Lomborg-errors: "Cool it!"

A new ice age over Europe?  
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 Comments to pages 87 - 92 in Cool It!

There is a system of sea currents bringing warm saline water from the tropics northward to the northern parts of the Atlantic Ocean. Like Lomborg (his note 585) I find it most easy just to use the term "The Gulf Stream" for all of these currents. They are part of what is popularly called "the conveyor belt" that serves to redistribute heat and salinity between different parts of the world´s oceans. The scientific literature has terms like the Atlantic conveyor belt, the North Atlantic Current, the  Meridional Overturning Circulation (MOC), the  TermoHaline Circulation (THC) and the ThermoHaline Ocean Circulation (THO). These terms are not absolutely synonyms. For instance, MOC may be said to include the effects of wind, temperature and salinity, wheres THC may be said to include onlye the effects of temperature and salinity.

To cite a text that describes the currents (link): "In reality, the Gulf Stream is the western boundary current of a large wind-driven subtropical gyre, that will continue to flow as long as the earth rotates and the winds keep blowing. But the wind-driven gyre is not part of the MOC. The part that branches off between 30°N and 40°N and flows further northward along the West European and Scandinavian coastline constitutes the MOC, and this branch is vulnerable to anthropogenic [man-made] climate change. In the media this branch is often identified with the Gulf Stream; in reality it is one of its many extensions."
Amounts of water flow are stated by a unit called a Sverdrup (Sv). 1 Sv is a water flow of 1 million cubic meters per second, or 31,560 km³ per year. This is approximately the same flow of water that is discharged into the sea from all rivers of the world. If this amount of water stayed in the sea, it would raise sea level by 94 cm per year. The flow of the Gulf Stream is even much larger than this. Off Florida, the flow amounts to more than 30 Sv. Further north, some of this bends off to the east into a gyre that leads its backward towards the tropics. But about half of it continues to the northern parts of the Atlantic. In the north Atlantic, the amount of water that sinks to deeper layers (about 1.5-3.5 km depth) is about 15-20 Sv.

In 1985, scientists published a study based on early computer models indicating that the Atlantic circulation had two distinct stable modes - one with the conveyor on and one with it off  - and that it was relatively easy for it to move from one mode to the other (Schiermeier 2006, cited by Lomborg). Since then, further computer models have been made to study whether the Gulf Stream is bi-stable (can exist in two widely different states, e.g. "on" and "off") or mono-stable (can only exist in one state, "on"). 
    Mono-stability implies that if it is perturbed, it soon reverts to its previous state. That is, even if so  much fresh water is added to the North Atlantic that the overturning circulation is stopped, once the fresh water addition stops, the overturning will start again. On the other hand, if it is bi-stable, then at given conditions with a given amount of fresh water addition, there are two possible states of the circulation - there is an "on" mode and an "off" mode, and which of the two is found does not depend on present conditions, but on what happened in the past. For instance, if so much fresh water is added to the North Atlantic that the overturning circulation is stopped, then the circulation is brought into "off" mode, and even if the fresh water addition is halted and conditions return to what they were, the circulation remains in the "off" mode. If the fresh water addition is large so that we approach the condition where we switch from one mode to the other, then random fluctuations may be enough to push the circulation into the "off" mode, and it will then stay there. That is, the risk of a mode switch depends not only on the amount of fresh water addition, but also on the degree of fluctuations. Furthermore, a rapid change in the amount of fresh water is more likely to cause a switch than a gradual change.
   To study the risk of a sudden shift from one mode to the other, i.e. an "abrupt climate change", some have used simple models with just two boxes, a box representing the cold northern part of the ocean and a box representing the warm part closer to the Equator. Results are that some of these simple models imply mono-stability, others bi-stability, depending for instance on whether and how wind stress is built into the model. More realistic studies apply complicated atmosphere-ocean general-circulation-models (AOGCMs). Again, some of these show mono-stability, others bi-stability. It is difficult to find out what details cause the difference; you would have to make many runs with slight modification of various parameters to find what parameters are decisive in determining the outcome. However, these models require so large computing resources that this is not practically possible. Instead, such studies must be made with earth system models of intermediate complexity (EMICs). These have a very coarse grained representation of world geography and reduced complexity in the modeling of the ocean, the atmosphere, or both. However, they still contain many details which can be varied systematically to study what details make the difference between mon-stability and bi-stability.
    Recent research with such models seem to have found one such crucial parameter; it has to do with what happens at the south border of the Atlantic Basin between South America and the south tip of Africa. There is a net loss of fresh water in the central and north Atlantic due to evaporation, and this fresh water is replaced mainly by import from the south into the Atlantic Basin. This import can take two different forms, depending on the difference in salinity between the east coast of South America and the west coast of Africa. A Dutch group of scientists have worked with EMICs in which this difference in salinity in this part of the ocean is varied. They found that with a low difference in salinity, the overturning circulation tends to be mono-stable, whereas with a high difference in salinity, the circulation is bi-stable. Now, all the complicated models (AOGCMs) that are referred to by the 4th IPCC report, have a low difference in salinity here. But actual observations show that there is a high difference in the real world. The implication is that all complicated models used up to now underestimate the instability of the overturning circulation. That is, even when the majority of all models, or the more complicated models, show a certain outcome, this outcome need not be the more realistic. The minority could be right, so to say. The whole issue is explained in more detail in this
link (see also de Vries et al. (2005): Geophysical research letters vol. 32 (link)).


    The water in the northern branch of the Gulf Stream will gradually become denser and heavier during its northward flow for two reasons: 1) it cools, and cold water is heavier than warm water. 2) it becomes more saline, partially because of evaporation, and partially because some of the water is brought out of the liquid phase by freezing. When it has become suffficiently heavy, it sinks.    
    In the case of global warming, this will shcnage somewhat. Air temperatures will rise more in the Arctic than further south, and so the northflowing water will cool less than before. Furthermore, in the warmer climate, more rain will fall at northern latitudes, making the sea water surface more fresh. This is partially due to increased precipitation directly on the ocean surface, and partially due to increased riverflow via Canadian and Siberian rivers into the Arctic Ocean, from where the water flows into the North Atlantic. Altogether, these processes cause so much freshening of the North Atlantic surface water that the mechanism behind the overturning circulation is weakened condsiderably. On top of this, there may be an extra fresh water pulse from melting of Greenland ice. However, this is not the largest contribution, and in some computer runs to study the risk of a halt of the circulation, the contribution from Greenland is not even included. Thus, the very large focus by Lomborg (and by Al Gore) on melt water from Greenland is somewhat misplaced.
    In the years leading up to the new milennium, there has been a considerable freshening of the North Atlantic, with about 19,000 km³ of fresh water added between 1965 and 1995 (Curry & Mauritzen (2005): Science 308: 1772-1774). By the turn of the millennium, this led to a fear that the erosion of the overturning circulation had gradually begun. However, around 2000, some new accumulation of more saline water was seen.
   All models forecast that there will be an increased influx of fresh water to the North Atlantic during the 21st century. The maximal estimates of this inflow, minus the loss due to increased evaporation, are about 0.14 Sv (Stouffer 2006, cited by Lomborg). The fresh water flux from melting of Greenland ice may some time far into the future reach values of up to 0.1 Sv. If increased ice discharge due to lubrication of the basis of the ice is taken inot account, a flux of this size order may occur earlier. This flux should then be added to the flux from other sources. The maximum total fresh water flux that might occur by the end of the 21st century might be about 0.2 Sv, but the most likely value is considerably lower.
    Now, how large af lux would be sufficient to bring the overturning circulation to a halt? In this respect, various models of intermediate complexity as well as various complicated models differ widely. Their sensitivity to differences in fresh water flow is very different.
   Rahmstorf et al. (2005: Geophyscial research letters 32: L23605) coordinated a study of the Atlantic overturning circulation using 11 different models of intermediate complexity. In all of these, the circulation was bi-stable under certain conditions. In 7 models, the circulation was clearly bi-stable under present conditions. The increase in fresh water flux that would cause a shutdown of the circulation varied from 0.1 - 0.15 Sv for the four most sensitive models (which were already bi-stable), the other models shut down only when the flux increased to 0.2-0.3 Sv or even up to 0.55 Sv for the single model with the most stable circulation. The conclusion from this is that once the fresh water flux reaches 0.1 Sv, there is a real risk of shutdown of the northern branch of the Gulf Stream. And if the flux reaches 0.2-0.3 Sv, the risk is very great. To this should be added the warning (see above) that many models have an unrealistic pattern of salinity in the South Atlantic, if this were corrected, they would become more sensitive.
   If you compare this with the forecasts of the increase in fresh water flow, you see an overlap. The most pessimistic scenarios imply a flow of up to 0.2 Sv, and the most sensitive models break down already at litlle more than 0.1 Sv. So a shutdown is a realistic possibility. Not in the near future, but probably at the end of the present century.
   The text of the 4th IPCC report from 2007 seems to downplay the risk of a shutdown of the overturning circulation. The text has that "none of the current models simulates an abrupt (nonlinear) reduction or a complete shut-down in this century. There is still a large spread among the models’ simulated reduction in the MOC, ranging from virtually no response to a reduction of over 50% by the end of the 21st century." This is a simplified message. First, it deals only with the outcome of complicated models (AOGCMs), and as we have seen above, all AOGCMs applied up to now have a parameter setting regarding the salinity in the South Atlantic which causes the model to underestimate the sensitivity of the overturning circulation. Some models of intermediate complexity (EMICs) do forecast a shutdown aalready at the end of the 21st century. Second, what is reported tends to be the `central estimates´. Imagine that we run a model one hundred times, each time with slight variations of some parameters. Imagine that the `central estimate´ is that the MOC is reduced by 40 %. This means that out of a hundred runs, fifty would show more than this and fifty would show less than this. In that case, there may still be a considerable number of runs which show a 100 % reduction. Such an ensemble of 100 runs cannot be run with the most complicated models, because of computation time, but it can be run with EMICs. One such study is a study by Challenor et al. (2006), published in a publication which Lomborg has consulted. See comments to page 122 bottom. Here, the authors found a near-total shudown before 2100 in 37-38% of the runs. Thus, when we consider the probability distribution of the outcomes, the risk of a shutdown before 2100 may be considerably above zero, even though the central estimate is just a moderate reduction. IPCC has chosen to disregard such probability distributions.
Altogether, it is very likely that there will be no shutdown of the overturning circulation before 2100, even though the probability of this happening should be considered to be larger than zero, and even though there will most probably be some reduction in the circulation. 


    The question of the possible shutdown of the Gulf Stream should be grist to Lomborg´s  mill, because here there has been quite some hype about a possible event that is not likely to occur before far into the future. If Lomborg´s intention is to present the scientific matter and to assure his reader that there is no imminent risk of a shutdown of the Gulf Stream, he could have done this very easily simply by referring to the latest IPCC report. There would be no need to manipulate any text - the points are free to be taken. Why did he not simply do that?
    It seems that in addition to assuring the reader that there is no danger, Lomborg has another main agenda. That is to describe how much the whole issue has been misrepresented by the media. In the British version of Cool It, he goes further and paints a picture of a whole bunch of untrustworthy persons, ranging from Al Gore and journalists to many scientists who appear to exaggerate the worries. Apparently he tries to create mistrust in the authorities. The available evidence is not enough to do this effectively, and so Lomborg himself exaggerates the unreliability of all opponents. In doing this, he produces a surprising lot of flaws and errors.
    So it is his attempt to paint the picture of untrustworthy media and untrustworthy authorities that Lomborg manipulates so much - especially in the British version of the book, whereas the American version avoids some of the controversial points. 

Flaws on particular pages in Lomborgs text:

Note to page 87: "This is the story Al Gore tells . . . although he gets the timing wrong." Comment: Lomborg has misunderstood this. The episode that Gore refers to is the onset of the period called the  Younger Dryas about 11,500 years before present (rounded by Al Gore to `around 10,000 years ago´.  This event was the start of a cold period lasting 900 to 1,000 years. It is different from the episode referred to by Lomborg at about 8,200 years before present.

Page 87 bottom: "an unprecedented amount of fresh water flooded the North Atlantic . . . " Comment: In the previous episode at about 11,500 years ago, which marked the start of the period called Younger Dryas, the amount of fresh water released was probably much larger.

Page 87 bottom - 88 top: "This pushed Europe into a little ice age for almost one thousand years." Flaw: Lomborg confuses the episode at 11,500 years ago and that at 8,200 years ago. The formed caused a return to an ice age climate for another thousand years. The latter caused relatively smaller temeprature drops for about 400 years.

Page 88 top: ". . . the IPCC expects Greenland to melt almost one thousand times less . . .  " Error: The figure of a thousand times is calculated by Lomborg and is not to be found in any IPCC publication. The basis of the calculation can only be seen in the British version of Cool It. Lomborg takes
the very lowest of all the IPCC estimates for the melting of Greenland ice during the 21st century (this figure corresponds to the amount of melt water that Greenland already now dumps per year, as an average of various studies (A.Cazenave &W. Llovel (2010): Contemporary sea level rise. Annual review of marine science 2: 145-173)). The amount of water released into the sea at the event 8,200 years ago was, according to Lomborg´s source, about 200,000 km³. This would suffice to raise the world´s sea level by 60 cm. The maximum estimate of how much ice melt from Greenland could contribute over a century, in case of very rapid global warming, is also 60 cm (6 mm per year, according to the one of Lomborg´s sources, Stouffer et al. (2006)). So the fresh water pulse at the event 8,200 years ago is not a thousand times the expected fresh water pulse from Greenland; rather it is about the same size as the very largest (and not very likely) forecast for the fresh water pulse from Greenland during 100 years from now on.

Page 88: " . . . what would happen if Greenland melted at triple the rate expected by the IPCC . . . " Comment: Lomborg gives us the impression that the model study is at the upper extreme of what the melt rate could be, and probably three times the most likely melt rate. This is indeed what is stated by the authors. Actually, concerning the rate of Greenland melt, the authors use a base scenario (A1B) which yields a contribution to sea level rise of 10 cm during the 21st century. A tripling of the basic rate would mean a contribution to sea level rise of 30 cm during the 21st century. This is approximately the same size order as the IPCC maximum estimate if we include the possible effexts of increased flow of ice sheets due to lubrication of their base.  In addition, the authors note that whereas some models are relatively sensitive to changes in the fresh water fluxes, their model is not so sensitive. The reasons for such differences between models are not very well understood. In any case, the effects to be expected are probably small relative to certain other models.

Page 88: " . . . who put the theory forward one week before world leaders met in Japan . . . " Comment: Lomborg infers that the theory was more or less invented in order to affect the political process in Kyoto. But Broecker published his theory in Nature in 1985, more than ten years before the Kyoto meeting. However, it is true that Broecker did publish another paper in Science in 1997 at the time of the Kyoto meeting.

Page 89 top: "In 2004, Fortune magazine revealed . . . " Comment: The article in Fortune did include certain caveats. It said that scientists generally refuse to say much about abrupt climate change, citing a data deficit, and it tells that the scary scenario was produced by non-scientists. It also says: "It doesn't pretend to be a forecast. Rather, it sketches a dramatic but plausible scenario to help planners think about coping strategies."

Page 89 top: "Now the Pentagon tells Bush . . . " Remark: Lomborg is right in criticising the article in The Observer, which goes much too far in painting a world-wide disaster which apparently "will" happen soon.

Page 89: " . . and by 2020 Europe´s climate is more like Siberia´s " Comment: It is obvious from the part of the text cited by Lomborg that this does not include South Europe, and on the next page of the document, it is said that this refers to Northwest Europe. The temperature drop by 2020 is indicated as 6° F (= 3.3° C).

Page 89: "Aggressive wars are likely to be fought . . . " Comment: The whole idea of the Pentagon report is to produce a scenario in which certain types of conflict and war would arise, in order that Pentagon could think over how they would prepare for such eventual events. The whole thing is conceived as a thought experiment, in which the exact dates are just inserted to dramatize the account. If no wars had been included in the scenario, it would not have fulfilled its purpose.

Page 90 top: "Yet, the problem with these terrifying forecasts . . . " Flaw: They are not forecasts. The authors of the Pentagon report write: "Rather than predicting how climate change will happen, our intent is to dramatize the impact climate change could have . . ". The Fortune article explicitly said "It doesn't pretend to be a forecast". And nobody will be in doubt that The Day After Tomorrow is a piece of fiction, not a forecast.

Page 90 top: "  . . .Europe probably cooled some some 2.7° F . . " Comment: Lomborg´s main source for this event, Barber et al. 1999, say 2.7 - 5.4° F for northwest Europe.

Page 90 top: " Model estimates project the same kind of drops from future disruptions . . . " Comment: Lomborg is very unprecise here. What kind of disruptions? However, the sources given in the note give an indication. Stouffer et al. (2006) deal with two scenarios. One is a moderate and rather likely situation - a 30% reduction of the Gulf Stream, which would reduce temperatures on continental Europe by 0.5-1.0° C, and somewhat more in Britain; the other is a complete shutdown of the northern branch of the Gulf Stream, which would cause temperature drops of about 4° C in central Europe and up to 10° C (18° F) in northwesternmost Europe. These are very drastic drops, giving conditions reminding of an ice age. The other study is Wood et al. (2003). For a total shutdown of the northern branch of the Gulf Stream, they get immediate temperature drops of 3-5° C in Britain and less on the continent.

Page 90: " . . . the MIT ocean physicist Carl Wunsch . . . " Comment: The reader´s letter that Wunsch wrote to Nature in 2004 was overstated. Wunsch is right that since the winds will continue to blow and the Earth continue to turn, the energy source for the Gulf Stream will remain. But as stated in the general comments on top of this page, this relates only to that part of  the Gulf Stream which stretches up to about 30° N. North of that, the northgoing branch of the current, which feeds the Meridional Overturning Circulation,  is more sensitive to changes in various forces, and this branch may be affected by man-made climate change. See also the comments to Wunsch here. Furhtermore, it remains true that it is not unthinkable that the Meridional Overturning Circulation may shut down, as demonstrated by reconstructed prehistoric events and indicated by certain model runs.
  However, the letter (which was repeated in an issue of the Economist) made him popular among climate skeptics, and was probably a main reason why he appeared in the film `The great global warming swindle´. However, Wunsch subsequently protested against the way he was used in that film, and the filmmakers had to make a revised version in which he was cut out. In a declaration printed in Science no. 328, May 7th 2010, p. 689, it is said that: "There is complete, comprehensive, and consistent objective evidence that humans are changing the climate in ways that threaten our societies and the ecosystem on which we depend." Among the many scientists who signed this declaration is Carl Wunsch.

Page 90 bottom: "The headlines were predictable . . ." Comment: Lomborg wants to give the impression that several public media try to overdramtize and to scare its readers. Is this impression true? Not quite. Some media report the story without proper reservations. This was the case for The Independent. The Times had an article focusing on negative news, but did remember to state at the end that the data from 1992 and 2004 could be mere aberrations. Also, National Geographic remembered to write that it is too early to say if we are just seeing natural variability, and that there is no reason for immediate concern. Sydney Morning Herald wrote that not enough measurements had been made to rule out natural variability. New Scientist (3rd Dec. 2005) quotes Bryden that he is not yet sure if we see a long-tem trend. BBC News (30th Nov 2005, link) has a large paragraph with the heading `Natural variation´, stating that the the trend could be down to natural variability. Altogether, out of six media, only one forgets to mention the necessary reservations. Considering the usual standard in public media, this is not very bad. 

Page 90 bottom: "New Scientist chose the news as one of its top stories from 2005 . . " Comment: New Scientist had a lengthy and not very misleading article on 3rd Dec. In a review of the main news of the year on 24th Dec. (this is the one referred to by Lomborg), they had a much shorter notice, in which the Gulf Stream issue took up only five lines. This short notice mentions that there is a ´worry´, but does not stress the uncertainty further than that.

Page 91 top: " . . . which included a reference to The Day After Tomorrow". Comment: The reader will probably believe that the reference to this film is yet another part of the sensationalism and scare-mongering. It is nearly the opposite. Twice in the article, the journalist stresses that The Day After Tomorrow is not realistic. She writes "Researchers have dismissed the idea the climate would ever change as rapidly as depicted in The Day After Tomorrow".

Page 90-91: Altogether, the whole paragraph starting with "The headlines were predictable . . ." gives an impression of the media coverage which is not accurate and somewhat misleading.

Page 91: "In New Scientist the headline was . . . ": Comment: New Scientist had a lengthy article on the subject already on 15th April 2006 by Stephen Battersby.

Page 91: "None of the current models simulates an abrupt reduction or shut-down." Flaw: Lomborg has truncated the sentence. The original sentence is: "none of the current models simulates an abrupt reduction or shut-down in this century." The missing three words at the end of the sentence are important and should not have been left out.

Page 91 bottom: " . . . but no models show a complete shutdown." Comment: See the paragraph at the top of this page titled:
WHAT IS THE RISK FOR A SHUTDOWN OF THE ATLANTIC OVERTURNING CIRCULATION ? Here it is said that the cited text refers only to the most complicated models, the AOGCMs, and that these may all be flawed in one crucial respect. There are a number of models of intermediate complexity (EMICs) that show a complete shutdown. In the most sensitive models (which are not more unrealistic than any others) a fresh water flux of little more than 0.1 Sv suffices to cause a shutdown of the circulation, and a fresh water flux of that size order is not unthinkable at the end of the 21st century or some way into the 22nd century. So although the risk of a shutdown is small, it is not zero. 

Page 92: "As the IPCC points out, Europe will still warm, even if the Gulf Stream shuts down completely" Error: The source for this statement is the 4th IPCC report (its question 10.2), but IPCC did not exactly say this. Their text is about "a gradual reduction of the MOC that continues even after climate is stabilised". As to what will happen in case of a clear-cut shutdown of the northern branch of the Gulf Stream, w
e know that Lomborg has consulted two papers on this issue by Wood et al. from 2003 and 2006.  He ignores what is said in these. They present a run with one of the advanced models (an AOGCM), in which a shut-down of the Gulf Stream is forced upon the model in the year 2049, following upon the degree of warming that has occurred up to that point. The result is shown in figures in these papers and is referred to as follows in the text: "We see that around the North Atlantic, the cooling effect of the THC change more than outweighs the effects of global warming, leading to a net cooling relative to the pre-industrial climate in those regions." and in another place:  ". . . such a shutdown would return northwestern Europe in particular to a climate that was substantially colder than pre-industrial, and the pontential rapidity and unpredictability of such a change could make adaptation particularly difficult." To put the predicted temperature drops of up to 3-5° C in Britain in perspective, they mention that during `the Little Ice Age´, average temperatures in central England were cooled by about 0.5° C. So the model´s cooling is much, much worse than in the seventeenth and eighteenth centuries. This part of the presentation was also referred to by an article in the Guardian that Lomborg has read.  Considering that Lomborg has read the papers by Richard Wood et al., and has read the Guardian article, it is gross sloppiness that he overlooks this.