To E/Acc: Greening the Desert for Cheap

Anthony Repetto
5 min readOct 3, 2023

~ evaporation is orders of magnitude better than desalination ~

If I buy the snazziest, most efficient (and expensive) desalination facility, I can clean 1 ton of water for 2kWh — that’s 7,200,000 Joules. Solar panels in the desert, where 24hr-averaged insolation is 320 Watts per square meter, can collect 80W/m3, which yields 2.5 GigaJoules of electricity per year, enough for 350 tons of water, to provide modest irrigation for a tenth of a hectare of land, a ratio of 1,000 to 1 area utilized! Yet, both the panels and the desal facility are big-ticket items, with fancy catalysts or electronics, and batteries, wires, maintenance. Then, your desalinated water must be piped to cities, farms, some of them dozens of miles away, which costs a lot of power AND requires immense amounts of infrastructure. There is an easier way:

Two-Step De-Desertification

First, make five percent of the desert black, in the middle somewhere, by dropping sooty paper wings from a cargo plane. Then, pump seawater into plastic troughs along the shore or on buoys, suspended a few meters above dense combs of fiberglass yarn, to drip in the wind. Done.

How that works:

Suppose you want to irrigate 1,000,000 km2 of desert. You will need some 400 Billion tons of water to rain, each year. If that moisture is being carried inland on a sea breeze, from warm waters, then it easily carries 10 grams of water per cubic meter, or more (at 20c, air holds 17g of water per m3 at 100% humidity). If you can get just 5g to rain, per m3, then that 400Bt of water every year (13,000 tons per second) will require only… 2 1/2 Billion cubic meters of air every second. Along a coastline of 2,000 km, with a wind-front some 500m high, that’s an extra 2.5m/sec inland breeze -> a 5 mph wind.

So, we pick a spot inland, considering Coriolis Effect and mountain ranges, to paint black. We need to paint enough of it black, absorbing additional sunlight, that we pull our 5 mile-an-hour wind from the coast. Our large, contiguous, blackened region will be able to convect if there is even a 2c temperature gradient; roughly 2kJ/m3 of extra heat. For the 2 1/2 Billion m3 we must convect, each second, we need 5 TeraWatts of solar power. [[That is slightly more than a quarter of humanity’s current energy budget, yet 5 TW is still 500 TIMES less heat than our “2% insolation retained due to CO2 emissions”; it doesn’t alter the trajectory of climate change.]]

The desert is mostly beige, reflecting a third of the sunlight. A moderately black pigment, even when dusty, boosts absorption by 30% — an extra 90 Watts/m3, which is MORE than your solar panels! To reach our goal of 5 TeraWatts, we need only 56,000km2 blackened. With paper wings, sprayed with a mix of soot and linseed oil (from flax seeds: organic water-repellant adhesive that degrades after a few years of sun exposure), you only need 0.5mm thick of cellulose covering that ground; 56,000km2 need 28 Mil m3 of paper, which is only a fraction of global annual paper production.

With pulp delivered to fabrication facilities near the target site, planes can drop across uneven slopes and difficult sand on short routes, minimizing the number of cargo planes needed. To drop all 28 Million m3 of paper (weighing some 56 Mt when soot-paint and bob-weight are added) in 4 years would require an average of just under one quarter of a m3 paper per second; if each plane completes a round-trip in 2 hours, you would need planes with a total cargo capacity of 1,800 tons — just half a dozen of the biggest cargo planes.

For a cost of fabrication in the range of $2,000/m3, this would cost $56 Billion in materials; the fabrication facilities would be billions more, as would the use of those cargo planes. Yet, we are still looking at a price-tag well below $200B.

Now consider: if that irrigation increases the value of the land, as well as providing agricultural output, for a total increase in value of just $1,000 per acre, then our original 1 Million km2 of desert is now worth $245 Billion dollars. Further, the sea breeze increases humidity, making the nights warmer (desert nights drop to just above freezing!) and the air more comfortable, while frequent rains and evaporation from plants, convection, all work to carry daytime heat high into the sky, where it radiates away into the icy black void of space: the ‘liveability index’ shoots upwards, so that cities built in the region will be worth hundreds of billions more.

That was step ONE!

Step two is the part that requires some solar panels, to operate pumps along the coast. Pump sea water into large, wide, flat troughs, suspended on aluminum frames a few meters above sea level. Pumping that water will take you just 12kJ/m3 of water, per meter lifted… which is 6,000 TIMES less energy than desalination. That water drips through tiny holes in the trough, where fiberglass yarns hang. The water drips down the yarn, while the wind blows through the dense forest of wet fibers. Your sea-breeze is now 100% humidity! If you ever need to add more water to the air (because it is hotter than normal, and so it can absorb more water per volume), then you just make the rows of fibers extend further inland from the beach, each row adding more moisture.

By increasing the humidity in this way, then you need LESS of the desert’s surface to be blackened by paper wings, which saves a lot of money there. Where you strike that balance, between evaporators along the coast vs. black wings inland, will depend on the proximity to the coast, mountain ranges and wind patterns, as well as the amount of pre-existing breeze — if there is already a strong, steady wind, then evaporators will yield a higher return by being exposed to such a large volume of air per second.

Further, when deserts begin to fill with lakes, those lakes will gather all the salt left from previous waters; use the pump, trough, and yarn to evaporate them again, to capture the salt, extracting it from the soil, to prepare the soil for agriculture. And, once plants take hold, they also slowly release humidity back into the air, which HELPS to drive MORE convection (water vapor is H-O-H, much lighter than air molecules’ N-N and O-O, water is buoyant in air). In this way, a fertile forest and plains will pull its own sea-breeze; you can ensure that this evapotranspiration creates the MOST rain if you are pre-humidifying the breeze with evaporator troughs along the coast.

Oh, and if we green 1 Million km2, even as low-intensity pasture, we can feed 200 Million more people. 30% of the land on Earth is desert, while farm and pasture yields have been in steady decline almost everywhere. You could even make money doing it; the King of Saudi Arabia spends more than $200B on a city in the shape of a line, and he has 1.2 Million km2 of desert along a coast! Is it time for some geo-engineering?

[[earlier explanations of the designs are googleable: “Make the Drought Evaporate” Anthony Repetto]]