Populations rise above the ability of the water supply to give them the amount of water they need. In countries labeled here:

Serious - ground water is being mined and will fail in the near future
In Moderate - there is a problem likely in the medium future.
Episodic - means water supply depends on good rain which does not always occur. Parts of developed countries, such as the Southwestern United States also have problems in the moderate to serious range. The whole of southern Australia is an area with very doubtful water supplies, prone to droughts that may be increasing with climate change.

One of the main symptoms of global warming will be the expansion of the global deserts towards the poles. Thus: southern Europe, midwestern United States; southern Australia are likely to become drier, and perhaps uninhabitable.

It is not clear yet whether the Monsoon rain belts may also move nearer the poles, thus the Sahel may become wetter. Even if this happens the loss of agricultural land will be much larger than any gain of tropical land.