I posted on how Europe might end up cooler rather than warmer as a consequence of global warming and its affect on ocean currents, click here for the post.
Following the post, a reader of this blog, Phil Done, suggested in response to a comment from Ian Mott, that Mott really should read up on the phenomenon at Wikipedia.
Mott, who likes working things out for himself, has had a read at Wikipedia and done a bit of a general google and emailed me his findings as follows:
A check of the first pages of google sites dealing with the claimed ice age that would be produced by the collapse of the 'Atlantic Conveyor' reveals some interesting stuff. Most carry vague descriptions of how this would take place and seem to indicate that it will be caused by a change in northern salinity levels due to melt water from the Greenland Ice Sheet that will prevent this less dense water from submerging and thereby altering the flow pattern. Most carry the claim that evaporation from the gulf stream currently make this body of water very saline and more dense than the rest of the ocean. All point to the disruption of this salinity level by fresh melt water as the primary agent of disturbed flow pattern.
A good example is http:www.enviroliteracy.org/article.php/545.htm which has a curious link to a graphic called "The Atlantic Thermohaline Circulation" at www.clivar.org/publications/other_pubs/clivar_transp/pdf.files/ . Now the most curious thing about this graphic is that the Gulf Stream is shown as flowing due east from New York to Portugal before heading north past the UK. The generally accepted route up the US east coast appears to have been an inconvenient fact to be ignored for the sake of the story. Even more curious is a "cold, saline bottom current" heading north past New Zealand, through the shallows of Vanuatu into the far north pacific where it surfaces between Alaska and Kamchatka where, curiously, it is supposedly warmed in this sub-arctic clime for the journey south.
The brightest spot was at www.awitness.org/column/global_warming_ice_age.html that rightly pointed out that the mini-iceage of the 1400's was no such thing and that the other common example provided 12000 years ago was actually a slight pause in the middle of a period of glaciation with little relevance to this scenario.
But it is the actual numbers involved that reveal the truth. According to www.encyclopedia.com/html/G/GulfS1tre.asp the initial speed of the Gulf Stream is 6.4km/hour over a width of 80km. This slows further north as it widens so for the sake of this analysis we assume the Gulf Stream/North Atlantic Current has a median flow of only 3.6km/hr, is 100km wide and about 500m deep. At this speed the entire trip from Florida to Iceland will only take 70 days. And even assuming zero rainfall, the maximum evaporation is only likely to be 70/365 days x 2000mm evaporation = 383mm evaporation per cycle. And this means the 500 metre thick water column is left to absorb the salt reserves of 0.383 metres of evaporated water before it heads south again. As normal ocean salt level is 3.5% then the 499.617 remaining metres of the water column absorbs 3.5% of 0.383 metres of water, a total of 13.4 millimetres of salt that is added to the 17.5 metres of salt in the column. This is an increase of only 0.07657 of 1%.
And as for the claimed impact of fresh water on the current, we have a total of 180 km3 of water flowing past any given point in the north atlantic each hour. This amounts to 1.577 million km3 each year which disperses into the approx 160 million km3 North Atlantic (40 million km2 x average depth of 3.926km). This north atlantic data is derived from the whole of atlantic data from http://en.wikipedia.org/wiki/atlantic_ocean .
The entire ice volume of Greenland is only 2.44 million km3 so a complete melt over a highly improbable twenty years would add only 0.122 million km3 to the annual current flow of 1.577 million km3, for a total of 1.699 million km3 pa. The existing salt in the NA Current, at 3.5% of volume, will be 0.055195 million km3. And this will only reduce the salinity level of the combined current and melt water by one 14th to 3.25%.
A slightly less improbable 100 year total melt, but still very rapid in climatic terms, would involve only 0.0244 km3 per annum. This would produce a combined flow of 1.6014 million km3 with a salinity of 3.45%. This range is only half the variation normally observed within a 1000 metre ocean profile. See www.windows.ucar.edu/tour/link=/earth/water/salinity_dept
In summary:
The North Atlantic currents are horizontal cycles that flow in a clockwise direction due to the rotation of the Earth. They are doing the same thing as the water in a northern hemisphere bathtub and for the same reason. They are, in most part, not vertical cycles with surface water flowing north and sea floor water flowing south so any theory based on a disturbed flow due to lower salinity from ice sheet melt water is a theory that ignores the primary determinants of current flow, the rotation of the earth.
The theory of increased salinity in the Gulf Stream/Nth Atlantic Current due to evaporation ignores the fact that precipitation also takes place in the same regions. Ocean salinity maps indicate that highest salinity is actually in the middle of the North Atlantic (Sargasso Sea with high air pressure and low rainfall), not in the northern regions.
The volume of water in the Gulf Stream/Nth Atlantic Current is of such magnitude that a complete melt of the Greenland Ice Sheet over a period of only 100 years, contracting North at 27km/year, would dissipate the fresh water to such an extent that salinity levels would only drop from 3.5% to 3.45%. This variation is well within the normal range of ocean salinity levels. To suggest that a minor change in the chemical composition of such a large body of water could override the influence of factors of such magnitude as the rotation of the Earth, itself, is pure fantasy.
Consequently, we must conclude that the Pentagon was rather charitable in describing the Atlantic Conveyor/Ice Age scenario as an extreme scenario and very low probability event. It is, in fact, a highly improbable event that not only extrapolates known effects to improbable extremes but also excludes mitigating factors that are of thousands of orders of magnitude greater.
I am not endorsing Mott's conclusions, but posting them for general discussion.
Interesting, Phil, it seems the author can't decide whether he is writing science or the sequel to Lord of the Rings. Armies marching along the sea floor etc, I was expecting a cutaway to Frodo and Bilbo any moment. The nearest thing to a fact was a volume of 80 amazons.
But what it does highlight is the fact that this vast current is suposedly being driven by a comparatively mild force. This mild sinking of salt water in the north atlantic is assumed to be driving a cycle that includes all the force, volume and velocity of the southern ocean.
Prove me wrong if you will, but logic would suggest that the most dynamic part of a cycle is the portion actually doing the work.
And this brings us back to this assumed accumulated salt load in the north atlantic. The original analysis in my post dealt with an assumed absence of rainfall onto the gulf stream. But if precipitation is assumed to be only 50% of evaporation then the salinity changes will be even less significant. And if we add the very clearly mapped influence of fresher water from the Gulf of St Laurence as it drains the excess of the Great Lakes, then this whole, "heavy with salt" argument is getting very thin indeed. In fact, all the fresh water runoff from all the US Atlantic sea board rivers would ultimately be incorporated into this assumed salty current. And that is why the salinity maps don't show it as being as salty as the mid latitude Sargasso sea.
Ian - you may be interested in this account. Not a journal reference of course.
http://www.zinkle.com/p/articles/mi_m1511/is_n12_v17/ai_18938018?pi=znk
Do you mean, Phil, that the world intellectual community has not been hanging on every word of this discourse? I guess I was misinformed. Seriously, my understanding is that most of the paleo record events, like the younger dryas, took place in periods of glaciation and much lower sea levels. And in the North Atlantic that means an exposed North Sea floor, much shallower floor south of Iceland and an interupted surface flow up the Norwegian Coast and down the Greenland Coast because of expanded ice sheets. This is not relevant to this integlacial conveyer collapse theory.
Ian - a couple of issues - the paleo evidence seems to support the conveyor stopping in the past. Othwerwise what is happening - alternative hypothesis? The debate stems from the paleo evidence combined with ice core evidence of rapid climate shifts.
You're re-writing a fair bit of physical oceanography on the back of an envelope (and that's not an insult - simply a great number of minds have studied the system. I agree the numbers seem large but it's easy to be wrong on this stuff if you're not well briefed on all the fluxes.
Other equivalent latitudes around the world are much colder so Britain owes it warmer climate to the Gulf stream.
Other major planet scale inversions do occur - El Nino for example. Why do these events defeat the predominant physics then and make the spectacular global changes we see? Why the does the circumpolar vortex change ?
Thanks Ender, I concede that the statement should have said something more like;
Given the volume and velocity of all currents operating in the area, vertical thermohaline cycles are not the dominant feature.
It also seem a little unrealistic to suggest that it is purely a saline dependent heightened density that makes a vertical column of water descend for only 4km, but with such power that it overcomes the drag induced by 5000km of ocean floor. This descending water column would certainly not be anywhere near the 3.6km/hour of the Gulf Stream. And it certainly would not be of such magnitude as to create a vacuum that would suck the Gulf Stream into it at a greater speed than the claimed active force.
It would seem far more likely that the descent of North Atlantic water is not based on a salinity/density excess but, rather, the simple fact that when a warm body of liquid or gas meets a cold body then the warm body will flow over while the cold flows under. It is a simple temperature based density excess.
And that is basic fluid dynamics. The northern water is, and will remain, colder than the Gulf Stream for the simple reason that it is from a Latitude that is less condusive to absorbing heat from the sun.
And on that matter we should ask, at what minimum temperature variance would a change in salinity/density override the normal stratification of warm over cold water? And is that minimum variance present in the North Atlantic, or likely to be present, given that the shape of the Earth itself can be assumed to be constant?
Thank you for testing this, Ender & Phil, but the more we look at it the more unrealistic the conveyor collapse theory becomes.
Antartic Ice, I seem to remember someone did an analysis of sea ice extent from whaling ship records, does any one have details?. From memory it seems that antartic sea ice expands or stays constant in some areas while there are large decreases in other areas. The records of single explorers in the artic or antartic are likely to be difficult to interperate without understanding the larger spatial pattens.
Ian - not sure what "our end" means - Australia, where you live or ??
I am informed by El Nino experts that in general there are no cyclical patterns. You can say there are trends i.e. more El Ninos since 1976, wetter decades like the 50s and 70s, but that's about it. {also Google Rob Allan, El Nino, CSIRO.} ANd then of course no two El Ninos are alike !? in terms of rainfall results.
Experienced climatologists worry about trends as you can easily have yourself on statistically speaking with our relatively short 100 year or so record. This of course has been the big trap with sunspot correlations. Fourier analysis will show that there are decadal and multi-decadal bumps. But you will arguments that it's just red noise and not a signal.
To make a claim you need a good quantitative statistical argument, a plausible meteorological physical mechanism or driver, and get some future predictions right. And you have to get excited about as many as you get wrong as you get right. And survive publishing.
But don't let that deter you (seriously!).
First thing to do is let someone who knows stats look at your stats argument.
Ian - "They are, in most part, not vertical cycles with surface water flowing north and sea floor water flowing south so any theory based on a disturbed flow due to lower salinity from ice sheet melt water is a theory that ignores the primary determinants of current flow, the rotation of the earth."
This is what you said. As far as I can see you were disputing the existence of thermohaline circulation - sorry if I took it that way but it seemed pretty clear to me - hence the question.
Ender, you appear to be assuming that I am disputing the existence of thermohaline circulation. Not so. But if surface melt water is being modelled in respect of its interaction with thermohaline circulation then the interaction with surface currents must also be modelled. It is not at all clear that they have been. As for the paleo records, that will have to wait until I get time to take a good look.
Phil, if El Nino is not a cycle then why do the climate records point to approximate 4,8,16 year cycles of rainfall events at our end?