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Comment by luckylion | Hacker News Reader

Are the Qattara Depression project and similar proposals too small in impact or more expensive than building better flood management, or why aren't we doing those?

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Much too small. Using Wikipedia's figures: volume of Qattara Depression = 1,213 cubic kilometres, volume of Greenland ice sheet = 2,850,000 cubic kilometres. And the project is only useful as a long-term power source if the contents evaporate leaving space for renewed inflow, i.e. it only sequesters a fraction of its volume.

Making Qattara a bay of the Mediterranean would be great as a way to bring life and human habitation to the desert.

It's incredibly expensive though.

Scott Alexander estimated it would "reverse one year worth of global-warming induced sea-level rise" here: https://slatestarcodex.com/2016/04/15/links-416-they-cant-li...

There are 360 million square kilometers of ocean. Of course sea level rise increases the surface area a bit but if you assume 1 mm (1 millionth of a kilometer) goes straight up then each mm of sealevel is 360 cubic kilometers of water.

We are depleting our aquifers at a rate of about 280 km^3 per year. US is down over 1000 km^3 since ~1900. I'm not sure how much of that ends up in the ocean, but reducing that number and taking it negative would go a long way toward slowing sea level rise.

The comment above says filling Qattara takes 1,213 cubic kilometres, which would mean about 3mm lower sea level. Which would be a real change, but also quite small in the bigger picture.

I guess depleting aquifers can lower land levels in those areas. I hear Jakarta has sunk 4 meters from that effect. But it's a separate thing from rising sea levels.

I was thinking more of the fact that if it’s not below ground it’s above ground, so most of that water has been evaporated and deposited into the ocean.

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