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Comment by gaiagraphia | Hacker News Reader

I'm absolutely fascinated with all ideas which attempt to 'green the Sahara'. I personally think there should be a Manhattan project-esque attempt to fight back terraform our way out of climate change, and the Sahara should be centre stage.

I've heard many times that the Amazon is dependent on the Sahara, but I don't think it's fair to sacrifice North Africa for an overabundance of life in South America. If you look at the globe from afar, the desert is like a scar which is spreading through the middle of the planet. It just seems unwise not to look at this as a threat.

Other similar project to the OP include the Qatarra Depression project (and similar projects in Ethiopia and Tunisia), the new Nile Valley project (Toshka lakes), and rediverting the Congo Rier to replenish lake Chad. Also some cool ideas with planting Mangroves in coastal desert areas.

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In terms of impact on climate change, most proposals for greening the Sahara are surprisingly just around the breakeven point, where negative impacts are about as large as positive ones.

For example you can do extensive reforestation enabled by water from desalination plants. Running those is energy intensive, but you sequester carbon in the living trees. But the sand and rock off the current Sahara reflect a lot of light right back into space. Trees capture more sunlight, turning it into heat that can be trapped by greenhouse gases. In the other hand trees bring moisture into the air, causing clouds, which reflect a lot of light. On the other hand, desert sand picked up by wind also reflects light.

It's a web of complex interactions that all approximately balance out. I'm not saying it's impossible to make it work, but you end up with supremely expensive projects with very uncertain benefits

Breakeven sounds like an absolute win when you consider the societal benefits. Getting the Saharan populations working on something positive together and building a more stable future for themselves is far better than being a war magnet where questionable ideologies thrive.

Surely the sand captures a lot of thermal radiation?

Sand is a terrible thermal conductor. It's one of the worst materials you can use if you're building a heat bank for a masonry stove or mass heater. The problem is that the air in between the grains of sand acts as insulation, so there is very very bad thermal transfer between what is on the surface and what is below.

That, coupled with the lack of moisture, is why deserts can be extremely hot during the day and freezing at night.

Good point.

Water is the crucial factor here in afforestation as a greenhouse gas, though eventually the moisture will evaporate and form clouds which they reverse their effect, and a tree trunk can hold carbon for a very long time. So I'm not sold that tree planting has as negligible effect as the original poster claims.

Also why walking on top of sand can burn your feet, file digging your feet in to just below that top layer is suddenly very cool.

So a bunch of copper rods drilled into the soil for a reverse heat pump?

Don't forget that the heat will transfer both ways. Heat pumps require some form of mechanical energy to move the heat (usually via some fluid) that you can shut off when the sun is down if you don't want that heat radiated back to the atmosphere.

Much better to just put up a bunch of mirrors so the light never gets absorbed as thermal energy into the earth in the first place.

I live in Texas. As soon as the sun goes away the temperature drops considerably.

I don't live in Texas, but I know the concrete in front of my house gets warm after dark. I was thinking it was similar to sand. But I'm guessing the crucial thing here is heat retained in the water which evaporates from the plants, and whether that is more significant than the carbon storage in its greenhouse effect.

I lived in Texas (Houston). One of the weirdest things I remember in summer was the crazy 90 degree heat (at least it felt like it) late at night (like 10p)

Houston is by the coast. It is pretty damp and humid. More swamp than desert.

The biggest impact is in the change of the albedo factor.

Specifically the biggest impact is the albedo in the infrared

It feels like the big negative effect here is probably the energy cost of the desalination plants. But if we can generate energy while getting free rain on top of that, it seems to me that would make a massive difference to this balance.

Since when do you need desalination plants? Play stupid games, win stupid prices.

Most of the time all you need is to increase the water retention of the soil for the water that is already falling onto the location and to prevent erosion and runoff.

It could be a worthwhile project to pursue, but these kind of ideas will draw fossil-fuel shills or fossil-fuel funded 'environmental' groups, with the common objections we hear: so much land is needed, some 'environmental' damage is happening, etc.

  We should first focus on projects that use no land:
  1. Vertical solar with farming
  2. On lakes/reservoirs to prevent evaporation
  3. There are lots of hydro dams, which have reservoirs, and hydro already has transmission lines
  4. On roofs of buildings, commercial and residential
  5. On sides of buildings (vertical solar)
  6. Floating Solar in the ocean. Italy is building 540MW[1], why can't every country do 100 - 1000 of these?
  7. Solar green houses
  8. On highways, lands are owned by federal govt, can build both transmission and solar without any issues
  9. On superfund sites
  10. On parking lots
  11. Cities could build wide side-walks for walk and bike with solar canopy. Every city owns 10 - 100 miles of walking trials.

This is by no means exhaustive, as we look around we'll find a lot more. My hope is that Solar advances fast and soon we can have solar fabric or have solar spray paints, then every surface can be converted to absorb energy.

[1] https://electrek.co/2024/03/01/italy-hybrid-floating-solar-f...

I forgot to mention farm subsidies. Farmers are in the business of capturing energy, using the green growing thingies, which they have to plant, nurture, harvest twice a year. Which is a lot of work for 3- 6% of energy captured by photosynthesis. Solar panels are 20 - 25% efficient, and they last 25 years (under warranty) and could last 50 years or more with slightly lower output.

Every country has farm subsidies. Ethanol subsidies support 40m acres of corn in US. Worldwide, this could be 100m+ acres for ethanol/biodiesel. If the subsidies were slightly shifted to incentivize farmers to produce solar, it is profitable for everyone.

So we sacrifice hundreds of thousands of species in the rainforest for… more humans living in the Sahara? That might be great for a population expansion of humans in Africa, but it sounds pretty horrible for the overall health of the planet.

Would it be a good trade off to double the species in the Amazon for doubling the size of the Sahara? Is the number of species the only metric on which the success of the planet should be based?

That whole band of desert - from Mauritania to China - is spreading. Desert is death, and is barely any different to Mars. Imagine the absolute crisis the world will face when the likes of Spain become like Algeria, and the current migration trends intensify tenfold.

We need to do a better job of adapting to what we have and living in ways which complement the terrain. The Sahara has lots of potential to support life and create sustainable thriving communities, but are being ignored as the world.

> Desert is death, and is barely any different to Mars

Fennec foxes are mighty cute [1].

[1] https://en.m.wikipedia.org/wiki/Fennec_fox

Fennec foxes survive on the remnants of a green Sahara, though. Once all that water underground is used up, the ecosystems will all die.

At the moment we're sacrificing the rainforest for profit. And to feed people in Europe, perhaps. A habitable Sahara sounds like a better tradeoff than that. Although ideally I'd prefer to keep the rainforest, of course.

The Amazon is being deforested at an alarming pace. If trends continue, it could be almost completely cut down within 50 years.

I don't think potential adverse affects on the Amazon should be a talking point for whether Saraha is re-forested.

It's also not like it's going to matter much for global warming. If Sahara was as densely forested as the Amazon - that wouldn't even take out 2 years of carbon emissions.

It's a one trick pony. Then what?

Yea, it's silly to talk about reforesting Sahara when we can't even manage to stop deforestation in the Amazon.

There is the Great Green Wall project. But I doubt this is still going strong.

https://en.m.wikipedia.org/wiki/Great_Green_Wall_(Africa)

There are also companies like Groasis that try to make it possible to grow trees in very dry land by using moisture from the air. https://www.groasis.com/en

there are groups still working on it: https://www.youtube.com/watch?v=WCli0gyNwL0

Absolute madness. Do you realize how much of humanity, and the biomes we rely on, are dependent on the Amazon? And even it were a good idea on balance we have no idea of the ripple effects and externalities. How about we just focus on stopping the changes we’re already causing and get to an equilibrium state with our planet first?

>stopping the changes we’re already causing

That ship sailed when the industrial revolution started. We - as a planet - industrialised, and the cost is a change in climate. Billions of people are catching up with what Europe kickstarted, and nothing's going to change that.

The equilibrium is going to be thinking about how the future world may look, and adapting our terrain and lifestyles to match it. The Sahara is the perfect test bed for this.

> That ship sailed when the industrial revolution started

According to this[0] at least, the main deforestation cause is cattle ranching and palm oil production. Not exactly activities caused by industrialisation.

[0] https://earthobservatory.nasa.gov/images/145988/tracking-ama...

I said stopping, not reversing

Don't worry, we'll cut down enough of the Amazon that it'll collapse, long, long before we green the Sahara enough to hurt the rainforest :)

> don't think it's fair to sacrifice North Africa for an overabundance of life in South America

Would note that the Sahara isn’t Mars. It has an ecosystem we would destroy in terraforming it.

North Africa, which starts above the Atlas Mountains that mark the northern border of the Sahara, has traditionally been one of the world's big grain producers. Very good farmland there, because the region benefits from the rain that the mountains stop from reaching the Sahara. The Sahara begins south of North Africa.

Imo the far greater potential is in the oceans. There are vast stretches of barren wasteland with low oxygen and low nutrients in the oceans. We could deliver those ingredients for life artifically.

Is there any scenario in which both the Sahara and the Amazon turn green simultaneously?

There's evidence of the Sahara being greener. I don't think there's any evidence of the Amazon not ever having had trees.

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