One of the best ways to stop the encroachment of the Sahara on surrounding areas would be to remove the human population of the scrubland around the desert such as the Sahel.

Their struggle to survive is the "last straw" for the local ecosystem and doesn't help things at all. However, those people live in the land of their ancestors of many generations and have no wish to leave, nor is there a place for them to go to.

Technology can help, but Africa is a land of foundered intentions and thwarted plans. Perhaps if we could stop the corruption, tribalism, and blood fueds, plus the imprint of AIDS and drugs, then things would improve?

But colonialism didn't really work, it just held back the floods for a while before releasing them even stronger. Gradually, we need to bring the benefits of modern civilisation to all the world, not just the near-to-home bits.

As for Mars, perhaps the dream of Terraforming is an escape from the even greater difficulty of sorting-out human cultures and politics here on Earth.

With a new frontier you can just keep on going and not have to stop and put things right...