Nuts to the Sahara!
What about Australia?
The Spice must flow - true.
Hmm, the fifth heading in that link involves “Scalar Methods.” That 1920s technology just keeps on working…
I believe Libya has constructed or is constructing huge underground tunnels to carry water from the under-desert aquifer to the more populated coastal regions.
Hey, and don’t forget the orgone methods, either! Here’s a more reputable source for information on airwells (scalar technology not included).
Problem 1. Grasses aren’t usually better suited to arid regions than grass. Indeed in the most arid regions there are invariably trees or equivalent plant forms where the grasses exist only as extreme ephemerals. Or to put it more simply you are better off starting with the woody plants.
In fact you are better off finishing there. The reasons that grasslands exist in tropical regions is because of fire. Fire kills trees but not grass. Grass produces grasslands at thee expense of trees. Unless you are prepared to spend a lot of money on fire control, and a lot more replanting all those areas when it inevitably fails then you should be trying like the dickens to keep the grass out. Initially that should be as simple as using drip irrigation. If rainfall increases then you will need to introduce lots of grazing animals to kill the grass.
But it’s very doubtful if rainfall will increase simply because of trees. Trees don’t ‘fix’ water. In fact trees destroy water, that’s what photosynthesis is all about. Every molecule of oxygen represents one water molecule that has been destroyed.
Trees can be beneficial in bringing up water form the deeper soil profiles and hence maintaining humidity from annual rains for longer periods after the rain has ceased. Trees in the absence of fire also provide leaf litter which has much the same effect. ** Bosda Di’Chi of Tricor** asked “What about Australia” and there is in fact a small pool of scientists who believe that human induced deforestation is largely responsible for the desertification of Northern Australia precisely because of those effect. However Northern Australia is a special case, and to understand why and why will help you understand why your plan would be unlikely to work for the Sahara even if it does work for Australia.
Deserts generally exist in two bands on the upper latitudes of the tropics or the low subtropics. That’s because the circulation of air currents produces a high pressure ridge right through those altitudes as warm air from the equator falls back again as it cools. You can’t get rain under a high pressure system precisely because the air has already cooled and thus all the water has already been extracted. You get rain when moisture laden air rises in a low pressure system, not when dry air that has already shed its water is falling.
The deserts of Northern Australia are exceptional because they are marginal on the high pressure ridge, indeed they vacillate between being under the ridge in winter and outside in summer. That makes the rain-depressing effect of the ridge fairly weak. Moreover Northern Australia has direct access to a massive, warm body of ocean water from equatorial Asia that gets damned by the Indonesian archipelago right off the coast. Once the subtropical ridge weakens in summer the continent also heats up causing a rising mass of air; a low pressure system. You have ready made rain generation system with a low pressure system right next door to a warm still ocean.
Those conditions make Northern Australia suitable to attempts to reclaim the desert by re-establishing forest and indeed this is the main reason why it’s believed the deserts exist because the trees were killed. Most of Northern Australia wasn’t desert only 50, 00 years ago, it was dense forest and scrub. With that sort of tree and litter cover the warm Asian ocean water could penetrate deep inland for extended periods. Moreover the cooling effect of foliage meant that effect meant that the land heated up less after rain, moving the high pressure system ever beyond the reach of the lat rains. Over the course of one summer numerous high pressure systems built up, but each one was restricted further south as the forests sprouted new growth in response to the northerly rains. And of course the moisture laden air only shed when it struck the high pressure system.
In effect Australia has a unique set of factors that make it ideally suited to a situation where trees mean rain. The Sahara doesn’t.
The Sahara, or most of it, is firmly under the subtropical ridge all year round. IOW it sits in a high pressure zone almost perpetually. Moreover it is bounded on the equatorial side by land. That prevents any build up of a warm body of water adjacent to it that could be a source of moisture such as Northern Australia has. Under those conditions there’s simply no potential source of rain. In Australia you can say the water exists in the atmosphere and with trees it can be encouraged to move further inland whereas currently it is restricted to the coastal regions. The coastal regions of Northern Australia are wet with rainforests receiving metres of rain annually whereas the coastal regions bordering the Sahara are arid.
It’s not like there is an excess of atmospheric moisture going to waste in the regions of the Sahara and trees can’t simply make moisture. They need to get it form somewhere and there is simply no Saharan source.
Blake, that’s a nice analysis, but at one point in human history, the Sahara did have trees, and the desert has been growing at a considerable rate a year for some time now. Clearly something must have been different in the past for this to have occured. One of the things which trees provide, is a collecting point for dew, and I’ve read that in several places where there’s lush forests, there’s actually not enough rainfall to sustain the forest, and that it’s the dew being collected on the leaves of the trees that provides the moisture for the forest to thrive, and that once the trees start to go, the whole forest rapidly begins turning into a desert.
I don’t know what the moisture content of the Sahara air is, but if there’s any moisture at all to be wrung out of the air by things like airwells and fog fences, one could get a source for water. How large a source that would be, and if it could be enough to modify global weather patterns enough once trees got going to support a lusher environment, I don’t know.
Tuckerfan large parts of the Sahara did indeed have trees only a few thousand years ago. There were some forests in the sahara and patches of them remain even today. These forests spread into the deserts along water cources and are what are referred to as gallery forests. However generally sharan trees didn’t form forests, they formed woodlands with a continuous grass layer, what is known as a savanna.
And that savanna form gives a pretty damn good idea what happened to the trees, and it’s got nothing to do with rainfall. The trees vanished at about the same time that saharan people started switching to agriculture. As a wise man once said “Nothing makes a desert like a goat”. People overgrazed the savannas while they cut the gallery forests down to cultivate the riparian areas. The goats and camels killed the tree seedlings and the people cut the mature trees down for fuel. The soil blew away and the people died or moved on. This is what chefguy was referring to above. There is some really excellent data in the deforestation and agricultural erosion in Palestine for example that turned it form the Biblical land of milk and honey with its great cedars to what we see today. And that transition occurred within written history.
That is what was different in the past. It wasn’t a lack of rain that killed the trees, it was over-exploitation. The trees weren’t bringing any more rain, they were simply tree species adapted to low rainfall. And because of that the planting of more trees can’t possibly create more rain.
As for trees being a significant collector of dew or forests sustaining themselves on dew collection, I’m sceptical. I’d have to see a reputable reference to believe it. Are you sure you aren’t thinking of cloud forest?
There are certainly cloud forests in this world, and they survive on the water droplets that form on their leaves. But that is radically different from the forests supporting themselves on the dew forming on the leaves. Clouds are not dew, they are masses of already condensed water droplets. They are, in their own way, precipitation just as fog is. I don’t doubt that the removal of trees from cloud forest would resulting significant drying simply because of the loss off all that vertical collection area, but once again this is cloud we’re talking about, not dew.
I don’t have enough engineering knowledge to comment on fog fences or other water sources, and it seems rather irrelevant to me. As Sandyhook so eloquently said, with enough money we can irrigate the moon using modern technology. There’s no doubt we could get water there if we had the will. I was really only intending to comment on the idea that trees mean water to the Sahara and to answer the question about Australia. I hope I’ve done that and explained why. Trees don’t bring water to the Sahara though they may to northern Australia due to global scale climatic and geographical effects.
Funny, I distinctly remember reading an article (SciAm? Economist?) that claimed that the Sahara rapidly shifted from lush forest to arid desert and back again quite a few times within the last mumble thousand years.
It caught my eye because they claimed that those two states were the only stable states and could shift from one to the other in as little as a hundred years or so.
You mean - take all the Macadamia trees in Oz to Africa?
I don’t think that they would survive, and the natives would not be able to eat them anyhow. Seriously - I broke a hammer shelling Macadamias. 
Si
On a more serious note, one of the problems with deserts is that they not only have high sunlight, but that the sunlight reflects back off the surface, heating the air and creating a convection that moves moist air up and away from the desert area. During the night, with no incidental radiation, temperatures drop rapidly.
One concept for desert recovery involves changing the albedo (reflective index) of desert areas with permanently green low water use plants, or physical methods, to change the air movement patterns.
But would there be any value in recovering the Sahara. The soil is poor - I can’t see it being productive without centuries of organic cycling, while maintaining the conditions for growth. Our “food basket” regions are the product of millennia of forest or marsh or flood silt creating useful soil. If you don’t have that, you need to add massive quantities of fertilizer and nutrients to get anything out.
Better to cover the Sahara with Silicon Solar Cells, and generate power from it. Of course, dumping solar cells on the Sahara will reduce the albedo, and it will probably never stop raining just after you spent a considerable proportion of GEP (Gross Earth Product) putting the cells there in the first place.
Si
Shalmanese I haven’t heard of anything quite like that. The Sahara itself has obviously existed in some form for along time as can be told form the presence of species adapted to desert conditions. The idea that it has ever vanished any time in the past thousands of years as opposed to hundreds of thousands or more likely millions of years makes no real sense. There would have had to be a sizeable area of desert always in existence to allow the desert species to survive, and you don’t get sizable areas of desert surrounded by lush forest.
It’s believed that parts of the Sahara have shifted from desert to savanna numerous times and the idea that savanna and grassland/desert are two stable states in the same environment is certainly true. It’s also possible to ratchet from savanna to forest in many instances due to fire effects.
But the idea that any system can ratchet from grassland to forest with no intermediate stable savanna form makes no real sense. The idea that any system can move form desert to closed forest in as little as 100 years is also impractical. It would take just one lightning initiated fire in that period to set the whole system back to square one, and if there were sufficient rainfall to support a forest then there would be plenty of lightning.
I might be and unfortunately, I don’t have the time at the moment to dig through the materials I have to see if I’m right or not.
Interestingly that is not so here - probably because of the proximity to the Gulf. In summer the heat remains 35-40c at night. The experience of seeing a black sky but feeling HOT air is unbelievable (it is not balmy, or even quite warm, or sticky-humid, it is mega-hot) - you really have to witness it for yourself.
Re “greeing the desert” - the amount of water invovled is aboslutely phenomenal, and the power to create it (as in here, through desalination plants) is prohibitively expensive. The late ruler, Sheikh Zayed, built this “forest” of palms and other trees, but it relies totally and utterly on irrigation pipes, and between the trees and pipes is just sand. Plus the trees look very unhappy and dusty.
I haven’t read anything on this - but it strikes me that dust is a major issue in reforesting desert: it chokes leaves. And here there is no rainfall, (or adequate rainfall) as in more temperate or tropical climates, to wash it off.
Blake: A quick google dug up the following reference:
Scheffer, M., S. Carpenter, J.A. Foley, C. Folke, and B. Walker (2001). Catastrophic shifts in ecosystems. Nature 413, 11 October 2001, 591-596.
which cited the paper “Simulation of the termination of the African Humid Period” that can be found here.
Looking at the graphs, the shift seemed to happen between 7500 years ago and 5500 years ago so 2000 years instead of 100 but still astoundingly short.
Shalmanese that article doesn’t suggest that the Sahara was ever forested. It actually says “During this phase, the African Humid Period (AHP), grasslands covered the Sahara/Sahel region”. Grassland=/=forest. Grassland could well include savanna but that’s still a world away from lush forest. When the article refers to the green Sahara it is speaking of grassland.
That a region can go from grassland to desert rapidly is not really that surprising, we’ve seen regions that have gone from grassland to desert within 50 years in the course of the last century.
Only the hammer? My, you were doing well.
Easily. It was interesting to see the effect in Mali, where the desert has reached Mopti and points south. The Niger has little water in it in many areas, which I would assume is due to climatalogical changes and irrigation. In Uganda, which is equatorial and a non-desert area, most rainforest has been cut down in order to plant cash crops of matoki (banana), coffee and tea, among others. There is, literally, a line that was drawn by the government to prevent further encroachment on the Impenetrable Forest and the gorillas that live therein. As one approaches Bwindi, it is a shocking sight.