It seems I read somewhere in the media this past year that the ocean conveyor had actually slowed down a bit due to this freshening.
Even if that’s not the case, then I figure we probably won’t have a sudden halt (if we have one at all in 100-200 years). Wouldn’t it be slowing down gradually over many decades (if at all)?
Comment by Lynn Vincentnathan — 3 Oct 2005 @ 2:15 PM
I haven’t read the papers and don’t know what is happening with salinity in the rest of the Atlantic, but looking at your map it occurred to me that if there was increased freshwater in the Northern Ocean due to ice melting and increase salinity in the tropical Atlantic due to increased evaporation, couldn’t a mixing effect at the southern edge of the Northern ocean as tropical water is circulated north show similar results?
Salinity could be a sleeper that will gain prominence. For example what about the effect on CO2 absorption? A recent TV news item said little Antarctic critters I think called ‘pteropods’ at the bottom of the food chain didn’t like ice melt and that could seriously affect fish harvests. Other possibly minor factors include irrigation runoff with soil leached salts (chlorides and sulphates) and the fact that large cities (eg Sydney) are looking to desalination of seawater.
This post was a good summary of thermohaline circulation (THC) and the difference between the two papers. I saw the Hatun et al paper, and at first glance I wondered if it contradicted conclusions like those of Curry & Mauritzen so I read it carefully. After I read the Hatun et al paper I thought the major point of the paper was that ocean circulation and the subpolar gyre is an important but little understood factor in the THC and more research in the area was needed.
IMO this issue is representative of climate change science in general, very interesting but very complicated. It’s good to have the working scientists at RealClimate as a source to help understand the science.
Comment by Joseph O'Sullivan — 3 Oct 2005 @ 11:14 PM
Recently there have been a number of media reports about a ‘tipping point’ having been reached in the Arctic as permafrost thaws, lakes disappear into the permafrost, and more bare ground is exposed to the sun. There is a recent press release from model runs in Hamburg predicting an ice-free Arctic summer: http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/2005/pressRelease200509301/
Meanwhile, there is some evidence that warming permafrost is going to release vast amounts of ancient methane to the atmosphere. So, all of the Artic feedbacks look positive? The phrase, “point of no return” has been used.
At the same time, hurricanes and tropical convection continue to transfer heat from the ocean to the atmosphere, and I assume that warmer sea surfaces will transfer heat to the atmosphere at a faster rate via mechanisms like tropical convection and hurricances. Hurricanes leave cool tracks; you could see the cooling in the Gulf after Katrina passed over. My understanding is that rates of equator-to-pole heat transfer are now larger then they have ‘ever’ been (past thousands of years)?
Things seem to point to massive change in polar regions. What will be the effect of all this on the thermohaline circulation? Is there a ‘tipping point’ there as well? Paleo studies show we’ve enjoyed a unusually stable climate over the histroy of civilization. What is somewhat unsettling here is the lack of apparent negative feedbacks that would lead to a new stable equilibrium state. The only possible solution seems to be to stabilize the atmospheric gas content, thereby reducing energy trapping in tropical/sub-tropical regions. This will only occur via deliberate human effort; the biosphere is not soaking up the excess CO2.
Where are the negative feedbacks here? Clouds? The Earth will get cloudy, reflect light back to space, and so cool off? Dynamic weirdness of just the right flavor could lead to a cooling trend as heat is ejected into space? Perhaps the real take home message is that we would be better off with far more extensive and detailed observations. However, I think a Las Vegas gambler would have no doubt about where to put the money at this point. Still, do we see a rush to abandon the coal fields and oil wells? Not really… but still, enlightened countries and people are shifting to renewables as fast as they can, although oil scarcity is perhaps the more immediate concern for many people.
P.S. Given that planetary-scale controlled experiments are impossible in climate science, ‘experimental proof’ will never be found. Note, however, that gambling casinos always rake in a tidy profit, year after year, based on purely statistical strategies.
RE #1, I think what I read was that the “chimney” where the water goes up or down due to salinity aspect has slowed or reduced a bit, and that this reduced the churning up of nutrients for phytoplankton…& sea life in general (which were showing signs of decline).
Comment by Lynn Vincentnathan — 4 Oct 2005 @ 11:08 AM
I read quite a lot about declines in phytoplankton recently. Phytoplankton in the deep oceans is in decline according to NASA, although just now this is being ‘off-set’ by algae blooms near the coast, probably linked to agriculture-run-offs – both developments being disastrous for the marine food chain.
I understand that the global warming on its own affects the churning up of nutrients, and the mixing of water – even without any changes to the THC at all (although a shut-down of the THC or even a slow-down would be absolutely disastrous for marine life). There is a good explanation on http://www.unep-wcmc.org/climate/impacts.htm .
My understanding (I hope I have got this right) is that colder oceans are vastly more productive in terms of plankton than warmer ones. This is because, in warmer waters, you have strong stratification, ie a big difference between warm water at the top and colder water below, with relatively little mixing between them. In colder oceans, the separating layer (thermocline) does not form, or only for parts of the year, so phytoplankton at the top receives nutrients from the deeper sea and provides oxygen for the the upper and deeper layers (as well as nutrients, when phytoplankton decomposes). This means that warmer seas are expected to lead to less productive oceans – something which is not proof of anything changing with regard to the THC or wider ocean circulation.
Here is an interesting series of images of ocean chlorophyll where you can see the seasonal phytoplankton activity (just looking at chlorophyll) : http://www.oceansonline.com/czcs_eatl.htm
These are images of the North Atlantic Bloom. I believe the story goes as follows: winter storms cause a lot of mixing; in the spring light triggers phytoplankton activity, and the mixed layer becomes shallower due to surface warming, so the phytoplankton are not mixed out of the photic zone, and these factors conspire to result in a bloom.
I suppose that to analyze this in detail you would have to couple an atmosphere-ocean type model to a ‘biosphere’ or ‘phytoplankton-sphere’ model. Is this even possible? Perhaps someone could explain the CO2 effect – do phytoplankton reduce CO2 in the atmosphere via ocean burial, or are they thought to be in some steady-state exchange with the atmosphere?
Here’s a nice quote by Spencer Weart (author of Discovery of Global Warming):
“The tangled nature of climate research reflects nature itself. The Earth’s climate system is so irreducibly complicated that we will never grasp it entirely, in the way that one might grasp a law of physics.”
How does research see the mechanism of the global ice mass taking up its latent heat of fusion as giving us a false sense of security regarding global warming?
I imagine that this would stabilise the overt effects of rising temperatures until, area by area, the ice at zero C turns to water at zero C. As that happens, the underlying global warming driver will be progressively loosing its energy sink, and not only will we see ocean rise, but a progressive escalation in the rate of atmospheric temperature rise as well. How does it look?
[Response:The temperature does indeed increase rapidly in areas where the ice/snow has retreated (melted). -rasmus]
Comment by Nigel Williams — 10 Oct 2005 @ 10:59 PM
OK, so are we not only looking for areas of ice retreat (which is obvious), but also getting a handle on the proportion of global ice that is in the I’m-Busy-doing-the-Absorbing-Latent-Heat-of-Fusion-Thing state?