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Comment by wongarsu | Hacker News Reader

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

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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?

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

Specifically the biggest impact is the albedo in the infrared

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.

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.

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.

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.

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.

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