Archive for the ‘meridional overturning circulation’ Category


More on Thermohaline Circulation

June 16, 2009

In a previous post “The Thermohaline Circulation Only Stops for Extreme, Unrealistic Models,” I compared the amount of fresh water used in “hosing experiment” models to drastically reduce the thermohaline circulation (THC, or Meridional Overturning Circulation, MOC) to the amount of water flowing over Niagara Falls, or flowing from all rivers into the Arctic,  or coming off of Greenland due to melting ice.

The key number was one Sverdrup, or 1 million cubic meters of fresh water per second.  One Sverdrup of fresh water artificially dumped into the Labrador sea, for 100 years would have the feared effect.  But it turns out that one Sverdrup of fresh water is 350 times the amount of water flowing over Niagara falls, and about 300 times the amount of water from melting ice that flows off of Greenland.  It was seen that there is not plausible source for this amount of extra fresh water to be dumped into the arctic.

An interesting letter that appeared in Science a year ago gives a little more perspective,  So I have reproduced it in full here:

Freshwater Forcing: Will History Repeat Itself?

IN THEIR RESEARCH ARTICLE “REDUCED North Atlantic deep water coeval with the glacial Lake Agassiz freshwater outburst” (4 January, p. 60), H. F. Kleiven et al. present compelling evidence for an abrupt deep-ocean response to the release of freshwater from glacial Lake Agassiz into the northwest Atlantic about 8400 years ago. Such data are particularly important in evaluating the response in ocean models of the Atlantic Meridional Overturning Circulation (MOC) to freshwater forcing. For this event, the freshwater forcing was likely large but short; Clarke et al. (1) estimate that the flood had a freshwater flux of 4 to 9 Sv [Sverdrups] released in 0.5 years.

In this context, we are aware of no possible mechanism that might reproduce such a forcing in response to global warming, and all available model simulations, including those with estimates of maximum Greenland Ice Sheet (GIS) melting rates, indicate that it is very unlikely that the MOC will undergo an abrupt transition during the course of the 21st century (2). Multimodel ensemble averages under Special Report on Emissions Scenario (SRES) A1B suggest a best estimate of 25 to 30% reduction in the overall MOC strength (2). In one example, 14 coupled models simulated a 100-year 0.1-Sv freshwater perturbation to the northern North Atlantic Ocean—17 times the recently estimated melt rates from the GIS [Greenland Ice Sheet]—and the MOC weakened by a multimodel mean of 30% after 100 years; none of the models simulated a shutdown (3). Another model simulated greenhouse gas levels that increased to four times preindustrial values and then remained fixed; the resulting GIS displayed a peak melting rate of about 0.1 Sv, with little effect on the MOC (4). One model simulation uses the SRES  freshwater forcing as an upper-bound estimate of potential GIS melting. In this case, the MOC weakened but subsequently recovered its strength, indicating that GIS melting would not cause abrupt climate change in the 21st century (5). Accordingly, we urge caution in drawing comparisons of the abrupt change 8400 years ago to future scenarios involving, for example, the melting of the GIS and its relevance to human societies.

1Department of Geosciences, Oregon State University, Corvallis, OR 97331, USA.
2Geophysical Fluid Dynamics Laboratory/NOAA, Princeton, NJ 08542, USA.
3School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8W 3P6, Canada.

1. G. K. C. Clarke, D. W. Leverington, J. T. Teller, A. S. Dyke, Quat. Sci. Rev. 23, 389 (2004).
2. G. A Meehl et al., in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, S. Solomon et al., Eds. (Cambridge Univ. Press, New York, 2007), pp. 747–845.
3. R. J. Stouffer et al., J. Clim. 19, 1365 (2006).
4. J. K. Ridley, P. Huybrechts, J. M. Gregory, J. A. Lowe, J. Clim. 17, 3409 (2005).
5. J. H. Jungclaus, H. Haak, M. Esch, E. Roeckner, J. Marotzke, Geophys. Res. Lett. 33, 10.1029/2006GL026815 (2006).

So, the event that occurred 8400 years ago involved 4 to 9 Sverdrups of fresh water.  This is THOUSANDS of times greater than the flow of the Niagara Falls today.  It is THOUSANDS of times greater than the amount of fresh water flowing from melting Greenland ice today. It is multiples bigger than the entire fresh water budget into the Arctic.

Note that in my previous post I referred to hosing experiments that pumped up to one Sverdrup of fresh water into the oceans.   The authors of the above letter refer to hosing experiments that used only 0.1 Sverdrups – yet they still point out how gigantic this is compared to actual sources of fresh water in the Arctic today.

So, when Al Gore ominously implies that that the Greenland Ice Sheet [GIS] is going to melt down and dump enough fresh water into the Atlantic Ocean to shut down the Thermohaline Circulation, remember the works of Clarke,, in the above letter: “we urge caution in drawing comparisons of the abrupt change 8400 years ago to future scenarios involving, for example, the melting of the GIS [Greenland Ice Sheet] and its relevance to human societies.”


The Thermohaline Circulation Only Stops for Extreme, Unrealistic Models

June 4, 2009

Return to Criticisms of Al Gore’s “An Inconvenient Truth”

Gore gives a cartoon description of the ocean circulation system when he explains what has become known as the thermohaline circulation, or the meridional overturning circulation.  In his simplistic scenario the surface ocean current that flows north in the Atlantic, bringing warmth to northern Europe will be halted by melting ice from Greenland, subsequently throwing Europe into an ice age. 

Here is Gore’s explanation in his own words from the Inconvenient Truth movie:

The Earth’s climate is like a big engine for redistributing heat from the equator to the poles.  And it does that by means of ocean currents and wind currents.  They tell us, the scientists do, that the Earth’s climate is an non-linear system – just a fancy way they have of saying that the changes are not all just gradual, some of them come suddenly, in big jumps… And so, all those wind and ocean currents that have formed since the last ice age and have been relatively stable - they’re all up in the air – they change. 

And one of the ones they’re most worried about, where they’ve spent a lot of time studying the problem is in the the North Atlantic where the gulf stream comes up and meets the cold winds coming off the Arctic over Greenland and that evaporates the heat out of the gulf stream and the steam is carried over to western Europe by the prevailing winds and the Earth’s rotation.  But isn’t it interesting that the whole ocean current system is all linked together in this loop, they call it the ocean conveyor.

vlcsnap-324533And the red are the warm surface currents, the Gulf Stream is the best known of them.  But the blue represent the cold currents running in the opposite direction…

vlcsnap-32114Up in the North Atlantic, after that heat is pulled out, what’s left behind is colder water, and saltier water, because the salt doesn’t go anywhere. And so, that makes it denser and heavier.  And so that cold heavy dense water sinks at the rate of 5 billion gallons per second.  And then that pulls that current back south.ani-21

At the end of the last ice age as the last glacier was receding from North America the ice melted and a giant pool of fresh water formed in North America, and the Great Lakes are the remnants of that huge lake.  An ice dam on the eastern border formed, and one day it broke, and all that fresh water came rushing out, ripping open the St. Lawrence there, and it diluted the salty dense cold water, made it fresher and lighter so it stopped sinking, and that pump shut off.

 vlcsnap-549956-smallAnd the heat transfer stopped.  And Europe went back into an ice age for another 900 to 1000 years.  And the change from conditions like we have here today to an ice age took place in perhaps as little as ten years time.  So that’s a sudden jump.  Now, of course, that’s not going to happen again because the glaciers of North America are not there… Is there any other big chunk of ice anywhere near there…?  Oh, yeah [Gore says ominously, as the image pans to ice covered Greenland] we’ll come back to that one…

Later in the movie Gore tells us that Greenland is rapidly melting.  The point being that it will provide a massive amount of fresh water that will stop the the thermohaline conveyor and  “would raise sea level almost 20 feet if it ‘went,’” Gore tells us.  He tells us about water seeping to the bottom of the ice sheets where it “lubricates where the ice meets the bedrock” causing the ice to slide toward the ocean.

Then he shows a series of pictures purporting to show the amount of melting in Greenland.  Gore says…


“In 1992 they measured this amount of melting in Greenland … Ten years later this is what happened…And here’s the melting from 2005″


Hosing Experiments

But what if…?  What if there were a huge amount of low density fresh water dumped into the North Atlantic where the high density water is supposed to be sinking, just like the giant Canadian lake crashing through the barrier of ice the Gore told us about?  This possibility is explored with computer models known as  “hosing experiments.”  In a hosing experiment a model that simulates the ocean and atmosphere circulation patterns is modified to artificially dump huge amounts of extra fresh water, as if from a giant hose, into some location in the ocean.   It has been found that when enough fresh water is forced in, the circulation can be slowed, but rarely stopped

How much fresh water do the hosing experiments use to nearly stop the thermohaline circulation?  Typically (or here), they use one million cubic meters of fresh water per second, for 100 years!!!  (One million cubic meters per second has its own unit name: One Sverdrup or 1 Sv).  How does 1 Sv compare to, say, the rate of water flowing over Niagara Falls?

Niagara falls168,000 cubic  meters of water fall over Niagara Falls every minute.  That is about 2,800 cubic meters of water per second.  So one Sverdrup of water is the same as about 350 Niagara Falls!  (1,000,000 / 2,800  = 357).  So, roughly speaking, if 350 Niagara Falls were dumped into the oceans around Greenland continuously for 100 years, then we could expect to see a significant slow down of the thermohaline circulation.

River systems discharging into the Arctic Ocean.

River systems discharging into the Arctic Ocean.

How does one Sverdrup compare to the freshwater discharge of ALL the rivers emptying into the arctic ocean?  One Sverdrup of fresh water amounts to nearly 32,000 km3 of water per year  (1 Sv  x 106 m3 s-1/sv x (86,400 s/day) x (365 day/year) = 31,536 km3/year).  The total fresh water discharge from all rivers into the arctic is only about 4,300 km3 per year.  So, typical hosing experiments that nearly stop the overturning circulation add a water volume about 7 times the amount of water from all rivers discharing into the Arctic Ocean combined.

What about Greenland?

Hosing copyGore ominously implies that the amount of fresh water needed to turn off the overturning circulation is just waiting to pour off of  Greenland, due of course (drum roll), to CO2 induced anthropogenic global warming.   His pictures of Greenland, shown above, imply that about half of Greenland’s 2.8 million cubic kilometers of ice have melted in the 13 years between 1992 and 2005.  This is wildly misleading.  Only a miniscule fraction of the area shown in Gore’s Greenland images actually melts every year.   This is evidenced by mass balance studies, which show Greenland loses on the order of hundred cubic kilometers of ice every year,  which translates into a measly 0.003 Sverdrups.

100 km3 /year= 1011 m3/year

(1011 m3/year) / (365 days/year) / (86,400 seconds/day)
             = 3 x 103 m3/second
             = 0.003 Sv

Put another way, one Sverdrup of fresh water is 86.4 km3/day.  So the hosing experiments pouring in one Sverdrup put about as much fresh water into the ocean each day (86.4 km3) as Greenland provides in a year (100 km3).

But if Greenland actually started melting, by some extraordinary circumstance,  300 times faster, then it would yield 1 Sverdrup, or 1,000,000 cubic meters, of fresh water every second.  What would happen after 100 years of melting at that rate?  Well, that’s a trick question, because at a melting rate that gives 1 Sverdrup of freshwater Greenland would run out of ice in about 90 years.  This is because Greenland has only 2.85 million cubic kilometers of ice, and one Sverdrup of water is the same as about 31,500 cubic kilometers of water per year.  Ignoring the difference in density between ice and water, then 2.85 million cubic kilometers divided by 31,500 cubic kilometers per year gives 90 years.


You don’t hear as much about the threat of the collapse to the thermohaline circulation today as you did a few years ago.  This is because it has become recognized as being a very far fetched possibility, even by most alarmists who want to maintain a shred of dignity.  But I have a feeling we will not see this wildly exaggerated threat removed from new editions of Gore’s “An Inconvenient Truth” anytime soon.

Return to Criticisms of Al Gore’s “An Inconvenient Truth”


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