If the melting polar icecaps are flooding the oceans with fresh water why don't we build desaulination plants?

Question

If we remove the salt from the excess ocean water and put the salt back into the ocean would it not cancel each other out? Plus all the fresh water that could be made could help in times of drought. In foreign lands where water is scarce it could be a life saver. There by delaying the cool down of the oceans and reducing the land consumed by the raising levels. This would not stop the melting polar ice caps but it would buy so more time to clean up our act.

Answer 1

The oceans are expected to rise about h = 20 cm by 2100 from current levels due to the melting ice in both the Arctic and Antarctic regions. [See source.] There are about A = 360 X 10^6 km^2 of ocean on planet Earth. [See source.]

Thus, the melting ice will be adding about V = Ah = 360 X 10^6 X 20 X 10^-5 = 7200 X 10 = 7.2 X 10^4 km^3 of fresh water to the oceans between now and the end of the 21st century. V in gallons is v = V X 2.6 X 10^11 gal/km^3 = 7.2 X 10^4 km^3 X 2.6 X 10^11 gal/km^3 ~ 20 X 10^15 gal of fresh water added over about d = 360 X 93 ~ 3,600 days. So your desalination plants would need to process enough sea water to make about v/d = 20 X 10^15 gal/3.6 X 10^3 days ~ 4 X 10^12 gals/day to cover the melting ice.

A typical desalination plant can change 100 gal of sea water into about 20 to 50 gal of fresh depending on the process and source of water. Let's say 33 gal, a mid point. Thus, 12 X 10^12 gals/day of sea water would need to be processed to get the v/d fresh water.

The world's largest desalination plant, in Saudi Arabia, produces about 128 X 10^6 gal/day of fresh water. Thus, using our conversion rate, it would throughput about 400 X 10^6 gal/day = 4 X 10^8 gals/day of sea water. That's clearly less than the 12 X 10^12 gals/day of salt water processing needed to offset the melting ice between now and 2100. In fact, we'd need n = 12 X 10^12/4 X 10^8 = 3 X 10^4 ~ 30,000 desalination plants of the largest kind to do the job.

But all of Earth has only around 7,500 plants, all of which are smaller than the largest one in Saudi Arabia. Bottom line, we don't have sufficient plants to offset the melting ice issue. And there is yet another issue...energy.

It takes enormous energy to desalinate sea water. Check this out:

"Energy use requirements for desalination plants are high. For example, an estimated 50 million kWh/yr would be required for full-time operation of the City of Santa Barbara's desalination plant to produce 7,500 AF/yr of water. In contrast, the energy needed to pump 7,500 AF/yr of water from the Colorado River Aqueduct or the State Water Project to the Metropolitan Water District (MWD) of Southern California is 15 to 26 million kWh/yr. These energy requirements may be compared to the energy use of a small- to medium-sized industrial facility (such as a large refinery, small steel mill, or large computer center) which uses 75,000 to 100,000 kWh/yr." [See source.]

In other words, it takes about twice as much energy to get fresh water from desalination than by simply pumping it up from aquafers and such. And, even if we had the energy sources, what do you think that extra energy would do to the global warming and the melting ice problem it brings us? You bet, it would add to the melting ice rate and make the problems that brings even greater.

So there you have it. Earth does not have sufficient desalination plants to offset the melt rate. In fact, it has less than 1/3 the number needed even if they were all the size and processing rate of the largest one in Saudi Arabia.

And to make matters (and global warming) worse, adding the 14,000 plants with their added energy requirements would likely exacerbate the ice melting rate. Adding desalination plants does not appear to be the answer you are looking for.

Answer 2

because, simply, we could never build enough desalination plants. The water that has been 'de-salted' would go into the municipal system, be used, cleaned, and returned to the ocean eventually. Even if we could stop this process, the rate pf desalination could not come close to equaling the rate that fresh water is released into the ocean from the melting, so the effect would continue at about the same pace.