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Comment by Merrill | Hacker News Reader

The core assumption is that 3 billion out of a total of 4.7 billion acres of desert currently existing can be reforested by planting trees and irrigating them for 20 years, after which they will continue growing without irrigation.

Is this possible, except for marginal areas bordering the deserts? Aren't most desert areas desert because of very low precipitation?

This may be practical for areas such as the example in the article of the Kubuki Desert reforestation project, where desertification due to the encroachment of dunes can be thwarted. But the major deserts such as the Sahara, Gobi, or Arabian Empty Quarter have very low precipitation, below what is needed for grass, desert shrubs, cacti, etc.

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In the article the claim is that the environment changes due to the presence of the forest and results in more precipitation.

i.e. the presence of precipitation and trees are linked. If you remove one you remove the other, and if you add one (trees in this case) you get the other.

Pretty amazing if true.

>But for the past 2,000 years or so, the climate of the Sahara has been fairly stable. The northeastern winds dry out the air over the desert and drive hot winds toward the equator. These winds can reach exceptional speeds and cause severe dust storms that can drop local visibility to zero. Dust from the Sahara travels on trade winds all the way to the opposite side of the globe.

>Precipitation in the Sahara ranges from zero to about 3 inches of rain per year, with some locations not seeing rain for several years at a time. Occasionally, snow falls at higher elevations. Daytime summer temperatures are often over 100 degrees Fahrenheit (38 degrees Celsius) and can drop to near-freezing temperatures at nighttime.

https://www.livescience.com/23140-sahara-desert.html

Precipitation is controlled by the humidity in the prevailing wind systems. Prevailing winds from the northeast drop whatever moisture they have on the southern margins of the Mediterranean or on Egypt and Sudan.

Most areas became desert after deforestation, many times due to earlier civilizations of humanity, many speculate that these were often largely influential in the falls of empires. A good example would be Iraq, when it was still Babylon there were a lot of trees, forests, and marshes there. The deforestation around the mountains eventually caused droughts, which turned it to desert and contributed to the fall of an empire. There is evidence of this with nearly every fallen civilization that wasn't directly caused by war, but even then often the wars happened due to scarcity caused by similar events in neighboring nations.

Any good books or films on this topic?

You probably heard before that large forests alter the weather around them. This is no exaggeration. Deserts are locked in a state of infertility because ay rain that falls quickly washes away, taking with it any top soil. So first you big holes to trap water underground, then you let the biomass build up some top soil, then you plant trees. When you have enough trees, they start pulling water from underground into the air, increasing moisture and starting a positive feedback loop. Then you start expanding into the surrounding desert areas. It's a lot of work, but possible, and being done.

>irrigating them for 20 years

And there is an assumption that there is enough freshwater to do this.

"A third of the world’s biggest groundwater systems are already in distress"

"Water scarcity already affects every continent. Water use has been growing globally at more than twice the rate of population increase in the last century, and an increasing number of regions are reaching the limit at which water services can be sustainably delivered, especially in arid regions"

https://www.unwater.org/water-facts/scarcity/

Correct, water scarcity is in fact the PRIMARY bottleneck.

The main "innovation" in this solution isn't "we must plant massive forests," it's that clean (solar-powered) desalination is now possible at very large scales, so we are able to provide the volumes of freshwater necessary for large-scale reforestation and conversion of deserts.

Forestry people have known for decades that trees were the best solution to CO2 in the atmosphere, but there wasn't enough forestable land. And while we can reclaim deserts, we couldn't do it at scale unless we irrigate. And we couldn't do it unless we could affordably create a new source of freshwater that wasn't already needed for humans and agriculture.

Now we have it. Now the solution in possible.

Should also mention that water requirements are generally much higher than needed to sustain an existing equivalent forest, as most desert soils have a high salt content near the surface, due to evapotranspiration drawing salts up and concentrating them, and lack of precipitation to ever flush/dilute them.

Not sure if any of these proposals plan to attempt to actively reduce soil (or aquifer) salt content, but for basins without sufficient excess water for an outflow, salt build up is a big problem. Managing the salinity requires more fresh water than what is needed to sustain the plane life.

Maybe theoretically possible if we have exceedingly cheap nuclear power. Fill the coasts with reactors that are optimized not for power generation but for transferring the waste heat to desalination facilities.

Adding sufficient acreage of forest would "artificially" create more precipitation, no?

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