Select Committee on Economic Affairs Minutes of Evidence

Memorandum by Professor Richard S Lindzen, Massachusetts Institute of Technology

  I am honoured to be able to share my impressions of the global warming issue with the members of this esteemed body. For the past 45 years I have been conducting research into various aspects of the physics of climate. I currently hold the Alfred P Sloan Professorship in Atmospheric Physics at the Massachusetts Institute of Technology, and have previously held professorships at Harvard University and the University of Chicago.

  It goes without saying that few laymen understand what global warming is really about. However, most of you have been assured that it is a very serious problem, and that almost all scientists agree. For example, your Prime Minister has written that it was quite wrong "to suggest that scientific opinion is equally split", and he went on to claim "The overwhelming view of experts is that climate change, to a greater or lesser extent, is man-made, and, without action, will get worse". The Prime Minister is certainly aware that there are many sources of climate change, and that profound climate change occurred frequently long before man appeared on earth. Moreover, given the ubiquity of climate change, it is implausible that all change is for the worse. Nevertheless, on the whole I do not disagree with the Prime Minister. Indeed, I know of no split whatever, and suspect that the Prime Minister is simply setting up a straw man in claiming that there is opposing opinion. Where the Prime Minister is, in my view, leading you astray is in suggesting that this agreement constitutes support for alarm.

  Indeed, when we analyse the nature of the scientific agreement we will see that it provides no support for alarm. However, given the proclivity of governments to respond to alarm with substantial support for science, we can understand the reluctance of the scientific community, such as it is, to object to the alarmist interpretation of their agreement.


  In order to analyse the meaning of the Prime Minister's claim, it is helpful to break the claim into its component parts. I won't suggest that there is no controversy over details, but there are few that would fundamentally disagree with the following.

  1.  The global mean surface temperature is always changing. Over the past 60 years, it has both decreased and increased. For the past century, it has probably increased by about 0.6 degrees Centigrade (C). That is to say, we have had some global mean warming.

  2.  CO2 is a greenhouse gas and its increase should contribute to warming. It is, in fact, increasing, and a doubling would increase the radiative forcing of the earth (mainly due to water vapour and clouds) by about 2 per cent.

  3.  There is good evidence that man has been responsible for the recent increase in CO2, though climate itself (as well as other natural phenomena) can also cause changes in CO2.

  I will refer to this as the basic agreement. To this extent, and no further, it is legitimate to speak of a scientific consensus.


  Various bodies have been unable to resist making claims that items 1 and 2 are causally connected. This is referred to as the attribution question. I will show that attribution is by no means widely accepted or even plausible. However, as we will see, the alleged attribution, itself, also provides little or no support for alarm. The reason why the basic agreement (even when supplemented by the claim of attribution) does not support alarm hinges on other points of widespread agreement, which the Prime Minister failed to mention (and very likely was unaware of).

  4.  In terms of climate forcing, greenhouse gases added to the atmosphere through mans activities since the late 19th Century have already produced three-quarters of the radiative forcing that we expect from a doubling of CO2. The main reasons for this are (1)  CO2 is not the only anthropogenic greenhouse gas—others like methane also contribute; and (2)  the impact of CO2 is nonlinear in the sense that each added unit contributes less than its predecessor. For example, if doubling CO2 from its value in the late 19th Century (about 290 parts per million by volume or ppmv) to double this (ie, 580 ppmv) causes a 2 per cent increase in radiative forcing, then to obtain another 2 per cent increase in radiative forcing we must increase CO2 by an additional 580 ppmv rather than by another 290 ppmv. At present, the concentration of CO2 is about 370 ppmv.

  5.  A doubling of CO2 should lead (if the major greenhouse substances, water vapour and clouds remain fixed), on the basis of straightforward physics, to a globally averaged warming of about 1C. The current increase in forcing relative to the late 19th Century due to mans activities should lead to a warming of about 0.76C, which is already more than has been observed, but is nonetheless much less than current climate models predict.


  This brings us, finally, to the issue of climate models. Essential to alarm is the fact that most current climate models predict a response to a doubling of CO2 of about 4C. The reason for this is that in these models, the most important greenhouse substances, water vapour and clouds, act in such a way as to greatly amplify the response to anthropogenic greenhouse gases alone (ie, they act as what are called large positive feedbacks). However, as all assessments of the Intergovernmental Panel on Climate Change (IPCC) have stated (at least in the text—though not in the Summaries for Policymakers), the models simply fail to get clouds and water vapour right. We know this because in official model intercomparisons, all models fail miserably to replicate observed distributions of cloud cover. Thus, the model predictions are critically dependent on features that we know must be wrong.

  If we nonetheless assume that these model predictions are correct (after all stopped watches are right twice a day), then man's greenhouse emissions have accounted for about six times the observed warming over the past century with some unknown processes cancelling the difference. This is distinctly less compelling than the statement that characterised the IPCC Second Assessment and served as the smoking gun for the Kyoto agreement: The balance of evidence suggests a discernible human influence on global climate. This is simply a short restatement of the basic agreement with the addition of a small measure of attribution. While one could question the use of the word "discernible", there is no question that human influence should exist, albeit at a level that may be so small as to actually be indiscernible. As we have already noted, however, even if all the change in global mean temperature over the past century were due to man, it would still imply low and relatively unimportant influence compared to the predictions of the models that are drawn on in IPCC reports.

  Another example of the misuse of the basic agreement to promote alarm consists in the opening lines of the executive summary of the US National Research Council (NRC) 2001 report: Climate Change Science: An Analysis of Some Key Questions. This hurried report was prepared at the specific request of the White House. The brief and carefully drafted report of 15 pages was preceded by a totally unnecessary 10 page executive summary. The opening lines were appended at the last moment without committee approval. Here they are:

  Greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. Temperatures are, in fact, rising.

  The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability.

  To be sure, this statement is leaning over backwards to encourage the alarmists. Nevertheless, the two sentences in the first claim serve to distinguish observed temperature change from human causality. The presence of the word "likely" in the second statement is grossly exaggerated, but still indicates the lack of certainty, while the fact that we have not emerged from the level of natural variability is, in fact, mentioned albeit obliquely. What, as usual, goes unmentioned is that the observed changes are much smaller than expected.

  The response from many commentators was typical and restricted to the opening lines. CNN's Michelle Mitchell characteristically declared that the report represented "a unanimous decision that global warming is real, is getting worse, and is due to man. There is no wiggle room". Mitchell's response has, in fact, become the standard take on the NRC report. Such claims, though widely made in your country as well as mine, have no basis: they are nonsensical.


  How is it that model based alarm has been "justified" despite the fact that the observed warming over the past century is much less than was anticipated by the models? As usual, the argument involves obscuring this latter fact. The argument also ignores the fact that the climate is capable of unforced internal variability. That is to say, the climate can vary without any external forcing at all. El Niño is an example but there are many others besides. Reference to any temperature history of the earth shows fluctuations that are not connected to any known forcing, and these fluctuations amount to as much as half a degree Centigrade.

  The most common defense is based on studies from the UK's Hadley Centre, and appears in Chapter 12 of the IPCC's Third Scientific Assessment. I would like to comment on this line of argument.

  In these studies, we are shown three diagrams. In the first, we are shown an observed temperature record (without error bars), and the results of four model runs with so-called natural forcing for the period 1860-2000. There is a small spread in the model runs (which presumably displays model uncertainty—it most assuredly does not represent internal variability). In any event, the models look roughly like the observations until the last 30 years. We are then shown a second diagram where the observed curve is reproduced, and the four models are run with anthropogenic forcing. Here we see rough agreement over the last 30 years, and poorer agreement in the earlier period. Finally, we are shown the observations and the model runs with both natural and anthropogenic forcing, and, voila, there is rough agreement over the whole record. It should be noted that the models used had a relatively low sensitivity to a doubling of CO2 of about 2.5C. In order to know what to make of this exercise, one must know exactly what was done. The natural forcing consisted in volcanoes and solar variability. Prior to the Pinatubo eruption in 1991, the radiative impact of volcanoes was not well measured, and estimates vary by about a factor of 3. Solar forcing is essentially unknown. Thus, natural forcing is, in essence, adjustable. Anthropogenic forcing includes not only anthropogenic greenhouse gases, but also aerosols that act to cancel warming (in the Hadley Centre results, aerosols and other factors cancelled two thirds of the greenhouse forcing). Unfortunately, the properties of aerosols are largely unknown. This was remarked upon in a recent paper in Science, wherein it was noted that the uncertainty was so great that estimating aerosol properties by tuning them to optimise agreement between models and observations (referred to as an inverse method) was probably as good as any other method, but that the use of such estimates to then test the models constituted a circular procedure. In the present instance, therefore, aerosols constitute simply another adjustable parameter (indeed, both its magnitude and its time history are adjustable). However, the choice of models with relatively low sensitivity, allowed adjustments that were not so extreme.

  What we have is essentially an exercise in curve fitting. I suppose that the implication is that it is possible that the model is correct, but the likelihood that all the adjustments are what actually occur is rather small. The authors of Chapter 12 of the IPCC Third Scientific Assessment provided the following for the draft statement of the Policymakers Summary: From the body of evidence since IPCC (1996), we conclude that there has been a discernible human influence on global climate. Studies are beginning to separate the contributions to observed climate change attributable to individual external influences, both anthropogenic and natural. This work suggests that anthropogenic greenhouse gases are a substantial contributor to the observed warming, especially over the past 30 years. However, the accuracy of these estimates continues to be limited by uncertainties in estimates of internal variability, natural and anthropogenic forcing, and the climate response to external forcing.

  This statement is not too bad—especially the last sentence. To be sure, the model dependence of the results is not emphasised, but the statement is vastly more honest than what the Summary for Policymakers in the IPCC's Third Assessment Report ultimately presented: In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations. In truth, nothing of the sort can be concluded. The methodology, by omitting any true treatment of internal variability, misses a crucial point. One can represent the presence of internal variability simply by plotting an horizontal line with the average value of the temperature for the period 1850-2000, and broadening this line to have a thickness of about 0.4C to represent the random internal variability of climate (in nature if not in the models). One can then plot the observations with a thickness of about 0.3C (corresponding to an observational uncertainty of about +/-0.15C). The two appropriately broadened lines will now overlap almost everywhere (a certain percentage of non-overlap is statistically expected) leaving no evident need for forcing at all.

  Thus, the impact of man remains indiscernible simply because the signal is too small compared to the natural noise. Claims that the current temperatures are "record breaking" or "unprecedented", however questionable or misleading, simply serve to obscure the fact that the observed warming is too small compared to what models suggest. Even the fact that the oceans' heat capacity leads to a delay in the response of the surface does not alter this conclusion.


  We still have not really addressed the interesting question of how modest warming has come to be associated with alarm. Here we must leave the realm where fudging and obfuscation are the major tools to a realm of almost pure fantasy. A simple example will illustrate the situation.

  According to any textbook on dynamic meteorology, one may reasonably conclude that in a warmer world, extratropical storminess and weather variability will actually decrease. The reasoning is as follows. Judging by historical climate change, changes are greater in high latitudes than in the tropics. Thus, in a warmer world, we would expect that the temperature difference between high and low latitudes would diminish. However, it is precisely this difference that gives rise to extratropical large-scale weather disturbances. Moreover, when in Boston on a winter day we experience unusual warmth, it is because the wind is blowing from the south. Similarly, when we experience unusual cold, it is generally because the wind is blowing from the north. The possible extent of these extremes is, not surprisingly, determined by how warm low latitudes are and how cold high latitudes are. Given that we expect that high latitudes will warm much more than low latitudes in a warmer climate, the difference is expected to diminish, leading to less variance. Nevertheless, we are told by advocates and the media that exactly the opposite is the case, and that, moreover, the models predict this (which, to their credit, they do not) and that the basic agreement discussed earlier signifies scientific agreement on this matter as well. Clearly more storms and greater extremes are regarded as more alarming than the opposite. Thus, the opposite of our current understanding is invoked in order to promote public concern. The crucial point here is that once the principle of consensus is accepted, agreement on anything is taken to infer agreement on everything advocates wish to claim.

  Again, scientists are not entirely blameless in this matter. Sir John Houghton (the first editor of the IPCC scientific assessments) made the casual claim that a warmer world would have more evaporation and the latent heat (the heat released when evapourated water vapour condenses into rain) would provide more energy for disturbances. This claim is based on a number of obvious mistakes (though the claim continues to be repeated by those who presumably don't know better).

  For starters, extratropical storms are not primarily forced by the latent heat released in convection. However, even in the tropics, where latent heat plays a major role, the forcing of disturbances depends not on the evaporation, but on the evaporation scaled by the specific humidity at the surface. It turns out that this is almost invariant with temperature unless the relative humidity decreases in a warmer world. Incidentally, this would suggest that the feedbacks that cause models to display high climate sensitivity are incorrect. The particularly important issue of whether warming will impact hurricanes, is a matter of debate. As the IPCC has noted, there is no empirical evidence for such an impact. State of the art modeling suggests a negative impact, while there are theoretical arguments that suggest a slight positive impact on hurricane intensity. This is all of significant intellectual interest, but it is not the material out of which to legitimately build alarm.

  Perhaps the most reprehensible attempt to generate alarm over global warming has been seen in connection with the recent tragic tsunamis in South Asia, where statements were made attempting to link this essentially geological event to global warming. However specious such links are, they follow what has become an almost self-parodying habit of those proclaiming alarm of attaching any severe, unusual or even common but not well known event to global warming while suggesting rather dishonestly that the event had indeed been predicted by models.


So where does all this leave us?

  First, I would emphasise that the basic agreement frequently described as representing scientific unanimity concerning global warming is entirely consistent with there being virtually no problem at all. Indeed, the observations most simply suggest that the sensitivity of the real climate is much less than found in models whose sensitivity depends on processes which are clearly misrepresented (through both ignorance and computational limitations). Attempts to assess climate sensitivity by direct observation of cloud processes, and other means, which avoid dependence on models, support the conclusion that the sensitivity is low. More precisely, what is known points to the conclusion that a doubling of CO2 would lead to about 0.5C warming, and a quadrupling (should it ever occur) to about 1C. Neither would constitute a particular societal challenge. Nor would such (or even greater) warming be associated with more storminess, greater extremes, etc.

  Second, a significant part of the scientific community appears committed to the maintenance of the notion that alarm may be warranted. Alarm is felt to be essential to the maintenance of funding. The argument is no longer over whether the models are correct (they are not), but rather whether their results are at all possible. Alas, it is impossible to prove something is impossible.

  As you can see, the global warming issue parts company with normative science at a pretty early stage. A very good indicator of this disconnect is the fact that there is widespread and even rigorous scientific agreement that complete adherence to the Kyoto Agreement would have no discernible impact on climate. This clearly is of no importance to the thousands of negotiators, diplomats, regulators, general purpose bureaucrats and advocates attached to this issue.

  At the heart of this issue there is one last matter: namely, the misuse of language. George Orwell wrote that language "becomes ugly and inaccurate because our thoughts are foolish, but the slovenliness of our language makes it easier for us to have foolish thoughts". There can be little doubt that the language used to convey alarm has been sloppy at best. Unfortunately, much of the sloppiness seems to be intentional. The difficulties of discourse in the absence of a shared vocabulary are, I fear, rather evident.

25 January 2005

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