(CNN) -- Lake Chad is shrinking rapidly, threatening the millions of people who depend on it for their survival. But some locals are fighting back in a bid to preserve their way of life.

From droughts causing bad harvests, to floods destroying farms and homes, life in Africa's Sahel belt can be a constant struggle.

The arid belt of land stretches from Senegal in the west, all the way across the continent to Ethiopia in the east. With the Sahara to the north, and the savannah to the south, it's a region that experiences extreme dry and wet seasons.

In the middle of it all is Lake Chad, the most reliable resource in this region of shifting extremes. More than 20 million people depend on the freshwater lake for their survival.

But it's been shrinking over the past 50 years and satellite images show it is now just a twentieth of its former size.

Huge expanses of water are now nothing more than a series of ponds and islands, and the once-fertile land that surrounds the lake is now dusty and barren.

"If there are solutions we must find them," said Farid Dembell, from the Society for the Development of Lake Region.

"The lake is in the process of disappearing and the lake feeds many people, not just here but in other countries like Nigeria, Cameroon and Niger," he continued. "They are all people who live on Lake Chad."

The way of life in this area stretches back centuries and many earn a living fishing in the lake.

Locals report that they are catching less fish and the ones that they do catch are smaller than they used to be.

A declining stock could have devastating consequences far beyond the water's edge, says Yakowra Mallom, from UNICEF.

"At the start we didn't know anything about the problem of malnutrition," she said. "But now the figures are enormous. The children are all malnourished. There are no more fish. There's no more milk, no maize, no vegetables or cereal."

Local communities say the changing weather is the biggest reason for the shrinking of the lakes shores. The necessary irrigation of farming land has also been a factor.

But some people are making efforts to save their livelihoods.

A small local group is trying to save the surrounding land by planting trees in the villages that have been worst affected by desertification. If they cannot bring back the lake, they hope there will at least be workable land.

Saleh Sagoubi heads up the Tree Planting Association, a volunteer organization that has around 50 young members.

"I was born here and I grew up here," he said. "I want the lake to come back, not just for me, but for the children of the future."

Sagoubi blames climate change for turning much of the once-fertile land of the Sahel into desert. His group is trying to hold back the Sahara with a "great green wall" of drought-resistant trees.

"To stop the Sahara we must make lots of effort day and night -- we must work," he said.

"The desert will be stopped one day by trees; they are our weapons of mass destruction."