Traditionally, the circulating water gets colder as it reaches the arctic circle and submerges, since cold water has a higher density than warmer water.
But: melting ice in the arctic releases freshwater. This reduces the salinity at the surface of the Northern Atlantic. Water with lower salinity is lighter. Therefore, this water no longer submerges, and the circulation is slowed down.
The studies differ on the timing of when that weakening began. Thornalley’s record, which spans those 1,600 years, suggests it started at the end of the little ice age, a period from about A.D. 1350 to 1850, when solar and volcanic influences depressed temperatures across the Northern Hemisphere and glaciers and ice sheets expanded. As the little ice age ended and temperatures warmed, ice melted and freshwater flooded into the North Atlantic. The results suggest the current state of the AMOC is the weakest it has been over that whole long record. Whether today’s state is just a continuation of that reaction or whether global warming has also started to chip in is not clear, he says. Caesar, meanwhile, put the turning point toward a weaker AMOC in the mid-20th century, suggesting it is due to the influence of human-caused warming. Her team’s record, however, does not extend as far back.
Indications of the process having started (edited) before humans could be responsible for it (few hundred years ago).
By my rudimentary estimation (based on numbers from https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/grl... Figure 3) I believe the annual ice melt in the Arctic each year is approximately 10,000 km^3 of ice. Let's assume all of it flows into the Atlantic. Remove 10% of that volume to approximate the liquid volume of the ice, then convert to kilograms of water = 9x10^15 kg. Average ocean salinity is 35g of salt to 1kg of water. That comes out to ~3.15*10^14 (315 trillion) kg of salt. You can pick a percentage that flows into the Atlantic, but it would still require on the order of trillions of kilograms of salt.
And the answer is that if we turned all of our energy to salt production, it wouldn’t even be a drop in the ocean, and our efforts would of course liberate more CO2 and contribute to global warming. Of all the possible ways we could geoengineer (all of them bad ideas frankly) this has to be one of the worst, least effective, most limited, and least likely to succeed.
If we’re going to hasten our demise, we might as well try sulfate aerosols or something with a more global effect. We’d still end up killing ourselves, but at least it wouldn’t be quite so much like trying to get a sailboat to move by blowing on the sails.
There must be some quantity of salt we can dump a year to raise salinity. We should be rigorous, and find out what that quantity is. If we can change a climate, we can change an ocean.
1/100 of 1% of the world's oceans is a cube 32 miles on each side [0]. This volume of salt would weigh 317 billion tons [1]. World commercial production is .25 billion tons of salt [2].
So, to raise salinity by .01%, we'd need 1268 years of normal production (plus some when you exclude sea-water extraction).
The assumption there is a feasible application of something in a location to remediate a world wide problem is attractive but I think built on faulty logic. To try an analogy, gluing a broken cup back together doesn't make it never have broken.
Yes, we probably can re-salinate a flow to attempt to alter its dynamics. We'd also have altered its salinity profile in ways which have repercussions on e.g. cod fisheries, or krill, or other things. Thats the problem with complex biological systems with lots of interrelated parts: they don't reverse with simple tweaks because once you change one part, you change lots of parts in unexpected ways.
(I did once study this stuff back in the 80s and was very bad at it, but what I recall of even two-actor predator-prey relationships is that they are remarkably complex, cyclical, and do not just return to the same stable place if you alter them trivially. I believe this is probably even more true for physical systems like weather and climate)
While you are right that reducing CO2 emissions is a must, I'm uncertain if we don't have to actively remove it as well. If it's not already too late anyway. With the methane hydrate already on the move...
