When reading about the projects that want to plant trees on the Sahel-Sahara border, for protection of the Sahel, I wonder what other possibilities there are.

Like, would it be possible to influence precipitation in the Sahara desert by enlarging the evaporation of seas and oceans.

This question would combine a number of different sub-questions.

First of all, would moistening the air help at all? When reading Wikipedia, it looks like the air in the Sahara is extremely dry, but there's also a lack of precipitation generation mechanism. Can rain be generated there if the air would be wetter?

Secondly, when it rains in the Sahara, where does the rain come from? The main wind over the Sahara desert is an eastern to northeastern wind. So it comes from over the Asian mainland, and can't carry a lot of water.

  1. Does it mainly come from different winds (like an occasional western wind)?
  2. Does it mainly come from evaporation of the red sea (though that has a small surface)
  3. Does it get carried all the way over Asia, so coming from the Pacific ocean?
  4. Anything other ideas?

Thirdly (this is more a physics question), is it feasible to alter the evaporation rate of a large water body (feasible as in comparison to the tree-planting project, so it can cost a few billions). The experiments I've found wrt the relation between water-surface temperature and evaporation seem to suggest an exponential relation. Which is at least nice,

Water doesn't have a colour, so it doesn't extract a lot of energy from the solar rays (at least not at the surface, but the energy gets spread over the entire depth). If we could let an energy-absorbing coloured surface (a black surface) float just a few inches under water, it could heat the water surface and make it evaporate more.

By making it evaporate relatively near the coast (as opposed to evaporation in the middle of the ocean), the majority of rain will also have to fall on land

When this technique could be applied to a decent portion of the Red Sea, the surface temperature could be enlarged sufficiently, which could let the Red Sea act as a big ocean, thus result in a wetter Sahara and Sahel.

This might of course be a very stupid idea (like the majority of fresh ideas).

I thought someone would have studied this thoroughly before me (at least before planing millions of trees), but I couldn't find a lot of info on it (not even on the basic questions like how strongly surface colour influences evaporation).

Thanks for reading through this, and trying to answer (a part of) it.

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    $\begingroup$ This is a lot of questions rolled into one. Example: the idea of your energy-absorbing coloured surface is a question in itself. Voting to close as too broad. $\endgroup$ – Jan Doggen Aug 21 '15 at 21:07

Changing ocean evaporation would impact weather & climate globally. While that may benefit some regions, it would be detrimental to other regions - robbing Peter to pay Paul.

One of the risks would be the effect on rainfall intensity, rainfall duration and the amount of rain falling in any location. It could have disastrous consequences for landslide prone regions or low lying areas prone to flooding.

Increased rainfall would result in accelerated soil leaching requiring soil nutrients to be replaced at a great rate to allow the continuation of farming and for forests to keep developing.

The accelerated soil leaching would also have an impact on the oceans and marine life and land based life reliant on marine life. Additional phosphates would result in increased algal booms, particularly in estuarine waters. Excessive algal blooms can make such waters toxic to life than can only live in such waters.

In addition to accelerated soil leaching there will be associated soil run-off and silting of rivers, estuarine systems and coast outflow regions. For marine wildlife parks such as the Great Barrier Reef, that would have a devastating consequence and may lead to the destruction of all biodiversity with such reefs.

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