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This oasis (above) next to the town of Mao, Chad, allows farmers to grow date palms and cultivate a few crops in the small fields around it. But rising temperatures due to climate change are a growing threat to oases such as this, and to the people and wildlife that depend on them.

The image is part of a series shot by photographer Tommy Trenchard for the photo essay “, which explores how these fragile ecosystems are disappearing. As temperatures rise, vegetation is retreating around oases and sand dunes are encroaching upon them. To try to hold back the sands, farmers in villages such as Kaou, also in Chad, are building barriers from palm fronds, as shown in the images below.

Mao and Kaou are located in the Sahel region, the semi-arid belt south of the Sahara desert that stretches right across Africa, from Mauritania to Eritrea. In 2007, the African Union launched the Great Green Wall initiative to try to prevent the desertification of the Sahel.

As part of this initiative, solar-powered water pumps have been installed in places such as Barkadroussou (below), not far from Mao in Chad, to help farmers irrigate crops. But the Great Green Wall initiative is controversial, with many questioning whether it will work.

Even where measures such as building barriers or installing boreholes do make a difference, with temperatures set to rise higher still, it is far from clear that oases like these will remain oases for much longer.