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My question is basically the title. Ignoring most of the feasibility of the enormous engineering and earthmoving requirements (I guess I mean to say, given an unlimited budget) in very hot areas, would this effectively solve the problem of sea level rise as a problem? If not, what am I missing?

I'm not sure if this question is within the scope of this board, but I like pushing the boundaries :)

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marked as duplicate by arkaia, Fred, Spencer, Jan Doggen, gansub Jul 16 '17 at 7:59

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ Side question: Would the resulting vegetative growth put a dent in the carbon output of the rest of the world? $\endgroup$ – Throsby Jul 15 '17 at 5:20
  • $\begingroup$ Do you have a specific area in mind? $\endgroup$ – Gimelist Jul 15 '17 at 5:29
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    $\begingroup$ Death Valley would be convenient because of the already-available gradient, but the Sahara would provide water to an area that already really needs it. Short answer: not really. It would be interesting to see the effects based on where the trench was made $\endgroup$ – Throsby Jul 15 '17 at 5:31
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    $\begingroup$ Related: Would it be geographically feasible to store water on land to counteract sea level rise? $\endgroup$ – Pont Jul 15 '17 at 5:46
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    $\begingroup$ The previous question is akin to asking "Can a human's lungs fill with liquid?". This question stands as "Would a human's lungs filling with liquid negatively affect the operation of the human?". The answers have crossover, but only to the extend of describing what is actually taking place, the first "Yes, this condition is called pneumonia." And the second "This condition is called pneumonia and it can kill people". The detail that it can kill people is NOT applicable necessarily (and for the purposes of this question/more importantly, is not addressed by the supposed "duplicate" question) $\endgroup$ – Throsby Jul 15 '17 at 12:47
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Yes, it would be a bad solution.

Aside from the construction and maintenance cost, the impact to the populations affected, and the impact on the environment around the destination, the benefits would be minimal, as roughly calculated in the answer to Volume of Land Below Sea Level

So to reduce the sea level by 1m the total area of these depressions needs to be around 5,000,000 km^2. This is around half the total land area of the USA. I don't have the figures to hand but given that these area would probably all fit into an area considerably smaller than Texas - I estimate that flooding land which is below sea level will reduce the sea level by exactly 'not very much at all' (give or take a millimeter - and very much less than 1m in any event).

There is also a rate of flow issue. You basically want to create rivers flowing inland at a rate to offset or reverse sea level rise, which means the flow rate of those rivers has to approach the volume of global sea rise. Global sea level is rising by an average of about 3.4mm per year, or 1,230 cubic kilometers per year according to NASA (and the rate is increasing). That amount is about the flow of the Congo River (the second largest river in the world) or roughly 19 percent of the flow of the Amazon River.

Fed by gravity, these are among the widest and deepest rivers of the world; if you're going to move that volume of water, you need to build channels to each basin with a net flow similar to that capacity, with that gradient, that won't leach saltwater into the groundwater of surrounding communities. (The engineering issues likely pale in comparison to the political challenges.)

And keep in mind that transferring water inland is only a temporary solution. Water evaporates and by spreading water over a larger area you're increasing its surface area; Death Valley has an evaporation rate of 140 inches or 3.5 meters per year. (In Southern California, the Salton Sea is drying out and causing a variety of salinity and dust issues due to lack of inflows.) So you're transferring ~60-degree seawater into a salt pan where it's going to heat up and evaporate and leave behind a toxic brine, then fall or flow back into the ocean, contributing in some small way to the thermal expansion of the ocean.

(Also, the net volume of CO2 released by building and maintaining these massive projects would also contribute to global warming in some non-negligible way.)

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