We face sea level rise due to the climate crisis and fisheries collapse due to overfishing.
Can we can address both at once by diking around coastal deserts, flooding them with the excess seawater and growing mangroves, etc. in the resulting shallow waters?
Perhaps there is an opportunity to reduce atmospheric CO2 while doing
something with a lot of seawater that would otherwise flood our
coastal regions, in a way that increases coastal fisheries, all while
using existing technology.
Coastal deserts could be surrounded by dikes and flooded with excess
seawater, like a salt water rice paddy. This would increase
photosynthesis, as the shallow oceans created absorb more sunlight
photosynthetically than the desert they would partly replace. It would
also remove seawater from the sea, and, on a massive scale, could
lessen coastal flooding from sea level rise. It could also increase
fish catches sustainably, as shallow waters are typically very
productive fisheries, and the increased area of shallow water created
by this could be fished, and might foster intensive sustainable
aquaculture, like mullet and milkfish ponds.
If windmills powered both dike creation and seawater pumping, then
perhaps setting this up could be nearly carbon-neutral - once
operating, the increased photosynthesis allowed would make this carbon-
negative, to the extent that fixed carbon is stored, instead of re-released.
About 70% of earth's surface is ocean: http://www.noaa.gov/ocean.html
About 14% of the earth's land surface is 'desert'(<25cm rain/yr.):http://www.galeschools.com/environment/biomes/desert/index.htm
So 14% of the 30% of earth that is land is about 4.2% of earth's surface. This is land receiving less than 25 cm of rain per year.
70/4.2= 16 & 2/3rds, so if a tenth of the earth's desert area was covered with seawater to an average depth of one and two thirds meters =167mm, it could reduce sea level by a mm -not much, but in the right direction.
How are you going to contain the water?
Dikes, or levees, as in the Netherlands, and as are being considered to protect Bangladesh and are used to protect low-lying cities like New Orleans. These might require certain soil types rich in clay to work well, which would need transport from source to dike, necessitating additional energy input.
How will you pump it?
Wind power, ideally, generated near where needed, to first power forming of the dikes, then to pump the seawater.
What effect on hydrological cycle?
While obviously there would be vast water surface areas behind the dikes, perhaps unobviously there would be less rise in sea levels, since so much sea water would be taken from an otherwise rising sea, and not flooding another millimeter of ocean's edge might avoid some of the increase in surface area flooded behind the dikes. So there would certainly be vast changes in the distribution of water surface, and thus evaporation, across the globe, and there would need to be detailed calculations of the effect on global water surface and weather patterns.
What about salinity control and contamination of fresh water?
There would obviously be massive direct impacts on previously-desert ecosystems, and any increases in evaporation would fall somewhere as increased precipitation. As moisture evaporated from the diked areas, salinity of the seawater held would increase. If this brine was sequentially pumped from lagoon to lagoon into the desert, it would mimic salt production but on a larger scale, and would yield brine or salt, depending on how far this was taken in the salt-making process. This brine or salt could be a disposal problem or an additional marketable product.
These last two points draw attention to probable major negative impacts of such a project. On the positive side, shallow water ecosystems are enormously productive in terms of sealife and photosynthetic activity. If such diked areas were established and managed aquaculturally, with fertility supplied so as to lead to intensive capture of sunlight energy, we can expect it would lead to extensive growth of commercially favored sealife, which might pay for the project, while feeding many who otherwise wouldn't eat.
Hence diking some desert areas and flooding them with seawater might yield a positive cash flow and other social benefits, so that it might be possible even without subsidy, although it would require massive investment.
The amount of carbon bound by photosynthesis in unfed maricultural systems probably exceeds that observed in wild estuaries, so let's use esturine productivity as an estimation of the amount of carbon involved. "Primary productivity varies from 25 to 1250 gm C/m2/yr in the marine environment and is highest in estuaries and lowest in the open ocean." (from: www.tulane.edu/~bianchi/Courses/Oceanography/CHAP10.PPT ) Taking 1,000 gm C/m2/yr as a conservative estimate of aquacultural productivity, and 3 million square kilometers as a tenth of the area of global deserts(roughly summed from a total of the largest desert areas chart at : http://en.wikipedia.org/wiki/Desert), to be diked, flooded and managed for aquaculture, 1,000 gm C /m2/yr * 3x10^6*10^6 square meters per square kilometer= an overall gross carbon fixation of 3 Pg C/year. For some idea of the human-caused atmospheric carbon changes, I'll quote the following:
"Fossil fuel CO2 emissions [in 2009 totalled] 8.4±0.5 PgC emitted to the atmosphere": (http://www.globalcarbonproject.org/carbonbudget/09/hl-full.htm)
Hence diking and flooding deserts with seawater, and managing these for aquacultural productivity, might involve significant amounts of global atmospheric carbon. If sold as aquacultural products, such carbon might quickly re-enter the atmosphere, and couldn't be said to have left the biosphere, even if we filled a supertanker with frozen fish and parked it in the arctic for a number of years.