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The area of the earth covered by desert is more than 20%. (Sites range from 20-33%, I don't know why the wide range).

With technology close to providing zero cost solar (not free, but declining each year by a substantial fraction) the ability to desalinate water and pump it over desert land will be viable at some point in time.

What would be the resulting downside to doing this? The new land would be a source of food to reduce starvation, and plants to help clean the air and provide oxygen. Would there be any negative impact to the earth's ecosystem?

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    $\begingroup$ California is already essentially reclaimed desert, as is Arizona. Take away modern irrigation and a few other items and it reverts back to desert. $\endgroup$ – JohnP Jan 7 '15 at 14:38
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    $\begingroup$ Incorrect, at least if you're talking about the Central Valley. Previous to European settlement, it was a fairly lush grassland environment, with many species adapted to the climate & weather patterns. Irrigation is used to grow non-native crops: any reversion to "desert" in the absence of irrigation reflects the facts that 1) native water supplies have been taken for irrigation; and 2) much of the native vegetation has been killed off, and doesn't have time to re-establish itself. $\endgroup$ – jamesqf Jan 7 '15 at 19:27
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    $\begingroup$ Actually, pumping enough water to change the climate of a desert would be a major logistical problem. Desalination OTOH would be almost incidental. And even unnecessary. Climate is sensitive to the amount of water, how much salt you are or are not also moving doesn't make that much difference. $\endgroup$ – Ville Niemi Jan 7 '15 at 20:30
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    $\begingroup$ @VilleNiemi I thought that I had read something about there being serious long-term effects on soil fertility from irrigation with salty water. The water you're actually pumping in may be fresh enough to support life, but as it evaporates you're concentrating more and more salt in the soil. $\endgroup$ – Random832 Jan 7 '15 at 22:30
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    $\begingroup$ Would also have the negative effect of killing all the sandworms. The spice must flow. $\endgroup$ – Hugoagogo Jan 8 '15 at 12:14
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Dust - and sand - is a vital part of the engine which drives our climate, they constitute the majority of nucleating particles in our atmosphere which allows moisture to condense and form rain, hail and snow under a wider range of conditions. In addition to that, the blow-off from deserts also fertilise the oceans to a high degree and create algae blooms which feed food chains.

Without these effects the result on global climate and local oceanic life could be catastrophic. How catastrophic - unfortunately we don't know yet, there's simply too much we don't know about how the process works.

However not all deserts are sand and dust - the Antarctic is technically a desert, which is why your sources disagree on the percentage.

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    $\begingroup$ "unfortunately we don't know yet" - I'd argue that the fact that we don't know yet, is actually pretty fortunate. $\endgroup$ – Eric Jan 7 '15 at 23:17
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    $\begingroup$ Interesting factoid: Sahara provides half of the nutrients to the Amazonian rainforest: earthscience.stackexchange.com/questions/2566/… Without this exchange, there probably wouldn't be a rainforest there. $\endgroup$ – congusbongus Jan 8 '15 at 0:59
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    $\begingroup$ Its easy to say "the effects could be catastrophic" when you also say "we don't know yet". Why the immediate jump to fire and brimstone? $\endgroup$ – B T Oct 27 '15 at 18:22
  • $\begingroup$ I think the leap from not having deserts, to not having enough dust for rain is pretty extreme $\endgroup$ – Andrey Jul 12 '17 at 14:04
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One counter-intuitive effect would probably be severe damage to parts of the South American rainforests if certain parts of the Sahara disappear. Apparently they are relying on the nutrients in the dust that is blown over the Atlantic. Link to one recent article on the phenomenon.

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A lot of negative posts, so here's a more positive view:

While terraforming ALL of the Earth's deserts away might be a pipe dream at the moment, there were serious engineering proposals in the 20th Century that might have altered the climate of the Sahara considerably, by flooding areas that were below the sea level:

enter image description here

The most interesting of these projects was the Qatarra Sea proposal in Western Egypt, which would have also generated electric power in the process. While the lake itself would likely be too salty to suport life, the shores would thrive.

Of course, the electricity potential of such a project would be dwarfed by the energy generated from littering the desert with solar panels.

As countries around the Sahara will get better governance & property rights protections, and become richer, they will probably follow the path of other better managed countries in making the desert bloom, with or without solar power.

http://i57.tinypic.com/33aatd5.jpg

Careful ecological management could greatly shrink the Sahara even without massive irrigation work. With fusion or near-zero cost solar, it's quite likely that more and more of the wastelands of Earth will be reclaimed. It would likely not be massive and wasteful open lakes, but subterranean irrigation micro-arrays going straight to the roots of cultivated plants (better known as Drip Irrigation).

There will be those who will moan and decry the loss of habitat for some desert spider or spiny shrub, but most likely the Chadians and Nigeriens will be too busy growing out their gardens to care.

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    $\begingroup$ You don't even need to irrigate those lands, just stop the over-grazing and other destructive practices, which created much of the desert in the first place, and continue to expand it. Much of North Africa was productive farmland as recently as Roman times, and the source of most of the city's grain supply: en.wikipedia.org/wiki/… $\endgroup$ – jamesqf Jan 8 '15 at 2:45
  • $\begingroup$ @jamesqf That is not (with few exceptions) due to destructive practices. The climate was simply that different. North Africa has been losing water since the last Ice Age due to "natural causes". $\endgroup$ – Ville Niemi Jan 8 '15 at 9:08
  • $\begingroup$ There was an article in the latest New Scientist mentioning this. The latest thinking is to use it as a form of pumped storage to balance supply and demand if desertec is built in North Africa.It's at newscientist.com/article/… but for subscribers only. $\endgroup$ – Chris H Jan 8 '15 at 10:09
  • $\begingroup$ @Ville Niem: Lots of scientific/historical work that disagrees. I'll just point you at a Wikipedia link: en.wikipedia.org/wiki/Desertification#History Follow the links, or search if you want more. $\endgroup$ – jamesqf Jan 8 '15 at 18:33
  • $\begingroup$ @jamesqf Seems we just disagree what "North Africa" means, I bundle the areas of human contributed desertification into "mediterranean area" and "fertile crescent", because the destructive practices followed those cultural divisions. And with "North Africa" I mean whole of "North Africa" including Sahara because the topic was terraforming deserts. Probably not worth discussing more... $\endgroup$ – Ville Niemi Jan 8 '15 at 20:52
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Just dig a hole in desertground, about 2 meters deep, than dig a hole on the same latitude, but not in the desert. You'll notice that the desertground is much cooler.

Sand reflects more light than a forest or moister ground, which absorbs more. This means, your idea would heat the earth up even more.

You should read about albedo which tells how a surface reflects light. The albedo factor of deserts is ~0.3 which means 30% is reflected, Forests have 0.05-0.18, so this is 6 to <2 times less.

When 20% of the earth will absorb 600% the sunlight it does now, the earh will absorb a total of 120% more sunligh (!) and thus warm up.

The gained space would soon be populated by people and within some generations, earths population growth will get even faster, so the food problem would be pushed a little but further in the future but is not solved.

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    $\begingroup$ What food problem? $\endgroup$ – Serban Tanasa Jan 7 '15 at 20:09
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    $\begingroup$ Quick correction: 20% of the land area is desert, but land area is only about 30% of the earth is land. This yields a total of 6% of the earth being desert. A reduction from .3 to .05 on this portion of the Earth would reduce the average albedo of Earth from about .35 to around .332, corresponding to the earth absorbing about 3% more light. $\endgroup$ – ckersch Jan 7 '15 at 20:54
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    $\begingroup$ @ckersch But increased evaporation of water at these latitudes could create more clouds, which have this annoying tendency to be white and reflective. Nothing is ever simple, is it? $\endgroup$ – Serban Tanasa Jan 8 '15 at 0:37
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    $\begingroup$ @SerbanTanasa: Maybe you did not notice because food is abundant in your country, but people are starving the world over. $\endgroup$ – Lightness Races in Orbit Jan 8 '15 at 18:36
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    $\begingroup$ @LightnessRacesinOrbit , but the problem is political and organizational, and has nothing to do with us literally running out of farmland. Not a "food problem", a "genocidal leadership" problem. $\endgroup$ – Serban Tanasa Jan 8 '15 at 19:19
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You need to distinguish between semi-desert like Arizona, and "real" desert like Sahara or Atacama.

Semidesert does have soil and can benefit from irrigation.

"Real" desert has little or no soil (no organic materials). Irrigating sand will NOT create fertile land. It will take decades to add organic materials and bacteria to create fertile soil on top of sand.

Also it would be wise to take into account any future climate changes we are already committed to due of already released CO2. Temperatures will increase and deserts would be more "deserted" and less hospitable.

Also, with climate change, currently productive lands will experience dry spells and become desert, so irrigation efforts should be focused there (if money are issue, as it always is).

Interesting side effect of more irrigated land would be more evaporation, which will (according to weather models) increase severity of weather events (more vapors = more energy in air = more energy for Nature to play with). Things like tornadoes.

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  • $\begingroup$ I don't have the data to make that distinction. I was asking 'at the margin,' i.e. with the understanding that deforestation is bad, is an incremental conversion of desert to fertile land 'good'? I understand your answer, some fraction of desert land is better suited to this process. By the way, decades are ok. There's no rush. $\endgroup$ – user6320 Jan 7 '15 at 15:44
  • $\begingroup$ Desert classification is generally based on rainfall, not vegetative cover. Even in sand, though, there are plant types that can grow given sufficient moisture. Here's some people that are working on doing just that: saharaforestproject.com $\endgroup$ – ckersch Jan 7 '15 at 21:08
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The safest answer is that we don't know. Life on Earth is often the result of an equilibrium and pumping to much water into the land will cause some problems. As other mentioned, we need deserts, as they play a role in our biosphere. I will say that we don't know for sure what will happen if we change millions of kilometres into marshes by pumping water into them but we could still try to regain the lands that were taken by the desert during the last decades/centuries.

Re-affecting the land cannot be done simply by adding more water, it is not always the solution. In northern China and in the Sahel, the main source of desertification is the erosion of the grounds. Erosion caused (in China) by overusing the land. A combination of deforestation and overusing the soil with poor agricultural practices has led to the desertification of what was once the birthplace of the chinese civilization. This problem is also present in the Amazon. The deforestation has not led to fertile lands but lands that are affected but erosion in a large scale. The ground is simply collapsing as there is a lot of water but nothing to retain the soil.

If it can be done, it is surely a slow process. Add water to fill rivers and wait for the plants to develop before adding more water. Increase the water with the increase of water retention given by the increase in plants in the area.

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    $\begingroup$ To expand a bit, much modern desert - areas around the Sahara, the Middle East "cradle of civilization", parts of the American West - are actually the result of human activity, as over-grazing and plowing destroyed the native, adapted grassland ecosystem. $\endgroup$ – jamesqf Jan 7 '15 at 19:31
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To expand on a point Sempie made, the colour of the ground determines how much light it reflects. White is the best reflector; black absorbs. This is why a black car will be hotter on the outside than a white car after a day in the sun.

This principle also applies to infra-red, which is the type of radiation via which we get our heat from the sun. White or near-white surfaces such as desert sand reflect a lot of heat. However, the reflected heat does not all go back out into space; some stays and diffuses evenly over the planet.

If you were to change the desert into a more fertile landscape, such as one of soil and trees, or fields for farming, your land would be hotter. However, the air near the land - I.e. the zone in which people walk around - would actually be cooler. This effect is why deserts are hot in the day and cold at night: during the day, the sand reflects the Sun's heat and heats the air, which is what people move around in and breathe. This makes it feel very hot. During the night, there is no heat to reflect and the ground has not stored any, so it gets very cold.

So, in summary, you wouldn't actually have a net temperature change. The difference between day and night, however, would be quite different, but this would make for very good farming land.

N.B.: Although the temperature wouldn't change as a direct result, other climatic effects will occur and may have an impact on temperature. See other answers.

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  • $\begingroup$ I think you are wrong about deserts and heat. Deserts (and I live on the edge of one) are not necessarily the sand dune-covered Sahara stereotype, nor are they necessarily much hotter than more humid lands at the same elevation and latitude. They cool off faster at night because of the lack of moisture in the air, which a) allows heat to radiate to space more easily; and b) does not release heat of condensation by becoming dew. $\endgroup$ – jamesqf Jan 8 '15 at 2:38
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We are already seeing most of the negative impacts of these kinds of changes already. The western states such as Nevada and California are now experiencing some of the worst droughts in US history.

Many projects starting in the first half of the 20th century have drastically changed the landscape such as the building of the Hoover Dam, and thousands of other dams all over the US, as well as other big projects like the Central Valley Project in CA which diverted millions of gallons of water to the central valley to irrigate farmland.

This prompted millions of folks to move to areas which were previously deserts. The land was not suited to support so many people; people who showered every day and used thousands of gallons of water a year per household to grow lawns.. lawns in the middle of the desert. This all lead to the water crisis they are experiencing now. There are many water saving regulations in place now, but it is too little, too late.

It is also very unlikely that we would ever be able to eliminate the world's deserts. The current rate that deserts are growing massively outstrip our ability to reverse it. A desalinization plant takes a massive amount of power to operate. Only wealthy nations such as Saudi Arabia are rich enough to build and operate them.

Desalinization plants must also be relatively close to the coast. The Sahara Desert is thousands of miles across. There would be no way to bring enough fresh water inland to turn it into a tropical paradise.

I'm sorry to say, but this is just a pipe dream (pun intended).

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Why can't all the organic waste we produce from foodstuffs isn't used to layer areas of desert with sand driven over it..e.g.from Cairo..This will help hold onto water that falls, though rainfall is irregular....

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  • $\begingroup$ This seems more like a comment than an answer. The question is about the results of eliminating deserts, particularly negative ones. This is suggesting a way to eliminate deserts better. That doesn't seem responsive to the question. Perhaps it is responsive to one of the other answers, in which case a comment would be more appropriate. $\endgroup$ – Brythan Dec 27 '16 at 23:56

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