Lomborg-errors: "Cool it!"

A new ice age over Europe?  
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 Comments to pages 117 - 125 in Cool It!

 GENERAL COMMENTS:
 
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.


 IS THE GULF STREAM BI-STABLE ?
 
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)).


 
 WHAT IS THE RISK FOR A SHUTDOWN OF THE ATLANTIC OVERTURNING CIRCULATION ?

    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.


 COMMENTS TO LOMBORG´S PRESENTATION

    In Lomborg´s text on the Gulf Stream, there is an extraordinary amount of flaws, as demonstrated below. He goes into many details about what various persons have said and written, and in doing this, he repeatedly misrepresents the evidence. 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. In that case, he would not have risked to make so many flaws. 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 many persons, and to paint a picture of a whole bunch of untrustworthy persons, ranging from Al Gore and journalists to many scientists who appear to exaggerate the worries. In other words, the agenda is 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.
    So the many flaws are not mainly flaws in representing the scientific evidence. They are flaws aiming at putting his opponents in a bad light.


Flaws on particular pages in Lomborgs text:


(COMMENT)
Page 117: "However, according to Gore, climate scientists are now realizing that the Gulf Stream is "surprisingly fragile".
Comment: Gore may be said to be right here.

(COMMENT)
Page 117: "About 10,000 years ago, when the last glacial . . . "
Comment: The episode that Gore is referring to here is the onset of the period called the  Younger Dryas about 11,500 years before present. 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.

(COMMENT)
Page 117: "an unprecendented 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 larger.


MISUNDERSTANDING
Page 117 bottom: "For the last and largest event . . . ". Lomborg has read in the sources that the event at 8,200 years BP was the largest event. However, he has overlooked what the sources state, viz. that it was  the largest event in the Holocene. The Holocene (the present period) started per definion 10,000 years ago. The other events at 12,500 and 11,500 years BP were much larger than that of 8,200 years BP as regards the drop in temperature (this drop is known with some certitude) , but they occurred while the world was still at the end of the ice age. The event 8,200 years BP may have been the largest in terms of the amount of fresh water that was suddenly released (but this amount is not known with certitude). Al Gore speaks of the event at about 11,500 years BP. The reason why scientists are so interested in the smaller temperature drop event 8,200 years ago, is that it set in at a time when temperatures were roughly the same as at present.

FLAW
Page 118 top: ". . . the IPCC expects Greenland to dump some 126 km³ of water per year . . .  "
Flaw: That amount is about what 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). With rising temperatures, the annual contribution will of course grow larger than this. Lomborg´s figure is based on the claim that the contribution from Greenland to sea level rise will be 3.5 cm. As explained in the notes to p. 78, this is however the very lowest of all the IPCC estimates, and it excludes the contribution from increased ice flow due to lubrication. When Lomborg, as he does here, presents this figure as if it were a central or authoritative estimate, rather than the minimum estimate that it actually is, he is deliberately biased.

(COMMENT)
Page 118 top and note 592: " . . some 126 km³ of water per year . . .  ". Comment:
Lomborg´s calculation is erroneous. 361 million km² is the total area of water on the globe, including fresh water. The area of the world´s oceans is 335 million km². Using this correct figure, we get 117 km³ instead of 126 km³. However, this is a minor error and it is not important, as the basic assumption of 3.5 cm is biased anyhow. So it is not counted here as an error.

ERROR
Page 118 top: ". . .almost a thousand times less than what happened in the story Gore tells us. . .  " Error: 
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.
    However, the event 8,200 years ago is not the story that Gore told us about. Gore told about the event 11,500 years ago, when the amount of fresh water released was probably larger.

(COMMENT)
Page 118: ". . . "The possibility of such extreme events precludes ruling out that  disruption  . . .  ""
Comment: The original quote, apparently changed somewhat by Al Gore, seems to have been taken from the following paper: R. Curry & C. Mauritzen (2005): Science 308: 1772 - 1774. This paper ends with the following statement: "The possibility of such events precludes ruling out a substantial slowing or shutdown of the overflows as a result of greenhouse warming." Readers who want a more precise explanation of what was meant here are referred to that paper. 

(COMMENT)
Page 118: " . . . 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. This is more than the 3.5 cm postulated by Lomborg, but less than the 25 cm which is the upper range of the IPCC estimates. 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. 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.

(COMMENT)
Page 118 bottom: " . . . who put the theory forward one week before the world 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.

(COMMENT)
Page 119 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."

REMARK
Page 119 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.

(COMMENT)
Page 119: "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).

FLAW
Page 119 bottom: "Yet, the problem with these scary 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.

(COMMENT)
Page 120 top: "But most of northern Europe saw temperatures 0.5-3° lower . . " Comment: The authors of the Pentagon report write: "The future scenario that we have constructed is based on the 8,200 years before present event, which was much warmer and far briefer than the Younger Dryas, but more severe than the Little Ice Age." So the intent was more or less to mimick that event, but of course the authors were free to construct whatever scenario they wanted, as long as it agrees with their intention to make a scenario that, although it is not likely, could happen in principle. They indicate a temperature drop in NW Europe of about 3.3° C, which seems within the range of possible actual temperature drops during the event 8,200 years ago, considering the uncertainty in temperature reconstructions.

(COMMENT)
Page 120 top: " . . with little effect in southern Europe. For all of Europe, the difference . . " Comment: The authors of the Pentagon report write that the temperature drops do not apply to south Europe, whigh micht even become a little milder. So Lomborg should not compare "all of Europe" with Siberia.

FLAW
Page 120: "Instead of the 3.5 cm expected for the coming century, they used 86 cm . . . " Flaw: As stated in comments to page 78 and 118, the 3.5 cm is at the very lowest end of the estimates, which range up to c. 25 cm as the highest. The authors mention as a very high rate of melting a contribution to sea level of 6 mm/year, that is 60 cm in 100 years, not 86 cm.

ERROR
Page 120: " . . . they used 86 cm . .  " Error: First, there is a small error of calculation here. Using a correct figure for the area of the world´s oceans, 335m km², a flow of 0.1 Sv in 100 years will give a rise of 94 cm, not 86 cm. More seriously, the figure of 86 or 94 cm refers to the flow of all fresh water sources to the North Atlantic, not just the melt water from Greenland. The authors state that some of the highest estimates of fresh water flux for scenarios with 4 X CO2 have a flux from Greenland of about 0.07 Sv, from melting of perennial arctic sea ice of 0.01 Sv and from other sources of about 0.14 Sv, which yields a total of 0.22 Sv. However, in the hosing experiment they apply "only" 0.1 Sv. If we assume that the contribution from Greenland remains nearly one third, this would be a contribution of 30 cm over 100 years (this is lower than the highest rate mentioned above, which would yield 60 cm). A contribution of 30 cm is not very unrealistic, considering that the top estimate (involving lubrication of ice flow) in IPCC´s 4th report is about 25 cm, with much uncertainty about the figure.
Note that in the model run, the fresh water flux of 0.1 Sv is applied only for 100 model years, after which it is switched off. In the real world, the flux would continue and grow after the first 100 years.

(COMMENT)
Page 120: " . . . the Gulf Stream weakened by 30 % . . ." Comments: One comment to this is that there were large differences between models. The weakening varied from 9% to 62%. Another comment is that there were other important effects than just the temperature change in western Europe. Most notably, the whole belt of precipitation along the Equator shifted southwards, e.g. with a reduction of rainfall in the Sahel region of about 200 mm per year.

(COMMENT)
Page 120: " . . . a cooling over western Europe of just 0.5-1.0° C." Comment: The paper says "most of western Europe". The cooling in Britain is 1.0-1.5° C.

(COMMENT)
Page 120: " . . . one group of modelers find a cooling of the UK by 3-5° C . . . " Comment: The authors have run a model scenario with gradual warming due to CO2 and then, in 2049, they force a large fresh water pulse onto the model in order to see what happens when the Gulf Stream then turns off. The authors write: ". . . scuh 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 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. The reader will not understand this from Lomborg´s text.

FLAW
Page 120: " . . . the researchers even tried increasing the melt of Greenland a further ten-fold to 860 cm  . . . " Flaw: On page 120,  Lomborg has already reviewed the first part of the paper by Stouffer et al., in which the apply a fresh water flux of 0.1 Sv, which reduces the Gulf Stream by about 30 % on average. In the last part of the paper, Stouffer et al. then apply a fresh water flux of 1 Sv, which causes the Gulf Stream to shut down nearly or completely in all models (half of the models have a complete shutdown). In some models, the Gulf Stream is reversible (it reverts to its original state after the flux stops); in other models it is irreversible (it remains in the shut-off state even after the fresh water flux has stopped - Lomborg avoids to mention that). In any case, once the Gulf Stream has stopped, a maximum cooling of up to 12°C occurs between Scotland and Iceland, while western and northern Europe becomes 4° - 10° colder. The experiment is unrealistic - a fresh water flux of that size is unthinkable - but it serves to show hos much temperatures could drop if the Gulf Stream turned off. Lomborg´s presentation is not correct, because there is no indication in the model studies that the flux of 1 Sv should come from melt of Greenland Ice. Rather, the models are especially sensitive to how much precipitation over the North Atlantic increases per unit of warming.

GROUNDLESS DEROGATION
Page 120 bottom: " . . . Al Gore and many others . . . the Gulf Stream is what keeps European winters some 15-20°C higher . . . " Flaw: Lomborg does not tell who postulates such at temperature difference of 15-20°. At least it is not Al Gore. In his book, Al Gore just writes that Paris, London and Madrid are much warmer than Montreal, Fargo or New York, respectively. He does not go into any detail about how much warmer, or in what seasons. And as to the cause of this warming, he writes: ". .  the heat drawn from the Gulf Stream and carried to Europe". This formulation is correct. As the reader will understand that Al Gore is incorrect, when he actually is not, this is here desginated as groundless derogation.

(COMMENT)
Page 121 top: " . . . the predominant wind direction . . . " Comment: The explanation given here by Lomborg is incomplete. According to C. Wunsch (J. of climate 18(2005): 4374-4380), near the equator, the northward heat transport is rather evenly divided between heat carried by the ocean current and heat carried by the atmosphere. North of that, the fraction carried by the ocean declines. According to van der Swaluw et al. (J. of climate 20(2007): 6023-6032), at 35° N, 22 percent of the heat transport is by ocean. However, the wind is also crucial in keeping the sea current flowing, partly because the wind stress pushes the water surface, and partly because the warm wind causes the hot surface water to evaporate, thereby increasing the salinity in the surface water; it is this increase in salinity which allows the sinking of the surface water in the north Atlantic, and this sinking acts as a pump pulling sea water northwards. Even as high north as 70° N, there is an important transfer of heat from the northward flowing water to the atmosphere; this heat transfer reduces the atmospheric pressure gradient and makes winds less strong than they would otherwise have been.

(COMMENT)
Page 121: " . . . 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 Overtruning 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.

(COMMENT)
Page 121 bottom: "Two papers discussed the stability of the Gulf Stream." Comment: No, actually, three papers had the Gulf Stream as their main theme. See comments to page 122 bottom.

(COMMENT)
Page 121 bottom: "The first paper used a simple model of the North Atlantic Ocean." Comment: This is only a half truth. The paper consisted of two parts. The first part used a relatively realistic coupled atmosphere-ocean general circulation model. The other half used a simple two-box model. 

FLAW
Page 121 bottom: "The trick is that with suitable choices of set-up, this box model of the Gulf Stream can easily break down."
Flaw: This is misleading. The choice of parameters is not set in such a way that the Gulf Stream shuts down more easily. However, there is a choice of parameters so that the probability of bi-stability is manipulated. That is, instead of just having one value of the parameter of wind stress, the model takes an average of different wind parameter values, and thereby the probaility of bi-stability is manipulated. That is, we are dealing with the question of whether the Gulf Stream, once it is forced to shut down by fresh water addition to the North Atlantic, will be able to revert to its original state once the fresh water addition ceases. We are not dealing with the sensitivity of the Gulf Stream to fresh water addition. Concerning this latter feature, the authors (Schlesinger et al., p. 46) write: "We calibrated the . . .  model so that it is about as sensitive to a fresh water addition as the University of Illinois  . . coupled atmosphere-ocean genereal circulation model . . "

(COMMENT)
Page 121 bottom and note 621: "This of course was part of the original interest in the model . . . " Comment: The reference in note 621 (from 2005) does not say anything about the model in the paper in Schlesinger et al. (from 2006) and is not really relevant at all to the discussion here. The reader may believe that the reference in note 621 is a documentation for the postulate about the `trick´ referred to in the preceding sentence. However, it is completely irrelevant to that sentence. 

(COMMENT)
Page 121 bottom: "This model was the original background . . . " Comment: This is not quite true. The original two-box model from 1961 invariably showed a bi-stable Gulf Stream, but in 2002 it was amended to include the gyre circulation in the North Atlantic due to westerly winds. When this is included, the model is more flexible and may show either a bi-stable og a mono-stable Gulf Stream, depending on the intensity of the wind effect. Thus, the criticism from Wunsch that the effect of westerly winds is ignored, is no longer true. 

FLAW
Page 122 mid: "They set up the model such that it sometimes collapses, sometimes does not . . . " (note 624): " . . . this statement explains the thrust of the model." Flaw: This is not true. As explained above with regard to the sentence " The trick is . . . " on page 121, the model is not set up in a way that has the effect postulated by Lomborg. Rather, it is adjusted to have the same probability of a Gulf Stream shut-down as a much more complicated and realistic model dveloped at the University of Illinois. So, if the model is biased concerning the probability of shut-down, then this bias originates from the more complicated and realistic model that is used to calibrate it, not from the simple two-box model itself.

(COMMENT:)
Page 122 mid: " . . . collapses about 50% of the time." Comment: This has little to do with what the paper actually says. With no mitigation (no tax), there is about a 46% risk of shut-down in 2100 and about a 69% risk of shutdown in 2200. 

ERROR
Page 122 mid: "They claim that a $27 CO2 tax can reduce the risk by 28% in 2100." Error: This is wrong. From reading the curves on their figure 5.7d, one sees that the risk is reduced from about 46% to about 28%, that is the risk is reduced by 18 percentage points, or 39% of the original risk. %. curve does not agree with the data source. It seems that there is an error in the scale on the vertical axis. This original source is found at this link.

(COMMENT)
Page 122 mid and note 625: "They tell us how a $100 tax is just 50 cents on the gallon . . . " Comment: First, Lomborg forgets to state that by $100 he means $100 per t Carbon, equal to $27 per ton CO2. Second, the paper does not exactly tell that $ 100 per t C is equal to 50 cents on the gallon, but instead, as said in note 625, that $ 10 is equal to 5 cents per gallon. It is somewhat misleading in the note to say "They tell us six times . . . " as if the authors exaggerate their stressing of how low the amount is. Actually, in their text the authors say that only once. But Lomborg counts also five places in the graphs where the 5 cent value is indicated. FLAW
Page 122 mid: " . . . but neglect to tell us . . . essentially shutting down the fossil fuel economy by 2060." Flaw: Actually, the authors do show that use of fossil fuels is essentially shut down by 2060. This is clearly seen in their figure 5.7a. There is no reason why they should focus particularly on this in their text, as the paper is about what could happen to the Gulf Stream, not about effects on the global economy. We have seen above that Lomborg also counts on what is shown in the figures (the indications of 5 cents per gallon). So, when Lomborg counts the figures as a part of what the paper says, then his claim about the shutting down of fhe fossil fuel economy by 2060 is not correct.

FLAW
Page 122 mid: "In the model the risk reduction is 28%, but  . . . if the risk is much lower - say, close to zero . . . " Flaw: Lomborg fails to state what he is talking about. Does he talk about the risk of a shut-down in 2100, or in  2200, or when? It is true that in many other models, the risk of a shut-down already in 2100 is very small, but the risk of a shut-down in 2200 is much larger. The paper by Schlesinger et al. mainly discusses the risk of a shut-down in 2200, and according to their model, this is  likely to happen if global temperature by then increases by about 2.3° C relative to the year 1900, which is very likely. Actually, a temperature rise of this order of size might occur already by 2100. As to the likelihood of a shut-down of the Gulf Stream already by 2100, this is not referred to in the text, but may be read from one of the graphs. Here, the probability is about 46% for the no-tax scenario. As also stated above, with a $100 per t C taxation, the risk is reduced from about 46% to about 28%. This is not a 28% reduction.


FLAW OF OMISSION
Page 122 bottom: "However, it is interesting to realize that, at the conference, there were two presentations  . . . resulting in two papers." Flaw: Actually, there were three papes on the stability of the Gulf Stream. The one omitted by Lomborg is P. G. Challenor, R.K.S. Hankin & R. Marsh: Towards the probability of rapid climate change, pp. 55-63. This paper tries to produce a probability distribution of various outcomes regarding the slowing down of the Gulf Stream. This can be done by running a good climate model thousands of times with different combinations of plausible parameter values. However, the computational power of our computers is not large enough for that. Instead, it is necessary to run models of intermediate complexity. This has been done, with 100 computer runs. As seen in the figure below, out of 100 runs, all but 9 produce an amount of overturning of water in the North Atlantic in the years up to now of at least 13 Sverdrup, which fits well with the actual value in the real world. Then, if CO2 emissions during the 21st century follow IPCC´s A1B scenario (which is likely), the overturning of water is reduced in all runs. Out of the 91 runs, 37-38 % show a reduction of the overturning to less than 5 Sv, which will have considerable climatic effects. The result depends very little on the exact rate of melting of ice on Greenland. Thus, the result of this type of analysis, which has not been performed before, is that the risk for a shut-down of the Gulf Stream is much larger than was anticipated by the authors themselves and by most other scientists. When Lomborg ignores this paper, his presentation becomes very biased.
Gulf Stream simulation


FLAW

Page 122 bottom: "The journalist from the Guardian sat through both, and even referred to the other in passing. . . " Flaw: The contents of the Guardian article is grossly misrepresented. The `headline grabbing´ article by Schlesinger is referred to over c. 12 lines, whereas the second paper by Wood et al. is referred to over c. 8 lines. This is a fairly even treatment of the two papers.

FLAW
Page 122 bottom: "But while the headline grabbing first paper used one simple two-box model . . . " Flaw: No, it used two models, one of which was a complicated atmosphere-ocean-genereal circulation model, and the other was a two-box model which was not very simple.

(COMMENT)
Page 123 top: " . . did a review of the major comprehensive computer models." Comment: No, it did not. It referred to the IPCC third assessment report from 2001, which presented outputs from nine models, representing the state of the art before 2001.

FLAW
Page 123 top: "The authors pointed out that the shut-off mode essentially comes from the simple box-models and thus asked if this instability . . ". Flaw: This is misleading and not accurate. The reader will understand the terms `shut-off mode´  and `instability´ as a situation where the Gulf Stream is shut off. However, the part of the paper that Lomborg refers to here deals with something else, namely whether the Gulf Stream is bi-stable (can be led into a new state of permanent shut-off without reverting) or mono-stable (will revert to the original state after having been shut off). Simple models, ranging from two-box models to models of intermediate complexity, often show bi-stability. Because of computational costs, it is not possible to explore this in full-size detailed models. Instead, in such models, the shut-down is forced upon the model, and it is studied whether the Gulf Stream can revert to its original state. In most complicated models, it can revert, but in two of the models, it does not revert. On this basis, to cite the authors, "it is not possible to say definitively from these model studies whether the present day THC is bistable . . ."

FLAW
Page 123 top: ". . their basic conclusion was straightforward and very different . . . " Flaw: Here, Lomborg mixes up two different questions, 1) if the Gulf Stream circulation is bi-stable or mono-stable, and 2) if the Gulf Stream is likely to shut down. On neither issue was the conclusion straightforward. Thus, concerning issue no. 2, the authors write: "In the present state of scientific knowledge it is not possible to identify a `safe´ CO2 stabilisation level that would prevent THC shutdown." They also said that "It should be noted that none of the [general circulation model] results used in [the IPCC third assessment report] fully include the effects of melting of the Greenland ice sheet . . ". This important reservation is omitted by Lomborg.

(COMMENT):
Page 123 top: "They projected that the Gulf Stream would slow down, somewhere between 0% and 50%." Comment: Lomborg does not make it clear to the reader that this concerns what would happen up to the year 2100. After 2100, the probability of a shut-down is considerably higher.

(COMMENT):
Page 123 top: "No GCMs have shown a complete shutdown . . ." Comment: What is meant here is none of the GCMs (global circulation models) referred to by IPCC in 2001. However, the next paper in the book from 2006 (Challenor et al., referred to above) deals with the probability that a model giving a moderate slowdown as the main result includes the possibility for a greater reduction or a total shutdown with some probability. As demonstrated by the figure from Challenor et al. presented above, even a model with a most likely output of a nearly 50 % slowdown includes a definite possibility that there could be a total or near-total shutdown. As said above, this paper was ignored by Lomborg.

FLAW
Page 123: "Thus, the risk of a shut-down was estimated at 50 % by a two-box model. But when we used the most advanced models  . . . the risk of a shut-down was estimated at zero percent." Flaw: This is a gross distortion of what the literature says. Most of the two-box models (but not all of them) show that the Gulf Stream is bi-stable; this is what they are mainly used to demonstrate. They do not by themselves give any realistic estimate of the risk that the Gulf Stream will shut down at a certain temperature. In the paper by Schlesinger et al. that Lomborg does not like, the calibration of the two-box model is made by reference to one of `the most advanced models in the world´. Then, only after this calibration, was it possible to study what range of parameter choices would lead to a complete shut-down. The result was then that a shut-down already in 2100 is likely, and in 2200 very likely. FLAW
Page 123: " . . . the risk of shut-down was estimated at zero percent." Flaw: No it was not. The authors concluded: "The currently very high level of modelling uncertainty makes accurate projection of the future of the THC difficult, beyond the rather vague statement that complete shutdown is `unlikely´ over the next century." `Unlikely´ is not the same as zero percent.

(COMMENT)
Page 124 top: "National Geographic told us . . ." 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.

(COMMENT)
Page 124: " . . . throwing in 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".

(COMMENT)
Page 124: "New Scientist chose it as one of their 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.

FLAW:
Page 124: Altogether, the whole paragraph starting with
"National Geographic told us . . ." gives an impression of the media coverage which is not accurate and somewhat misleading.

(COMMENT)
Page 124: "In New Scientist the headline was . . . ": Comment: New Scientist had a lengthy article on the subject already on 15th April 2006 by Stephen Battersby.

REMARK
Page 124 bottom: "The Guardian in the UK did report . . .": Remark: In this case, Lomborg´s criticism seems warranted.

(COMMENT)
Page 125 top: "New Scientist told us . . . ": Comment: Lomborg could also have cited another sentence from that article which is not quite as reassuring: "
Climate models do not predict any substantial slowdown in Atlantic currents until near the end of this century."

FLAW
Page 125: "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.

ERROR
Page 125 bottom: " . . .IPCC tells that even if the Gulf Stream shuts down compeletely Europe will still experience warming" 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, we 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.