[bluGill]

You don't need to dig them constantly. In a few years your village has enough, and now you just need to maintain them which takes less time and so can be done in the cooler parts of the day. And these green places will be cooler than the desert so even in the heat of the day the whole is cooler.

Of course as the village population grows you send the "young men" out to the edge and have them dig a few more half-moons at a rate of one/day. However they are building next to previous green areas which are already lowering the local temperature, and if sometimes it takes 2 days to build one - who cares, there is no hurry.

[macspoofing]

If you want to treat this as a nice community project, akin to a community garden - great! Just don't think that this makes ANY difference on the macro scale of solving the encroachment of the Sahara, in the same way that a set of community garden initiatives would not solve, in any way, the problem of feeding a huge urban population. For both those things, you need things done at a massive industrial scale. If you want to feed a largely urbanized population of 350 million, you need industrial-scale agriculture, backed by a massive transport and distribution network. If you want to hold back the Sahara by creating a green wall spanning thousands of kilometres, and hundreds of kilometres deep, that is likewise a massive industrial undertaking.

>Of course as the village population grows you send the "young men" out to the edge and have them dig a few more half-moons at a rate of one/day.

Uh huh - because that's what young men want to do, dig ditches for no wage or subsistence-level wage.

What's going to happen is those who can, will simply leave.

[bluGill]

Africa has a large population living on subsistence level wages. The large number of people scales very well right now.

> because that's what young men want to do, dig ditches for no wage or subsistence-level wage.

It is an investment - see that bare desert over there, work off energy by digging it. Then when they "grow up" they discover that bare desert is their land to farm and their time was really a good investment in their future and gives them a good remainder of their life, and their kids a chance for an even better life.

Those who can will leave (unless they love farming), but today the majority don't have any better place to go. Stay in the village at the edge of the desert and dig in these green places means a better life than going to the city and trying to find a jobs (remember these people don't have skills in demand in the city either, other than their ability to dig ditches in the city, at least in the desert they can take ownership of the land they make better and have a farm.)

Ideally in 50 years Africa will improve conditions and industrial scale farming will take over, sending 98% of the people (there decedents) to a city where they work not as basic labor but as skilled labor (doctors, engineers...). However they are not in this position now and this looks like a step on the path to getting there.