region, one of the poorest in the country, has experienced an explosion in agriculture that has turned it into one of the main markets of origin for vegetables and greens in the country. In a certain sense, it is repeating the history that starred the greenhouses of the Spanish southeast. In fact, the intensification of agriculture is the main cause of the expansion of oases. Also in Africa, cases of expansion are due to artificial irrigation of lands that were previously desert.

At the same time, oases, integral parts of arid regions, are suffering the impact of climate change and the desertification it is causing. The greatest losses have occurred in African orchards, but also in large areas of Asia. Researchers estimate that changes to the oases have directly affected about 34 million people worldwide.

The researcher from the University of Alicante, Jaime Martínez Valderrama, has recently been to Xinjiang, where he is collaborating on a project with some of the authors of the global map of oases. He recalls that the image we have of oases are those of the Sahara or Arabian deserts, which are sustained by groundwater thanks to a natural spring or an excavated well. “99% of the liquid fresh water on the planet is underground,” he highlights. In fact, the largest aquifers are under the sands of the Sahara. “It’s rainwater that fell 40,000 years ago,” he says. Thanks to technology inspired by oil exploitation, the old and traditional wells have been replaced by sophisticated pumping and extraction systems that are making it possible to feed the demand for cereals, fruits and vegetables in the expansive megalopolises of the North African coastal strip, but compromising the future of the Saharan oases. “They are no longer four palm trees, but true food production systems that, in these arid areas, with lots of sun, good temperatures and now the water ingredient that was missing, are so profitable that they are expanding,” adds Martínez, also a scientist. of the Arid Zones Experimental Station of the CSIC.

The oases of central Asia and northwest China are something else. “They are also maintained by groundwater, but their main source is snow from the mountains,” recalls the Spanish researcher. With mountain ranges ranging between 4,000 and 7,000 meters, enormous rivers flow down their slopes and end in endorheic basins, that is, they do not flow into the sea, but rather into inland lakes or, as in this enormou
s region, into deserts. It is these waters that are leading what is happening in China. They call it there oassificationexpansion at the expense of the desert thanks to the exploitation of water resources.

“The oases are expanding, but the water reserves on which they depend are being depleted”

Jaime Martínez, researcher at the University of Alicante and the Arid Zones Experimental Station of the CSIC

“The oases are expanding, but the water reserves on which they depend are being depleted,” warns Martínez. “Not only because of an abuse of resources beyond their regeneration capacity, but also because, climate change is causing there to be less snow in the mountains, and melting ice is the origin of the entire system,” he adds. “With global warming it doesn’t snow the same, it melts faster, and that is a big problem. In the end you see that the factors that drive desertification are the same as those behind development. Will this expansion be bearable in a climate change scenario where water resources are going to decrease? It is not known,” concludes the Spanish expert.

Gui, the Chinese scientist, says the same: “There is no doubt that climate change will change the water cycle on a local and global scale, bringing enormous uncertainty about the sustainability of the oases. Its future will depend on the water situation in this context of global change and, more importantly, on people’s behavior.”

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