Hacker News Reader

> But several thousand eyars ago the Sahara was arable land.

I think the desert status of the Sahara has more to do with a combination of oceanic gyre patterns and the structure of the Hadley circulation. The Sahara and other latitudes around 30N are constantly under a high-pressure zone of descending hot dry air which is part of the northern Hadley cell. The exception is over India where the Eurasian landmass pulls the ITCZ (ascending side of the Hadley cell) way far North and causses the monsoons.

I don't think flooding the Sahara would change its aridity any more than the Arabian peninsula, despite being surrounded by water, is still a desert away from the immediate coastline. It's a matter of air moisture, not sea or ground moisture. You could try irrigation, though. The Libyans abortively tried that under Gaddhafi.

I'm not an expert in any of this but from what I know, portions of the Arabian peninsula sure do get plenty of rain for agriculture from the red sea in the western part.

>don't think flooding the Sahara would change its aridity any more than the Arabian peninsula

There's something to be said about the lake effect. It certainly happens in U.S and Canada where areas just east of the Great Lakes get much more precipitation than other parts

I think the lake effect requires air which is colder than the water. The lowermost level of air warms and accepts water vapor, which immediately rises, re-freezes, and is entrained as ice and blown out onto land. Lake-effect precipitation does form over seas, but you need the air temperature above the surface layer to fall below zero.

As for the rain in Saudi Arabia, there is a mountain range along the coast south of Mecca, and I think the area gets rain because of orographic uplift. When the wind blows across the Red Sea it does pick up a lot of moisture, just not enough to cause rain, because the air is so hot and can hold so much moisture overall. But it gets forced up over the mountains, cools, and passes saturation humidity, and the excess water becomes rain. You can see where the mountain range ends on an annual precipitation map of SA.

Sorry to rain on your parade.

Some people had ideas to flood the Qattara Depression in northwest Egypt.

https://en.wikipedia.org/wiki/Qattara_Depression_Project

It would have provided hydroelectric power and as the water evaporated and formed clouds it may have increased rainfall.

The United States wanted to use nuclear bombs to excavate the channel to the Mediterranean Sea.

Consequently, use of nuclear explosives to excavate the canal was another proposal by Bassler. This plan called for the detonation in boreholes of 213 nuclear devices, each yielding 1.5 megatons (i.e. 100 times that of the atomic bomb used against Hiroshima). This fit within the Atoms for Peace program proposed by President Dwight Eisenhower in 1953. Evacuation plans cited numbers of at least 25,000 evacuees.

Project Plowshare was pretty crazy. Use nuclear bombs for excavation. There were ideas to create artificial harbors by using a bombs.

https://en.wikipedia.org/wiki/Project_Plowshare

> Obviously this would have an impact on the ecosystem

Anticipating the impact is probably a fool's errand, since climate is extremely complex and changes on one part of the world affect others. It's not just about displacing people in the area, you risk altering the patterns that bring rain to places as far as Germany or help grow crops in Brazil. Sahara sand plumes play a role in replenishing phosphates in the Amazon, for example. Even blanketing some parts with solar panels will probably alter global weather patterns significantly.

We'd need to flood it with salt water. Best example as to why this shouldn't be done is the California Salton Sea ecological disaster.

What if we do not flood it, but instead make circulate a lot of salt water in open-roofed canals (but with leak-proof floor and walls) crisscrossing the desert?

Then the humidity would trigger rains and help grow a vegetation, and the salt residues would remain inside the canals.

Then at some point the vegetation would self-maintain the necessary humidity level.

There are lots of ideas around for desert greening; see the linked article for a list. Canals are unlikely to work well, but seawater farming is kind of close to your proposal. It requires greenhouses to be built though, so I don’t know how well it scales.

Years ago I read about a system that seemed successful. Ridges were built up at the desert’s edge, and salt-tolerant trees and shrubs were planted on hem to keep the ground stable. This change in microclimate promoted more greening in the valleys between, which increased overall moisture levels. Once this is established, more ridges can be constructed further out to repeat the process. It’s not an easy thing to search the web for, and I don’t recall how long it takes for one ridge to be productive, but I recall thinking it was shorter than I expected (still on the order of years, of course).

https://en.m.wikipedia.org/wiki/Desert_greening

Canals would silt up very fast. Also, I don't know about leak-proof water canals when humanity can't seem to make leak-proof sealed oil pipelines, and that's with significant financial incentive to do so.

> Then the humidity would trigger rains

Not necessarily, given the weather conditions there it's quite plausible that all the evaporation would form clouds which would only rain elsewhere and not in that desert.

Interesting, I had to read up on that. Could you elaborate how that is related to that though and what makes it a bad idea in particular?

From what I just read up on, the Salton Sea over the millennia would periodically flood, become a small lake or dry out to desert levels. The change that occurred was that humanity changed the rhythm of this artificially to a 'flood it' state for a prolonged period of time.

They are now complaining that when they changed it to the 'small lake' state again, well, shrinking lakes create a certain situation.

The problem I do see is really the way the 'flood' state was created and other usages of the lake. Runoff from agriculture with way too much fertilizer, waste dumping etc. and now they wonder why the dust is toxic and stinks.

How would that apply to flooding the Sahara w/ sea water perpetually?

>> Second, a lot of ice (eg Antarctica, Greenland) is on land so the first point doesn't apply to all ice as some deniers have tried to claim.

But what about ice that is 'on land' but also below sea level? Iirc roughly half of Antarctica's glacier ice is sitting dry ground that is below sea level. Whether melting that ice will raise or lower sea levels is more complicated, with variables for amounts above/below various points.

After deglaciation, land will also rebound over the long term, further displacing volume the water/ice occupies.

Earth is a finite volume. So if the land rebounds in one area, does it not have to subside elsewhere?

It's more decompression, to my understanding

https://en.wikipedia.org/wiki/Post-glacial_rebound

You may want to look into what the permaculture people are doing. That is something that can be done today and has worked many times. Many are seeing really good results. The issue for that area is most of the vegetation was clearcut for firewood. Basically with permaculture they try to put the system back to the way it was before it was clearcut.

there's another fun aspect that I think is neat. water is free to move around a little more than ice (especially ice on land). The earth is spinning, so the inertia of the water will sort of fatten the equator and thin the poles. Sea level might go down a little bit at the poles and go up around the equator.

That did notmcross my mind. But this depends on whether there is enough water to cause this phenomenon or it might simply have no effect.