Objective: a large-scale desalination and irrigation project in Mauritania, aimed at restoring the encroaching Sahara desert to farmland.
“Approximately three quarters of Mauritania is desert or semidesert. As a result of extended, severe drought, the desert has been expanding since the mid-1960s. [...] Despite all sorts of economic reforms, including exports in minerals, agriculture and other resources, Mauritania continues to remain as one of Africa’s poorest countries.”—http://en.wikipedia.org/wiki/Mauritania
Re-open desalination plants at Nouakchott, which were closed previously due to high operating cost, expanding the operation to at least 100 times its former size.
Pump excess fresh water far out into the desert, to the east, and then to the north, allowing it to drip out, slowly, dispersing over a very large area. Some will seep in, and support local wells; some will run off; some will evaporate, carrying the humidity with the prevailing winds, changing the local climate, and hopefully having cascade effects on the surrounding regions, downwind.
The project will employ local workers, re-enable subsistence agriculture, tree planting, and help stabilize the politics. Science conducted will gather knowledge of climate change, its causes and effects.
All you need is energy; and lots of it.