As an alternative to black plastic sheeting in Alvia's idea, you could use biochar. This has several advantages, in that it will attenuate flooding, hold water for dry periods, act as a substrate on which soil can begin to form in currently desertified areas. It also locks up carbon in the soil.
I agree that the desalination/irrigation idea isn't sensible. However, the practical limitation to the irrigation process is desalination, not water supply or the cost of moving the water. It would seem sensible to look at pumping seawater into the Sahara, and letting evaporation do the desalination for you. If you pumped water onto absorbent surfaces (think large bath towels) in the hottest, windiest, driest part of the desert, then evaporation rates would be enormous. The resulting moist air should be able to make rain whenever it rises or cools significantly. You'd probably have to include a return flow which took away saltier water and returned it to the sea, to prevent the build up of large salt deposits. Steps would need to be taken to prevent contamination of aquifers with saltwater. However, there are lots of plants that seem to be able to tolerate high salt levels, so maybe these could be grown on salt-contaminated areas. A 2009/9/22 Alvia Gaskill <[email protected]>: > > Nor might it have to take as long as several centuries to get things going. > From reading both of the papers, it is clear that the surface albedo has an > important role in the strengthening and weakening of the monsoon which in > turn supplies the rainfall for the plants. Revegetation of the Sahel has > been a work in progress for decades, with the fluctuations in climate > usually drowning out any man-made efforts. And as the other paper pointed > out, the water requirements for a man-made greening of the entire Sahara are > unacceptably large. I might add that there have been numerous > proposals/patents over the years to vegetate the Sahara, the one that > started this discussion just the most recent. The authors or someone else > sent it to Holdren as it appeared in the OSTP/FOIA dump I reported on a > while back. > > I proposed whitening parts of the Sahara to increase the surface albedo. > But it is the relatively high natural albedo of 0.3-0.4 that is the end > product of the natural desertification observed since the humid period ended > rather abruptly 5500 years ago. Someone should look into seeing what the > inpact of applying a BLACK cover to the region just outside the Sahel would > be on the monsoon. Black plastic lasts much longer under solar radiation > than white due to the fact that the carbon black that gives it the black > color absorbs most of the UV that breaks down the polymer. I've seen pieces > of black plastic in Durham that have been outside for more than a decade and > are still intact! > > Since the black plastic surface would absorb much more solar radiation than > the existing natural surface or even green vegetation, it would give a much > more enhanced effect. Of course, the black plastic can't evaporate any > water, but it could be applied concurrently with tree planting, the land > around the trees covered with the plastic. I would predict that a much > smaller area would have to be covered to get things going in terms of the > feedback effects. We can't make the sun stronger, but we can make the land > warmer. The additional GHG forcing from the plastic vs. the carbon > sequestered by the vegetation would have to be factored in. I would think > it would be much less once a sufficient area is revegetated. While some > water would be required for the tree or shrub planting, irrigation via > desalination or other means would not be necessary as the monsoon would > supply the water. The area in question stretches about 2400 miles from the > Atlantic Ocean to the Red Sea. A 10-mile North/South band of black surface > would cost around $15 billion to install and could be completed in a few > years. > > The concerns expressed about impacts on dust/nutrient flows would need to be > addressed, but it appears that iron and phosphorous are in excess now for > the Amazon and also for the Atlantic, so a decrease could be tolerated. > Since it would likely take more than a century to complete the revegetation, > there would be ample time to determine any impacts. Only a few tropical > waves are affected by dust storms, so the effect on hurricane development > would be minimal. The alteration of the surface and strengthening of the > monsoon might also offset some of the weakening predicted from use of > stratospheric aerosols. > > > ----- Original Message ----- > From: "Stephen Salter" <[email protected]> > To: <[email protected]> > Cc: "geoengineering" <[email protected]>; > <[email protected]>; "Climate Intervention" > <[email protected]> > Sent: Tuesday, September 22, 2009 6:34 AM > Subject: [geo] Re: Geoengineering the Sahara > > > Hi All > > The Sahara ploy may not be that hard. > > A very interesting paper at the techie Copenhagen meeting was from Kerry > Cook. She has a climate model that lets you plant trees. If she plants > them in Africa from the present forests up to 17.9 deg north they > quickly die back. But if she grows them slightly further north they > spread all the way up to the Mediterranean, just as they used to be 5500 > years when the Sahara was packed out with hippos. > > This means that you do not have to provide water for the whole area or > go on paying the $2 Trillion every year. Some papers are attached. > > Stephen > > > Emeritus Professor of Engineering Design > School of Engineering and Electronics > University of Edinburgh > Mayfield Road > Edinburgh EH9 3JL > Scotland > tel +44 131 650 5704 > fax +44 131 650 5702 > Mobile 07795 203 195 > [email protected] > http://www.see.ed.ac.uk/~shs > > > > Oliver Morton wrote: >> I blogged this last week >> >> http://heliophage.wordpress.com/2009/09/15/terraforming-the-sahara/ >> >> Text >> >> An interesting paper in Climatic Change: Irrigated afforestation of >> the Sahara and Australian Outback to end global warming by Leonard >> Ornstein, Igor Aleinov and David Rind Doi: 10.1007/s10584-009-9626-y. >> (Mason Inman has a nice write up with some background and comment over >> at ScienceNow; [update] and corresponding author Len Ornstetin >> chronicles the idea’s rocky research road on his own site). The >> central idea is that with enough irrigation you can turn big deserts >> into big forests: forests big enough to suck up a large part of total >> carbon dioxide emissions for decades or even centuries. I think that >> you can take this notion as a serious plan, a thought experiment, a >> jeu d’esprit, a warning or a jumping off point, depending on >> predisposition. Aspects of all that in what follows. >> >> Here are the basic numbers: The Sahara is about a billion hectares in >> area, on which you could fit a trillion eucalyptus trees. Those trees, >> if working flat out, could each put on twenty kilos of biomass a year. >> If roughly half that biomass is carbon, that would mean a net annual >> sink on the order of ten billion tonnes of carbon. That’s about the >> amount that humans currently emit. >> >> To create such a forest in a century, you would have to plant as many >> hectares of trees every year as are currently lost to deforestation >> worldwide. And, even harder, you’d have to provide them with what they >> need to order to grow. You need a great many things to turn a desert >> into a forest — soil nutrients, microbiota, possibly pioneer plants, a >> compelling reason for doing the work, and so on — but the biggest >> hurdle, pretty obviously, is water. Eucalyptus, the authors say, needs >> about a metre of rainfall a year. For a billion hectares, that’s 10 >> trillion tonnes of water. The authors assume, reasonably for all that >> I know, that if you have smart irrigation getting the water to just >> where it is needed you can get away with half that amount. Even so, >> even the vast aquifers beneath the Sahara don’t contain the amount of >> water required, so it will have to come from desalination plants on >> the and be pumped it up to where it is needed (the average elevation >> of the Sahara is about 450m). The size of this undertaking — more than >> 50 new Niles, flowing in reverse — may explain why the authors feel >> they need to use that fine old-school term “terraforming” for their >> undertaking. The power requirement, if I’m reading their figures >> right (4.04kWh/m^3 fresh water delivered), is a bit to the north of >> 2.2 terawatts, about 40% of it for desalination by reverse osmosis and >> about 60% for pumping. >> >> The world’s electricity generators currently provide about 18,000 TWh >> of energy, which averages out at 2TW of constant supply. So in energy >> terms the desalination and pumping needed for the Sahara forest would >> use a bit more electricity than the world currently generates for >> every other purpose. This unavoidably sounds nutty. But that is at >> least in part because of the nuttiness of the situation, rather than >> its proposed solution — the nutty situation in which we burn fossil >> carbon at tens or hundreds of thousands of times the rate at which it >> is sequestered over geological time. If humanity insists on putting so >> much carbon dioxide into the air every year that it would take a brand >> new forest the size of the Sahara to suck it all up, then that’s where >> the madness starts. That creating such a forest would have to be a >> large undertaking — large in terms of the whole world economy — is >> just a consequence of the initial folly. >> >> And in practice the investment would be smaller. A nice thing about >> forests is that they can go some way to creating their own weather, >> and the authors have looked at this effect with some climate modelling >> work. If a forest with irrigation dampened soil is imposed on the >> Sahara, rain begins to fall, in some places as much as a metre of it >> every year. This rainfall doesn’t obviate the need for irrigation, >> because it is strongly seasonal — basically an extension of the West >> African monsoon of April to November. But it might significantly >> reduce the irrigation requirements. Maybe you could get away with just >> a terawatt… >> >> The Sahel, to the south of the Sahara, also gets damper in those >> enhanced and extended monsoon rains, which is definitely a plus, I’d >> guess, and the African Easterly Jet, a feature which is driven in >> large part by the temperature contrast between the desert and >> surrounding land, seems to more or less vanish. Since a large number >> of Atlantic hurricanes get their starts as kinks in the AEJ, that >> might be a pretty significant change, too. Beyond that, the rest of >> the world seems pretty much unaffected. In particular, the authors say >> that their models show no additional warming that might be laid at the >> door of the change of albedo which comes with replacing light desert >> with darker trees. (I think this fits with the 2007 Bala et al paper >> in PNAS, which suggested that warming associated with afforestation >> would be due to changes in boreal, rather than tropical, forest >> cover). >> >> There is, however, a fly in the ointment. The Bodélé depression in >> Northern Chad is only a small part of the Sahara, but it is the >> world’s greatest source of mineral dust, with the winds drawing some >> 700,000 tonnes a day off the surface. According to Koren et al in >> ERL, 2007 40 million tonnes of dust a year travels from the Bodélé to >> the Amazon rain forest, half the total annual mineral inputs into the >> forest basin (the dust fertilises the mid Atlantic, too, and it may >> play a role in abating hurricanes too — Jim Giles wrote a lovely piece >> on this for Nature some time back). There’s a real chance that this >> dust is crucial to maintaining the soil fertility of the forest, and >> even if the Bodélé itself were left unirrigated and unforested, the >> increase in precipitation all round it, and the wetter atmosphere >> downwind of it, would probably shut it down as a dust producer. If >> growing a forest in the Sahara hurts the one we already have in the >> Amazon it obviously becomes a less attractive proposition (though if >> we are going to lose the Amazon forest anyway, things might look >> different…). That said, if you are pumping trillions of tonnes of >> water across continental scales, then paying to air dump a few tens of >> millions of tonnes of fine-particle mineral fertiliser upwind of where >> you want it is hardly going to break the bank. >> >> Something the authors don’t look into is that the higher the CO2 level >> in the atmosphere gets, the easier this all becomes. Higher carbon >> dioxide levels make plants more water efficient, all other things >> being equal. All other things are not, necessarily, equal — higher CO2 >> also makes things hotter, which plants don’t much care for. In a world >> with some solar radiation management, though (such as aerosols in the >> stratosphere) all things might indeed be kept equal, or at least >> temperature might be. Martin Claussen has been working for some time >> on the idea that the Sahara is a “tipping element” in the climate >> regime, one that can be pushed from a dry state to a wetter one >> relatively easily. In a more carbon rich but not-too-hot world the >> circumstances might be right for it to tip the other way, and it might >> take rather less than a 50-Nile terraforming project to nudge it over. >> >> In the final analysis, I don’t think I take this paper very seriously >> as a practical proposition. Doubling global electricity generation for >> a single project seems far fetched. For such a thing to be put >> anywhere near the top of one’s list of African infrastructure >> investments would require that a great many other large and important >> development initiatives (provision of power, water, roads, cold >> chains, vastly improved agronomical advice, etc to the vast majority >> of the population, for starters) would already have had to have been >> put in place. But it’s kind of nice to imagine a world in which we >> were wealthy and together enough to have actually taken the pressing >> need for those changes to heart, and were thus in a position to >> consider greening a great desert too. >> >> And regardless of practicalities I think there’s real value in taking >> the analysis further. A big idea like this throws off many fascinating >> questions that force you to look at the earth, and what we know about >> it, in new ways (or old ways but with a new twist): >> >> What polycultures would you build the new forest with? (all-eucalyptus- >> all-the-time is fine for first calculations, but doesn’t sound like >> anyone’s idea of a proper landscape. Baobabs? Laurels? And what fauna >> might be good, or bad?) >> >> What genetic engineering — reduced flammability, higher albedo >> leaves, more refractory soil carbon, who knows what else — might help? >> >> How much bioenergy with carbon capture could be built into the scheme, >> perhaps initially to power some of the inland the pumping stations? >> >> Can biochar help? (and a million other soil-creation questions) >> >> What are the best silvicultural ways to make the new woodlands pay, as >> that is something people by and large like their environments to do, >> and can there be room for some agriculture too? >> >> How could local people best be convinced this was a good idea? And >> what are the property title reforms that would be prerequisite? >> >> If the AEJ stops, do hurricanes stop too? Or does some other mechanism >> initiate them, maybe somewhere else? And does the dust really have an >> effect? >> >> When the Sahara was wetter and less dusty in the past, did the Amazon >> actually suffer from lack of nutrients? (I think there is actually >> some research already out there on that — but can’t offhand think >> where) >> >> How can the transformation be made stunningly beautiful? >> >> What regions and landforms do you want to keep as monuments/heritage >> sites/national or world parks? There would undoubtedly be a real >> aesthetic/biodiversity loss in the removal of the desert, not to >> mention risks to some utterly wonderful buildings. >> >> How to stop the Fremen becoming soft and decadent now that Arrakis has >> become a land of milk and honey? >> >> and so on. >> >> In particular, it would be nice to see some analysis of halfway >> houses; where in the Sahel and points north might merely huge, as >> opposed to planet-sized, afforestation be attempted, and what would be >> the costs and benefits? It is possible to transform land on very large >> scales, if not quite this large: 40m hectares of the Brazilian cerrado >> have been brought into agricultural production over the past fifty >> years. Can afforestation/silvicultural interventions on such scales >> ever make sense? And where else might be suitable for such things? >> >> And on the topic of where else: My apologies to any Australian readers >> for not going into the paper’s analysis of foresting the Outback in >> addition, or as an alternative, to the Sahara. Basically the arguments >> are largely the same but the costs and effects are a bit smaller. >> There’s also a risk of interfering with El Nino that would definitely >> merit further attention. If anyone wants to blog more on that aspect >> of the subject send me a link and I’ll post it up here. >> >> >> On Sep 21, 5:37 pm, John Nissen <[email protected]> wrote: >> >>> http://www.popsci.com/environment/article/2009-09/scientists-concoct-2-trillion-year-plan-geoengineer-sahara-desertScientists >>> Concoct a $2-Trillion-Per-Year Plan To Geoengineer The Sahara Desert >>> >>> >> >> >> > >> >> > > -- > The University of Edinburgh is a charitable body, registered in > Scotland, with registration number SC005336. > > > > > > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en -~----------~----~----~----~------~----~------~--~---
