Can we turn deserts green?

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1:03I'll start with the pouring I guess. Yeah that is some salty water. Give it a little mix. Now we pour this salty water into the kettle. Okay. Okay. So that's salty water. And now I turn this kettle on. now we just wait for it to boil this is CrowdScience the show where we boil questions in the kettle of curiosity I'm Alex Lathbridge so you're probably wondering why I'm boiling a kettle of salt water making this BBC studio smell like a dock well it's because of a question from listener Youssef yeah my question is can we re-green the desert so basically trying to put water back in the deserts the reason for my question is the climate change how it's affecting us and also with the sea levels going up and up maybe it could be a solution to actually divert the water into the deserts wow i hadn't even thought about that by doing it you're lowering sea levels there are so many parts to this okay if we manage to bring water into the desert, perhaps re-green parts of it.

2:23Would you want to move there? Would you set up a little plot? Absolutely. Okay, where? Which desert would you go to? I've never actually been to the desert. I mean, although I came from Algeria originally, I'm from Algiers, which is the north of the country, and I've never been down there, but all the people who have been there, they just, how beautiful it was down there. You know, all the dunes, the mountains and, you know, the solitude.

3:00So Yusuf wants to know if we can bring water to the deserts of the world, despite not actually having been to one. But what actually is a desert? You might be thinking of the huge sand dunes and desolate landscapes of the Sahara or Gobi, places that receive less than 250 millimetres of water per year. But deserts exist on a wide spectrum with varying levels of plant, animal and human life. This makes it sort of difficult to define, but generally speaking, a desert is an extremely arid environment where the limited amount of water makes it difficult for most life to thrive. And deserts are growing.

3:46Desertification is a process where fertile land declines over time. According to the United Nations, roughly one million square kilometres of healthy land is degraded every year. Desert is a natural phenomenon on the earth. If the desert is stable, it is fine. We can live where it is. Desertification means a natural land like a grassland or shrubland somehow becomes drier and drier and then converted to desert. This is Yan Li, a professor of geographical sciences at Beijing Nongu University in China. He started thinking about what might bring more water to the desert by focusing on what mechanisms take that water away in the first place, the very conditions that lead to desertification.

4:36concepts that were first understood in the 1970s by a scientist named Jules Chani. Chani, he observed that desertification in Sahara is becoming worse. Chani found that human activities play a significant role in desertification. When you have too many livestock, those animals eat all the grass, so the grass is degraded to bare ground or to sand. And when this happens, the albedo of the desert surface changes. So albedo means the reflectivity of the surface. If the albedo increases, that means more solar radiation is reflected back to the space. Something with high albedo is lighter in colour, or even white, and reflects sunlight.

5:26Think snow. A low albedo surface would be darker and could soak up more heat. Think like pavement on a hot summer day. Then Charley found that with the increased albedo, they will inhibit cloud formation and precipitation. When the albedo is higher, less heat is absorbed by the ground. This reduces evaporation into the atmosphere, which reduces cloud formation and therefore leads to less rainfall. What if we do it oppositely? What if we can decrease surface albedo? Will that increase precipitation? So then the solar panel is a perfect candidate. So Yan and his team ran a simulation, a model where 20 % of the land across the entire Sahara Desert was covered in solar panels.

6:16Because the solar panels, they have a dark surface, it means they have lower albedo or lower reflectivity. But they didn't stop there. They also ran a version of the simulation where they covered the Sahara with both solar panels and wind turbines. But why add the wind? If we have wind turbines installed on the surface, it will change surface roughness. You can understand roughness as surface friction. So with more friction, the more energy could be transferred into the atmosphere through turbulence. And the turbulence creates weather and creates those clouds. So by making the surface of the Earth more rough, it will help send that hot air with more moisture up into the sky, creating clouds and eventually rain.

7:04If we change the albedo of the Sahara, as well as the surface friction, thereby creating more clouds, how much more rainfall could this bring in? The average precipitation of the entire Sahara desert will be doubled if we have such massive solar and wind farms there. Through these simulations, Yan and his team modelled what would happen in the Sahara, but might it work elsewhere? Would this same phenomenon happen in the Gobi, in Mongolia and China, or in deserts in Saudi Arabia? It seems only the Sahara desert, we can have this significant precipitate increase. So we think that there are some reasons to explain this.

7:48First, the Sahara Desert has the highest surface albedo of all this desert. So that means if we put dark solar panels on Sahara, it will lead to the largest surface albedo decrease compared to other deserts. Another reason is because the Sahara Desert, it is close to the ocean. So the ocean moisture can easily go into the Sahara Desert. But for the Gobi Desert and the desert in the Middle East, they are far away from the ocean. So even you create more warm air or increase the convection activities there, it will not have a huge impact on the clouds and precipitation. Essentially, you can't make water out of nothing.

8:37But even if you're on the coast, you've got to find a way of bringing that water in. And here is where Yan and his team came up with another way to change the albedo of the desert. Vegetation actually played a very important role or essential role for the results we found. When the Sahara deserts, if they have more rainfall, the vegetation will recover. And with this recovered vegetation, the vegetation can further decrease surface albedo because the vegetation, they cover the bare ground and the bare desert, decrease the albedo. With the additional vegetation, it will also make the surface rougher or more friction.

9:21It's become a positive feedback loop. For water to come to the desert, you really need vegetation to keep the ground as rough and unreflective as possible. But to keep vegetation alive, you need water. So it might seem like this chicken and egg situation, but it points to a key concept to changing water that's available in the desert, the water cycle. You may have heard the water cycle because the water is cycling or recycling in the earth system. That means if we have surface water coming out from the vegetation or from the soil, those water vapour will go into the atmosphere and it travels through the winds.

10:04And at ideal condition, those moisture will fall into the ground again as precipitation. So every part of the world is connected through this moisture recycling process. So larger scale renewable solar and wind farms could actually change the weather itself, altering the albedo and increasing the winds, leading to more rain and vegetation, which leads to more rain and vegetation in a positive feedback loop. But here's the thing, right now it's just a simulation. And the amount of solar panels and wind turbines would be enormous, covering about 20 % of the Sahara. But for deserts where there are already clouds, is there a way of getting the water out exactly where it's needed?

10:57In what's often called the driest place on Earth, the Atacama Desert, there's a technology that's been pulling water out of the air. The technology of fog harvesting, it was created in Chile around 50 years ago. The technology is very simple. The idea of this technology is to harvest water from the clouds in the desert. This is Virginia Carter, a geographer and assistant professor at the Universidad Mayor in Chile. She's also an expert in building fog collection systems. Fog harvesting is exactly as it sounds. In deserts across the world where clouds are present, it's a way of pulling water out of the sky and onto the ground.

11:43But how do you catch a cloud? First you need the cloud, then you need a mountain which is facing this cloud, then you need wind and then you will find these collectors, so two posts and the mesh being supported. And then many or million little drops are gathered together and then are going down by gravity. So then you can use a pipeline and then you can harvest the water. Usually we use water tanks in the middle of the mountain. So that is the way. Literally the mesh is harvesting the water from the clouds. Here's the thing, to catch a cloud you've got to make sure that you're not too high and not too low.

12:31That sweet spot is known as the zone of interception. This is the altitude of which fog harvesters can collect the most water. So once you've placed your collector in a decent spot, how much water can it actually harvest? In the very north of Chile, the average is two litres by one square metre. Then you can have other places with seven, seven litres by one square metre per day. So if you want to collect, for instance, 200 litres per day, you can choose different sizes of collectors. And also you can choose how many collectors. Our listener is really interested in how we can re-green the desert.

13:19So could fog harvesting help with that, help with this sort of agricultural irrigation that could make it easier to grow plants in these areas? Yes. In fact, we are conducting a project in another place of the Atacama Desert. We are cultivating hydroponics using fog water. So if you are thinking in future strategies for Atacama or other places, of course it's possible. Also, we are thinking to promote the use of fog water for the regeneration of ecosystems. So, yes, you can use it for population, for irrigation, for cultivation, etc. How does the water taste? It's sweet for me, you know, it's delicious.

14:12I started with this when I was 22. I was studying geography. And I remember how happy I was when I drank my first glass of water. now it's amazing to see the communities catching this water because they don't have any other waters. While fog harvesting is incredibly promising there are a couple of drawbacks. The amount of water you can harvest is a lot smaller compared to other techniques and you need to be in a place that has fog which is usually near the coast. But there's one technique that could bring a lot more water to dry regions that are far from the coast. We'll be finding out about that next.

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15:59You're listening to CrowdScience from the BBC World Service. I'm Alex Lathbridge, and this week we're answering a question from listener Yusuf, who wants to know if we can make the deserts green. So far, we've found out how solar panels and wind turbines could help change the weather in the desert into something more hospitable for agriculture. and even though we can harvest fog, it's probably not enough to achieve listener Youssef's vision of re-greening the desert. So what if we could use seawater? Well, greening the deserts is a, I have to say, is an ambitious objective but certainly we can green parts of the desert if we can provide the water that's sadly missing and that's where desalination comes in.

16:43This is Chris Sansom, a professor at the University of Derby in the UK. His research looks at how we can use the sun to desalinate water. Desalination is a process that's commonly used around the globe to remove salt and other minerals from seawater to create fresh water. This is important because salty water isn't very good for making soil that you can grow things in. The way that most of the large desalination plants in the world work at the moment, they work by forcing seawater through membranes that trap the salt and the water goes through the membranes. Sounds simple. It's quite complicated.

17:23It involves huge amounts of pressure, huge pumps to pump the seawater through the membranes. And that's called reverse osmosis, RO. And that's the way that most of those desalination plants in the Middle East, that's how they work. There are something like 30 of those plants in Saudi Arabia today, but each one uses a huge amount of energy. They use something like 10 ,000 barrels of crude oil each day for each plant. Or you can go down the route of thermal desalination. Basically, it's like putting salty water in a kettle and boiling it up.

18:05OK, so water has boiled. You then collect that steam in a cold chamber where it condenses out. We caught some of the steam that came out in a little bowl, leaving you something that's close enough to pure water. This should be fresh water with no salt. I'm going to give it a little try. That is actually very clean. There is no salt in that. And I think I actually might leave the kettle here because the next person to use it, they make a cup of tea. That's going to be a really surprising cup of tea. The desalination units that Chris is developing rely on energy from the sun to boil water rather than using fossil fuels.

18:45We're looking to develop a small desalination system that could be used in refugee camps, disaster zones, where basically you need drinking water very quickly. So in that case you can make it using plastic lenses that focus sunlight onto a pipe through which runs a fluid which can then heat up and then can evaporate brackish water or seawater to give you h2o basically scaling up then you start to look at very very large concentrating mirrors in the deserts to concentrate a lot of sunlight onto very large pipes which can then evaporate a lot of seawater and condense a lot of steam to give you very large volumes so how much water are we talking here from the sun in a hot country we get about one kilowatt per square metre.

19:42A kilowatt hour would give you about two litres of water an hour. For the larger units, you can just keep scaling up. The more mirrors you have, you can just keep adding those areas of mirrors, more pipes, more seawater going in. You can scale that up to any size you like. So really, the sky is the limit as far as that's concerned. And so would this be enough? Would it provide enough water to help re-green desert areas? Certainly. Assuming that you have the source of seawater, so you're either by the sea or you're piping the seawater to where you need it, yes. The world already desalinates approximately 34.8 billion cubic metres of water per year.

20:29And those needs are going up. I think to simply to keep pace with population rise and climate change, that probably has to double or treble by 2050. And for your questions about greening the desert, then many times that, maybe 10 times that. And so these desalination plants are taking salt out of seawater. What happens to the salt? If you fly over desalination works, you will find very often large areas of salt that you will see quite clearly because it's white on the ground in the deserts, just dumped basically. Yeah, it is a byproduct of the process and at the volumes in which that salt is produced, there is nothing else you can do apart from just leave it there on the surface or bury it.

21:22While desalination is incredibly powerful, the way that it's done today with reverse osmosis is very energy intensive, often relying on fossil fuels. And the solar powered units are still in development. And no matter how you do it, you're still left with huge amounts of leftover salt, which can damage the environment around these plants.

21:50But all these methods to produce fresh water, they all have their trade-offs, right? So desalination, one of them is this production of really salty water that you've got to somehow get rid of. This is Alan Condron. He studies climate change and more specifically, how melting ice affects ocean circulation. I'm a research professor at the Woods Hole Oceanographic Institution, which is in the United States of America. So you desal them and you're just left with a bunch of salt. Right, you know you are. Who's got space for salt? No, it's a huge problem in the Middle East is that you end up injecting sort of really brackish, very salty water back into the ocean.

22:29And that's actually a pollution in itself for the marine ecosystems. You can't tolerate really high salt levels coming back out of these desal plants. Now, Yusuf wanted to know if bringing seawater to the desert could help reduce the impact of rising sea levels. So who better to ask than an ocean climate expert? So the rate of sea level rise is going up. We know it's accelerating. But on average, since about 2018 to now, about 3.7 millimetres a year. It's something like one sort of trillion cubic metres of water a year. All right, a little bit of quick maths. 34.8 billion cubic metres of water is desalinated each year.

23:15That's only about 3 % of annual sea level rise. At the risk of people hundreds of years in the future calling me a fool, it's looking like it would be nearly impossible to scale up desalination to keep pace with increasing sea levels. So, sorry Yusuf, it's looking like at least that part of the equation doesn't quite work out. But Alan is no stranger to trying out unconventional ideas. I developed a model that stimulates icebergs and how they melt and how they drift in the ocean, being pushed by the ocean currents and by the winds. So then we took this model and I said, OK, well, let's see if we can use it to tow icebergs to locations that have water shortages.

24:03is. Alan then tested two scenarios. One towing Antarctic icebergs to Cape Town in South Africa and then to the UAE. So I mean we're talking about icebergs that are getting close to a kilometre in size and if you want to move something like that you're going to need like 10 really high performance tugboats to move that. So our closest tow was from Antarctica to Cape Town. That's 2 ,500 kilometers the sort of speeds that we were sort of looking at or guessing that these icebergs could be moved at which is around about close to a knot you know you're looking at a tow time of somewhere between like 50 to 100 days and then if you go even further and you say you wanted to move it to Saudi Arabia or the UAE then it gets kind of mind-bogglingly you know long transit times of say 200 to 400 days of pulling this iceberg and i'm no expert on the sea and the temperatures but i'm pretty sure antarctica very very cold and as you get closer to perhaps continents the water may be a little bit warmer and the ice that you're towing might melt yeah that is a problem especially um so for our uae ito you're looking at pulling an iceberg through the indian ocean for quite a few months, you know, you're looking at sort of 25 degrees C plus water temperatures, that's going to take a huge toll on the iceberg as it melts.

25:34The route to Cape Town, our shorter route, as you start to move northwards from Antarctica, the water actually stays cold for quite a long time. It's not until sort of the last month or so of the iceberg to Cape Town that you start to move away from the sort of protection of the Antarctic waters and you start to pick up more sort of subtropical Cape Town waters. So what happens with the iceberg once it gets to somewhere, once it gets to port, what do you do with it then? That is the sort of million dollar question, right? As you get quite close to your destination, you know, the ocean gets shallower.

26:09So if you've got this really, really big iceberg, in some locations, quite a few actually, the bottom and the iceberg will then hit the seafloor. It might sort of be stuck 50, 100 miles offshore or something. And so how do you do that? I'm gripped. So that's when I think you're starting to touch into the realms of science fiction a little bit. One of the ways put forward is if your iceberg's very, very big, you can essentially put machinery on it and drill into it. Similar to like open cast mining, where you would kind of go down into the iceberg in a sort of circular way, sort of digging, sort of burrowing in to the middle of the iceberg and sort of extracting the ice as you do that.

26:51The other idea? Drop the iceberg off in a dry dock, drain the seawater out, let the fresh water melt and then collect it. Ta-da! Usable water. You know that example, your iceberg needs to be a lot smaller though, right? Because you're thinking about something that's maybe, you know, the same size as like a cruise ship sort of size vessel or something like that. So the model didn't look at a couple of key considerations, mainly the cost and the fuel involved in towing an iceberg across the world. Not to mention you've got melting icebergs that could collide with ships, they could get grounded when they get close to shore, and what do you actually do with the iceberg once you get it there?

27:34But I still wanted to know if Alan thought that this might be possible one day. It always seems like there's somebody just about to actually go and do an actual tote. It never seems to quite materialise. You know, yes, you know, I have a model that simulates icebergs and how they melt in the ocean. But, you know, like, it's just a model, right? Every model needs testing and is proving and validating.

28:07so we've heard some really interesting ways of getting water to the desert desalination fog harvesting or even changing the weather itself but if we want to re-green the desert we need to understand what kind of green it should be in other words we need a plant scientist to understand what kinds of plants could survive in such an environment. My name is Zinnia Aide Gonzalez-Garranza. I have been working in the University of Nottingham for the last 25 years. I would say instead of trying to green the desert or to bring water to the desert, I think we need to protect the life that is there. Zinnia has studied many desert plants, including the mesquite tree, native to Mexico and the southwest of the USA.

28:59Mesquite is a tree that is native from the Americas and is very well adapted to the semi-arid zones of the Sonoran Desert and all the deserts in Latin America generally. Mesquite is a tree that can actually help other plants to grow because when it sheds its leaves, those leaves decay and make richer soil than the typically sandy or rocky desert has and that provides a more nutritious home for future plants. They also fix nitrogen in the soil, which is a key element for plant growth. Sounds good, right? I think one of the problems that we have is that we tend to think, OK, we will generate these crops that are resistant to drought and to heat.

29:44We don't know what is the extra problems that will generate. And that's the case, for example, with the introduction of the mesquite in Africa. Throughout the course of her work with communities in Mexico, Kenya and Tanzania, Zinnia has seen the unintended consequences of introducing new species to the desert, even species like mesquite that are already well suited to dry environments. One such example can be seen in South and East Africa, where mesquite was introduced to help stop creeping desertification. So they thought, well, this is a good idea. Let's bring those trees. We stop the desertification.