Ah, I see, there is no shallow water in northern seas. One wonders how the land manages to pull itself out of the water then - shear underwater cliffs?
It's not really a question of there being NO shallow water in northern seas. There are lots of shallow continental shelves, but close to the continent the water also has more wave activity and a very different kind of local ecology which can disrupt growing coral. Shallow tropical seas happen to be ideal for the growth of coral, and northern waters do not really have comparable regions even if they do have continental shelves.
Still, it may be premature to give up on the migration of coral. Global warming has just begun. Maybe when the warming has progressed sufficiently, new ecological niches for coral will indeed emerge. The drowned city of New York may prove to be perfect. We'll know in a few more decades.
[...] and northern waters do not really have comparable regions even if they do have continental shelves.
What if we just opened up all the dikes in northern Europe?
- derumi (del-me)
"Bobby Fischer? Man, that guy is crazy!" - Mike Tyson
If we as a species should ever decide that the survival of coral is more important than that of ourselves, then yes, we should open the dikes of northern Europe and see if we can create some nice new shallow seas in which coral might be able to flourish. There may be some PETA fanatics who would approve of this strategy given that the coral polyp, for all its vegetative appearance, is actually an animal, although living in symbiosis with a plant. And animals must be treated ethically.
Some questions - does the warmth of the water effect current and wave action? Does the coral itself impact wave action and water temperature?
I am not suggesting that suddenly we are going to find coral reefs off Manhattan, but I find it very hard to believe that if the southern edge of a particular area becomes inhospitable to coral, that you wouldn't find that the northern edge would then expand as it warms.
The answer to your first question is no, not much. The answer to your second question is yes, a lot. Corals actually play a large role in creating those nice shallow seas that we associate with tropical paradise. That nice white sand that extends from the beach out to the reef is many thousands of years of broken down coral. Reefs act as a wave break, which helps stabilize all the sand and allows the shallows to remain shallow. There are lots of species that contribute to reef formation (anything that leaves behind a skeleton or a shell, basically), but corals are the dominant life form once the shallows have been stabilized.
Someone earlier mentioned migration, but I don't think that is the real problem, given that when corals spawn, the larvae remain in the water for several days before settling. Many of the world's various coral species would be probably be able to migrate to cooler waters in the face of increasing temperatures. They wouldn't be forming reefs for an awfully long time, but they'd survive.
Still, corals are screwed.
Even though the bleaching events that we're now seeing are a result of rising temperatures, the real killer in the long run will be the direct effect of CO2. The higher the concentration of CO2 in the atmosphere, the more acidic the water of the oceans becomes. Past a certain threshold (around 500ppm CO2), the concentration of carbonate ions in the the ocean becomes too low for corals to build a skeleton, and they die. Right now, we are on track to cross that threshold, perhaps within our lifetimes.
Everything that needs to be said has already been said, but since no one was listening, we must begin again. -Andre Gide
Temperature variations are smallest near the equator, because the heat input from the sun varies the least with the season there. If the earth starts heating up and the corals move north (or south in the Southern Hemisphere), eventually you get to a situation where everywhere it is cold enough during the winter for corals to survive, it is too cold for them to survive during the winter.
The main problem with moving corals north, however, is that corals grow almost exclusively on top of other corals. In order to grow, they need water, very, very low in sand and suspended sediment, because otherwise they get covered by sand and sediment and also because the sunlight can't penetrate the water column down to where the coral grows. Consequently, coral will almost never grow right on the edge of a continent, but will grow some distance offshore or around an island. Most of these locations exist because in the past coral grew there until it was too close to the surface and then stopped growing, which provided a feedback which kept the water depth shallow. As a result,there are far more locations with shallow water far from a continent in tropical areas than in temperate areas. Even where there are islands, the depth of water in northern areas tends to drop off much faster, leaving the area of an appropriate water quality and depth susceptible to wave damage.
Since the concentration of CO2 in seawater is very small, it actually dissolves by the reaction (which can be obtained by setting H+ concentrations equal in the two reactions for H2CO3 losing an H+)
CO2 + CO3(2-) + H2O == 2 HCO2(-)
Interestingly, this has the consequence that the amount of CO2 in seawater saturates, and can nearly be given by C1 - C2 / P(C02), which is to say that increasing CO2 from the preindustrial concentration to twice that much will increase the amount of CO2 in ocean water by twice as much as doubling it again from 2x to 4x.
The shell building reaction is Ca(2+) + CO3(2-) == CaCO3
In normal times, this reaction runs towards dissolving calcium carbonate in deep water and building it in shallow water. Under relatively constant CO2 concentrations, this reaction will restore carbonate levels and allow shell building to continue even at lower PH and higher CO2 levels. The problem for corals and other calcium carbonate shell building species isn't so much that the PH drops when CO2 levels are high so much as that this reaction takes thousands of years to equilibrate in normal times, and in extreme events return to normal carbonate levels is delayed further by running out of shells in the deep ocean.
To find an event similar to what is happening now, you need to find a time where carbon dioxide not only increased to as much as it is now, but increased at something close to the same rate. When this happens, low carbonate levels make it difficult to build shells, and you see extinctions of many of the animals who build calcium carbonate shells, while some species see their populations crash but then recover when conditions improve.
