Many scientists believe that one of the consequences that will come along with global warming is the expansion of world's deserts, most notably Sahara but not all scientists share the majority's opinion. In fact some scientists even believe that deserts are slowly retreating and how water and life will slowly start to grow in these dry places thanks to warming trend.
The evidence that supports this theory (up to a certain point) are satellite pictures of North Africa that seem to show the pattern of areas of the Sahara in retreat. According to this theory global warming would increase the temperature on Earth which would result in more evaporation of the oceans, and in the end result in more rainfall. And the more rainfall there is the more deserts are likely to retreat making way for new life to flourish.
Though this theory sounds very logical and is supported with satellite images of North Africa it still looks to be far from reality, especially if we look at what's happening in the ground. Droughts have been frequent in the last few decades in Africa, and it is really difficult to see some benefits from global warming despite these satellite images.
It also has to said that if there's one thing that really varies a lot then this is the amount of rainfall in deserts. In order to really see a pattern of deserts shrinking we should be having satellite images of at last last couple of centuries. At this point and given current evidences this is only an assumption and not an actual proof. Yes, the last few years there has been higher than average rainfall but such short period is really negligible and insufficient to make any solid conclusion about global warming effect on deserts.