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Greening a desert. Would this be worth a try?


mistermack
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I've dreampt up a speculative scheme for greening a desert. Do you think it would work? (or be cost effective?)

You need a desert that extends right up to a significant sea or the ocean. Like lots of Australia, or Namibia etc. Australia would be the best to try it on. 

Imagine you set up a huge array of pumps along, for example, the Great Australian Bight. Connect the pumps to spray nozzles, and start pumping when there is an onshore wind. Adjust the jets for a spray that evaporates most of the water, before it falls back into the ocean. So basically, you are manufacturing water vapour, from sea water, with the salty residue falling back into the ocean. 

So with an onshore wind, all of that humid air is carried inland, where it rises on thermals, forms clouds and rain, greening the land. If the wind blows the other way, you switch it off. 

Obviously, it could only be done on a national level, so that the benfits could be taxed and provide financing for the running of the system. 

Besides providing land that could now be used for agriculture, it could raise the level and quality of groundwater, cool the climate with more cloud cover, and actually get rivers flowing and lakes filling. 

If it worked. Any thoughts?

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11 minutes ago, mistermack said:

I've dreampt up a speculative scheme for greening a desert. Do you think it would work? (or be cost effective?)

You need a desert that extends right up to a significant sea or the ocean. Like lots of Australia, or Namibia etc. Australia would be the best to try it on. 

Imagine you set up a huge array of pumps along, for example, the Great Australian Bight. Connect the pumps to spray nozzles, and start pumping when there is an onshore wind. Adjust the jets for a spray that evaporates most of the water, before it falls back into the ocean. So basically, you are manufacturing water vapour, from sea water, with the salty residue falling back into the ocean. 

So with an onshore wind, all of that humid air is carried inland, where it rises on thermals, forms clouds and rain, greening the land. If the wind blows the other way, you switch it off. 

Obviously, it could only be done on a national level, so that the benfits could be taxed and provide financing for the running of the system. 

Besides providing land that could now be used for agriculture, it could raise the level and quality of groundwater, cool the climate with more cloud cover, and actually get rivers flowing and lakes filling. 

If it worked. Any thoughts?

I think one difficulty would be that the salt would not drop back into the ocean. As each droplet progressively evaporated and shrank, the salt would become more and more concentrated, leading eventually to tiny particles of salt that would be carried inland by the breeze for long distances. I would propose instead to combine your idea with solar stills to generate fresh water for your pumps, returning brine to the ocean.

In such hot deserts, one almost invariably gets a daily onshore breeze after about 11am, due to the rising of hot air over over the desert drawing in cooler air from above the sea. This normally persists until just before sunset. So it would be a fairly reliable process, I think.  But I have no idea whether enough evaporation could be achieved to change the climate this way. I think I read some years ago that another way is simply to use the fresh water for irrigation and promote the growth of plants which, by their transpiration, start to alter the climate that way - and generate moisture-retentive soil, by the detritus on the ground that they create. 

Edited by exchemist
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1 minute ago, exchemist said:

I think one difficulty would be that the salt would not drop back into the ocean. As each droplet progressively evaporated and shrank, the salt would become more and more concentrated, leading eventually to tiny particles of salt that would be carried inland by the breeze for long distances. I would propose instead to combine your idea with solar stills to generate fresh water for your pumps, returning brine to the ocean.

I did think of that, that's why you would have to adjust the spray and pressure so that the drops don't fully dry out. It would be a bit less efficient, but you couldn't allow salt dust to blow inland. You could possibly site the spray a bit of distance from the coast to ensure that no salt makes landfall. A problem might be that the salty brine might produce local ecology problems in the ocean, but I doubt that they would be widespread, as the tides and currents would soon disperse it.

You could even power the pumps with wind generators, the daytime onshore wind would be available at the same time as the demand.

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This company sounds interesting: https://www.eurekalert.org/news-releases/666725

Quote

 

A PROCESS that turns seawater into fresh water at around a third of the cost of conventional desalination is promising a new way of providing clean drinking water, claims one US company.

Called rapid spray evaporation (RSE), it is being developed by AquaSonics International, based in Atlanta, Georgia. The company has produced portable units capable of converting up to 11,000 litres of water a day and is now scaling up the technology for much larger desalination plants.

 

 

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You might want to consider unforeseen consequences. Depending on the scale of the operation - say, the Sahara - you might risk turning the Mediterranean into another Dead sea.

https://www.thoughtco.com/salinity-definition-2291679.

Quote

Salinity levels can affect the movement of ocean currents. They can also affect marine life, which may need to regulate its intake of saltwater.

But here's an alternative:

Quote

Sea water over the Sahara desert will evaporate due to the high temperatures. However, pumping sea water and releasing it underground within the Sahara desert may counteract global rising sea levels and promote vegetation amenable to high temperatures that may in turn reduce warm air streams that would otherwise result in eastern super storms or hurricanes. https://www.climatecolab.org/contests/2014/adaptation-to-climate-change/phase/1300633/proposal/1305906

 

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15 minutes ago, Phi for All said:

This company sounds interesting: https://www.eurekalert.org/news-releases/666725

Thanks, that's an interesting link. I have a couple of reservations though. They are quoting a situation where you have large quantities of waste heat going spare, like the reverse osmosis plant, and that's how they are quoting a cheap price of a third of established systems. 

In other systems, using hot air would be quite energy intensive I would think. But even if it's just a bit more competitive, it has a big and growing market to aim at. 

Because of the energy requirement, I doubt that it would be economic at climate level in the way I was aiming at. But the principle of spray evaporation is clearly a goer. I was inspired by the huge decorative fountains that are installed in various locations, but in reality, all you need is a spray that evaporates a good proportion of the feed water. It could be just a metre high, or whatever gives the most evaporation for the least input power. 

Maybe you could spray a fine layer of water onto a black surface, and let the sun and wind dry it off.

7 minutes ago, Peterkin said:

You might want to consider unforeseen consequences. Depending on the scale of the operation - say, the Sahara - you might risk turning the Mediterranean into another Dead sea.

I think even if that was a factor, it would take millions of years. 

 

8 minutes ago, Peterkin said:

But here's an alternative:

I don't think that's been thought through. Seawater would ruin the Sahara underground water deposits, and it wouldn't promote growth, it would kill vegetation.

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The reason that I thought that this might be feasible, it that it uses free energy from the environment to break the water/salt bonds, and to condense the evaporated vapour. The energy input of the pump is just a method of increasing the surface area of the water. The molecules of the dry air would be knocking the salt/water bonds apart, just like they do on the surface of the sea. It might be that having tried it, they would find that it would take too much energy to atomise the water, to make it an economic prospect for a country.

But if it did work, the economic benefits for a country like Australia would be enormous. Australia has such vast tracts of land, nice and flat, ready made for productive agriculture, and all it's lacking is water. Any of this new rainfall would be ultimately available for use, because if it fell on open land, it would either evaporate and fall again as rain, or soak down to the ground water level, and improve the level and quality of that. The ground water across much of Australia is already quite salty, and getting more salty with use.

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I am a bit skeptical of the promise of large scale weather modification. I think the reason that central Australia is desert is a persistent lack of on-shore winds capable of carrying moist air into the interior, not lack of evaporation over oceans when we get them. There is a preponderance of high pressure systems due to Hadley cells, that drop dry upper level air down over desert areas, with prevailing low level winds blowing coast-wards and/or blocking on-shore winds as a result. I think that is the case for the Sahara and other desert regions too. Mountain ranges and their rain shadows can be a big factor for some deserts too.

Greening desert fringes can have an impact but I suspect it works because human activities - introducing grazing livestock, including gone feral (goats, donkeys, camels and rabbits in Australia) - is why they lost their vegetation; in combination with managing those pest species it is possible to bring back vegetation. Whilst vegetation can increase local precipitation it isn't a huge effect; these regions are still highly dependent on occasional rainfall events. There can be - at least over the medium term - some regional greening from changed weather patterns; North West Australia is getting more rainfall out of those rainfall events when they occur - not necessarily more of them or regularly. How that plays out with much raised temperatures is still a question. Other regions of Australia - agriculturally productive ones - are getting less rainfall.

I am seriously concerned about the longer term prospects with global warming; 1 C of global average warming is making about 1.4 C of warming on the ground in Australia, so 3 C (which we are on track for with us reaching zero net emissions from a lot more serious commitment than we are seeing) could mean temperatures rise over 4 C. I think we will reduce our emissions a lot but not enough reach zero within the timeframe needed, so 4 C or 5 globally averaged, with temperatures on the ground raised 5 - 6 C seem not just possible but likely; our greening efforts are going to be in big trouble, along with the health of economies capable of undertaking them. That is still assuming reality is nearer the mid-range for climate sensitivity; if it turns out higher it can be worse again.

Arid zone plants and animals are only tough in comparison to those in milder conditions; in many cases their survival prospects have been borderline all along - surviving but only just is the rule.

Edited by Ken Fabian
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11 hours ago, Ken Fabian said:

I am a bit skeptical of the promise of large scale weather modification. I think the reason that central Australia is desert is a persistent lack of on-shore winds capable of carrying moist air into the interior, not lack of evaporation over oceans when we get them. There is a preponderance of high pressure systems due to Hadley cells, that drop dry upper level air down over desert areas, with prevailing low level winds blowing coast-wards and/or blocking on-shore winds as a result. I think that is the case for the Sahara and other desert regions too. Mountain ranges and their rain shadows can be a big factor for some deserts too.

Thanks. You would obviously do some pretty intensive surveys of the onshore winds situation before siting anything like this. I would imagine that a country like Australia would already have all of the wind info in it's Meterological department going back years, so you would just have to analyse the existing data to see if it was suitable. Other countries might have less detailed info, but it wouldn't be hard to get a rough idea of whether an installation was a non-starter. 

I know that some deserts do have regular onshore winds, because they have animals that rely on a fog that rolls in every morning, rather than rainfall. That indicates that as you say, there is humidity in the air, but when the sun gets strong, the dew point rises, so the fog droplets evaporate again. 

If as you say, there is dry air coming in at altitude and dropping down on desert areas, then that would probably mix with and reduce the humidity of any existing moist air, dropping the dew point and preventing it from forming clouds. But the dew point depends on humidity, so if you have raised the humidity artificially, the balance might be shifted enough to allow cloud formation, where there was none previously. 

If you were contemplating giving this a go, you would need to model the results before you started, and have a trial to establish how much you can raise humidity using various spray methods. 

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Correction above, the temperature rises above the dew point, not what I wrote. 

I found this about Australia in Encyclopedia Britannica, there's a marked difference between winter and summer, but there appears to be planty of onshore wind in the summer, in the north. They do say though, that the humid layer is often shallow, and so doesn't hold enough humidity to provide much rain. 

1093548964_OzMonsoons.JPG.759c4978ba0b0da0688e5d501e6ccdb7.JPG

https://www.britannica.com/science/monsoon

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I wonder if you could use a version of this as energy storage? 

Site the pumps where there is a good record of onshore winds, and where there is a good sized mountain range inland. So the vapour you create gets forced upwards by the mountain, and is forced to fall as rain. You can then build a series of dams, and store the water for hydro-electric generation.  

So in a desert spot, you would be creating fresh water, and harvesting electrical energy all in one go. So the potential would be there for the system to power itself. 

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Australia is seeing increased rainfall for the North-West but reduced rainfall for most of the rest, especially South, which looks related to changes to High pressure systems from Hadley Cells, ie they are tracking further from the Equator (ie South) due to global warming. Cattle grazing is likely to benefit in the near term but it is mostly not well placed for intensive agriculture. Not so clear about the longer term; it gets very hot there even without extra warming (Australia's hottest place is there) and extreme heatwaves could undo the benefits.

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Soils should ideally  be covered to make use of any rainfall. Soil microbial systems and health depend on that mulch to modify soil heat and humidity . Uncovered soils have poor ability to retain moisture. The microbial systems are leached and sterile for the most part, with pioneering species often needed to start the processes for successive growth.

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12 hours ago, naitche said:

Soils should ideally  be covered to make use of any rainfall. Soil microbial systems and health depend on that mulch to modify soil heat and humidity . Uncovered soils have poor ability to retain moisture. The microbial systems are leached and sterile for the most part, with pioneering species often needed to start the processes for successive growth.

Agreed. There are some really good items on youtube about re-greening semi desert by ensuring that the soil remains covered, and the results look pretty dramatic. It might be that grazing goats by humans is responsible for the spread of a lot of desert. Besides eating every green thing, they will also eat anything that ever WAS once green, and you end up with bare soil. 

On 8/22/2022 at 4:30 AM, Ken Fabian said:

Australia is seeing increased rainfall for the North-West but reduced rainfall for most of the rest, especially South, which looks related to changes to High pressure systems from Hadley Cells, ie they are tracking further from the Equator (ie South) due to global warming. Cattle grazing is likely to benefit in the near term but it is mostly not well placed for intensive agriculture. Not so clear about the longer term; it gets very hot there even without extra warming (Australia's hottest place is there) and extreme heatwaves could undo the benefits.

I do take the point of the Hadley Cells. But exploring it further, if Hadley cells are dropping dry air on Australia inland, they are almost certainly doing the same onto the ocean around the continent as well. So onshore winds when they do blow will not be very humid which might be a big factor in the dryness of the continent. If that's the case, then humidifying those winds artificially, when and where they do blow, could make a substantial difference. 

It would have to take some intensive modelling at University level, to work out if it was a viable strategy, and I'm pretty sure that it would never happen anyway. Although, climate modelling has instigated some gigantic spending lately, without any certainty of the benefits, so maybe it's not a totally lost cause. 

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On 8/16/2022 at 3:55 PM, mistermack said:

You need a desert that extends right up to a significant sea or the ocean. Like lots of Australia, or Namibia etc. Australia would be the best to try it on. 

Both low lying areas subject to offshore cold water currents/upwellings and predominantly dry trade winds? They are deserts for a reason. Could add the Atacama to this list. (Ekman transport can be an important mechanism in these cases)

On 8/16/2022 at 3:55 PM, mistermack said:

Imagine you set up a huge array of pumps along, for example, the Great Australian Bight. Connect the pumps to spray nozzles, and start pumping when there is an onshore wind. Adjust the jets for a spray that evaporates most of the water, before it falls back into the ocean. So basically, you are manufacturing water vapour, from sea water, with the salty residue falling back into the ocean. 

So with an onshore wind, all of that humid air is carried inland, where it rises on thermals, forms clouds and rain, greening the land. If the wind blows the other way, you switch it off. 

The lower few hundred feet of steady onshore winds are typically in approximate thermal equilibrium with the ocean, aren't they? So where is the energy to come from to evaporate the water? Even if the air is at lower relative humidity (such as a descending Hadley cell), evaporation is going to chill it further. This doesn't sound like a good recipe for creating a rising thermal. More a recipe for fog.

On 8/16/2022 at 3:55 PM, mistermack said:

If it worked. Any thoughts?

If it worked, nature would already be doing it, I think. As it does here in southern Nigeria during the rainy season. But come november the rainbelt will have moved south to Angola, the Hadley cells will shift what little air movement there is to a north-easterly flow, and it will be both cool (for us) and bone dry. Trying to 'make' it rain here at the turn of the year would be an exercise in futility. Everything would be working against you. 

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The clue is in the title, that this is a speculative idea. So thanks for your thoughts. 

17 minutes ago, sethoflagos said:

The lower few hundred feet of steady onshore winds are typically in approximate thermal equilibrium with the ocean, aren't they? So where is the energy to come from to evaporate the water? Even if the air is at lower relative humidity (such as a descending Hadley cell), evaporation is going to chill it further. This doesn't sound like a good recipe for creating a rising thermal. More a recipe for fog.

I don't think you've quite got the picture. You site powerful pumps along the coast, with spray heads some distance from shore. On days when there is a significant onshore wind, you turn on the pumps. The spray heads produce a fine mist of sea water, sprayed upwards into the air. Because the droplets are so small, most of it evaporates before it falls back into the ocean. Evaporation DOES cool the air, but as the wind is constantly blowing, the cooled air moves inland, carrying with it the water vapour that you have produced. Once inland, rising thermals turn it into clouds. The clouds reflect solar energy, producing some cooling. And produce rain, affecting the climate. 

That's my off-the-cuff idea for humidifying onshore winds. There may be better ways, or it might be a waste of energy, depending on the outcomes. The sad thing is that there's no way to test it in a small-scale trial. Not one that I can think of anyway. Modelling might provide an answer as to whether it would be viable, or a waste of time.

It sounds like an unlikely thing to try, but on the other hand, if it worked, the long term gains would be gigantic.  

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4 minutes ago, mistermack said:

It sounds like an unlikely thing to try, but on the other hand, if it worked, the long term gains would be gigantic.  

Yes, if it worked.

 

Here is a simple (and very cheap) experiment to test it.

Even in the UK we have rivers where there are weirs with substantial falls.
Nothing like the Victoria or Niagara falls but enough for must test experiment.

Where there is a reasonable flow of water tumbling down a few tens of feet there is usually to be found a substantial fine upward rebound spray zone.
So take a humidity meter and measure how quickly the humidity drops off to ambient away from this zone.
This would provide a crude assessment of how much and how far the 'climate' is affected by such a spray zone.
River gauging information would give an idea of how much water you would need to pump.

 

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We already use sprays as humidifier in buildings. You can go online and buy them, and they will probably provide figures on power consumption and humidifying performance. 

They aren't designed for large scale, though, or for sea water or for use in a wind. But it is a starting point. 

You could use balloons to track the winds on site, to find out where your humidity would end up.

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23 minutes ago, mistermack said:

I don't think you've quite got the picture. You site powerful pumps along the coast, with spray heads some distance from shore. On days when there is a significant onshore wind, you turn on the pumps.

I do get the picture. 

1) If there is a significant onshore wind, it is already humid and will cause rain inland. There is no problem to address.

2) You cannot significantly increase the humidity of this wind without increasing its temperature.

3) The evaporation of brine requires even more heat input.

4) I've not even mentioned the astronomical pumping costs.

This quote from https://en.wikipedia.org/wiki/Namib gives an idea of the practical realities you are trying to reverse:

Quote

The Namib's aridity is caused by the descent of dry air of the Hadley Cell, cooled by the cold Benguela current along the coast. It has less than 10 mm (0.39 in) of rain annually and is almost completely barren. Besides rain being scarce, it is also unpredictable. The western Namib gets less rain (5 mm) than the eastern Namib (85 mm). This is due to several factors. Winds coming from the Indian Ocean lose part of their humidity when passing the Drakensberg mountains, and are essentially dry when they reach the Namib Escarpment at the eastern end of the desert. Winds coming from the Atlantic Ocean, on the other hand, are pressed down by hot air from the east; their humidity thus forms clouds and fog.

 ie The 'significant onshore wind' and 'rising thermals' simply don't happen. If they did, it wouldn't be a desert.

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3 hours ago, sethoflagos said:

1) If there is a significant onshore wind, it is already humid and will cause rain inland. There is no problem to address.

Not if the Hadley cell has dropped dry air on the ocean. As I mentioned earlier in this thread.

 

3 hours ago, sethoflagos said:

2) You cannot significantly increase the humidity of this wind without increasing its temperature.

You can buy a humidifier for a building. They don't have to use heat. There are many different types. Two of them are spray humidifiers, and impellers. The impeller type has a rotating disk which flings the water against a diffuser, breaking it up into tiny droplets. So your above statement is wrong.

 

3 hours ago, sethoflagos said:

4) I've not even mentioned the astronomical pumping costs.

Nobody said pumping was free. How much is astronomical? 

3 hours ago, sethoflagos said:

The 'significant onshore wind' and 'rising thermals' simply don't happen. If they did, it wouldn't be a desert.

That's just stating the obvious. If there was no significant onshore wind, you wouldn't embark on a project in the first place. My suggestion was for siting an installation where there WAS a reliable onshore wind. 

If there was an onshore wind for a significant period, then an installation would be turned off whenever there was no wind. That's obvious. You would only be using energy when the wind was significant and onshore.

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3 hours ago, mistermack said:

Not if the Hadley cell has dropped dry air on the ocean. As I mentioned earlier in this thread.

What your proposal seeks to do is increase the contact area between brine and the lower couple of hundred feet of the atmosphere in order for it to approach equilibrium at 100% RH more rapidly. My point is that nature already does this quite effectively. The missing step is getting heat into the system. This is required to both increase the water holding capacity of the airstream and reduce its density sufficiently to allow it to rise. 

The amount of heat the sun is putting into the ocean to try and do this is measured in Terawatts. And in the cases we have mentioned that isn't enough to overcome the local conditions which are dominated by cold ocean currents. How many TW do you propose adding to this equation? 

4 hours ago, mistermack said:

You can buy a humidifier for a building. They don't have to use heat. There are many different types. Two of them are spray humidifiers, and impellers. The impeller type has a rotating disk which flings the water against a diffuser, breaking it up into tiny droplets. So your above statement is wrong.

Have you ever heard the phrase 'latent heat of vapourisation'? Have you any idea what it means?

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22 hours ago, sethoflagos said:

Have you ever heard the phrase 'latent heat of vapourisation'? Have you any idea what it means?

I don't find your responses particularly informed or useful. Others have posted cogent posts, but yours are becoming rude, and certainly not worth reading. Byeeee.

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  • 2 weeks later...
On 8/28/2022 at 11:18 PM, mistermack said:

Agreed. There are some really good items on youtube about re-greening semi desert by ensuring that the soil remains covered, and the results look pretty dramatic. It might be that grazing goats by humans is responsible for the spread of a lot of desert. Besides eating every green thing, they will also eat anything that ever WAS once green, and you end up with bare soil. 

 

Definitely the case, though goats do have a place in managed systems. Some land care groups are actively teaching system management  to subsistence farmers as part of their work. Australia has used heavy stocking rates of goats to clear black berries ( an introduced noxious weed here) and tangled undergrowth from open forest surrounding a  dam, larger than our Sydney harbor, preventing chemical run off into the system.

Goats are browsers, so used in open, mature forests they can be used to control woody weeds and undergrowth while trampling whats not eaten to cover the soil while waiting for grasses and regrowth. Mature trees are rarely touched while there is sufficient accessible brush. Most of the damage caused by goats is when brushy land cover is already depleted and or stocking rates are too dense. 

Some regenerative farmers will rotate goats on select parcels seasonally or confine goats to smaller lots for hand feeding excess foliage and trimmings when open forests need resting. 

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