Of course, model internal variability is not correct, and anthropogenic forcing includes not only co2 but also aerosols, and the latter are unknown to a factor of 10-20 (and perhaps even sign). Finally, we add have little quantitative knowledge of natural forcing so this too is adjustable. Recall that the hadley centre acknowledges that the aerosols cancelled most of the forcing from CO2. Yet, the argument I have just presented is the basis for all popular claims that scientists now believe that man is responsible for much of the observed warming! It would appear that the current role of the scientist in the global warming issue is simply to defend the possibility of ominous predictions so as to justify his belief. To be fair to the authors of Chapter 12 of the ipcc third Scientific Assessment here is what they provided 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. Studies are beginning to separate the contributions to observed climate change attributable to individual external influences, both anthropogenic and natural.
A recent article. Science (Jaenicke, 20056) even essay proposed a significant role to airborn dandruff. Other articles have been suggesting that the primary impact of aerosols is actually warming (Jacobson, 20017, Chen and Penner, 20058). Of course this is the beauty of the global warming issue for many scientists. The issue deals with such small climate forcing and small temperature changes that it permits scientists to argue that everything and anything is important for climate. In brief, the defense of the models starts by assuming the model is correct. One then attributes differences between the model behavior in the absence of external forcing, and observed changes in global mean temperature to external forcing. Next one introduces natural forcing and tries to obtain a best fit to observations. If, finally, one is able to remove remaining discrepancies by introducing anthropogenic forcing, we assert that the attribution of part of the observed change to the greenhouse component of anthropogenic forcing must be correct.
This was remarked upon in a recent paper. Science (Andersen, et al, 20035 wherein it was noted that the uncertainty was so great that estimating aerosol properties by tuning them to optimize agreement between models and observations (referred to as an inverse method) was probably as good as any other method, but that. This is as strong a criticism of model procedures as is likely to be found. The authors are all prominent in aerosol work. The first author is the most junior, and when it was pointed out that the article reflected negatively on model outputs, he vehemently denied any such intent. In the present example, the choice of models with relatively low sensitivity, allowed adjustments that were not so extreme. New uncertainties are always entering the aerosol picture. Some are quite bizarre.
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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 report 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 dnp be noted that the models used had a relatively low sensitivity to a doubling of CO2 of about.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. 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 outputs, aerosols and other factors cancelled two thirds of the greenhouse forcing). Unfortunately, the properties of aerosols are largely unknown. In the present instance, therefore, aerosols constitute simply another adjustable parameter (indeed, both its magnitude and its time history are adjustable, and even its sign is in question).
Although we are far from the benchmark of doubled CO2, climate forcing is already about 3/4 of what we expect from such a doubling. Even if we attribute all warming over the past century to man made greenhouse gases (which we have no basis for doing the observed warming is only about 1/3-1/6 of what models project. We are logically led to two possibilities: Our models are greatly overestimating the sensitivity of climate to man made greenhouse gases,. The models are correct, but there is some unknown process that has cancelled most of the warming. Note that calling the unknown process aerosols does not change this statement since aerosols and their impact are unknown to a factor of ten or more; indeed, even the sign is in doubt. In arguing for climate alarmism, we are choosing the second possibility.
Moreover, we are assuming that the unknown cancellation will soon cease. How is the second possibility supported given that it involves so many more assumptions than the first possibility? The ipcc third Assessment Report (TAR) made use of a peculiar exercise in curve fitting using results from the hadley centre. It consists in three plots which are reproduced in Figure. In the first panel, we are shown an observed temperature record (without error bars and the outputs of four model runs (using their coupled ocean-atmosphere model) with so-called natural forcing for the period. There is a small spread in the model runs (which presumably displays model uncertaintyit most assuredly does not represent natural internal variability).
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The reason for this is that in these models, the most important greenhouse substances, water vapor and clouds, act in such a way as to greatly amplify the response to anthropogenic greenhouse gases wallpaper 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 textthough not in the summaries for Policymakers the models simply fail to get clouds 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. In Figure 1 we see that treatment of clouds involves errors an order of magnitude greater than the forcing from a doubling of. While the ipcc allows for the possibility that the models get water vapor right, the intimate relation of water vapor to clouds makes such a conclusion implausible. Let me summarize the main points thus far:. It is not the level of CO2 that is important, but rather the impact of man made greenhouse gases on climate.
We may, therefore, seek to determine how the current level of man made climate forcing compares with what we would have were co2 to be doubled (a common reference level for gcm calculations). 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 related - 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 (i.e., 580 ppmv) causes a 2 increase in radiative forcing3, then to obtain another 2 increase in radiative forcing. At present, the concentration of CO2 is about 380 ppmv. The easiest way to understand this is to consider adding thin layers of paint to a pane of glass. The first layer cuts out much of the light, the next layer cuts out more, but subsequent layers do less and less because the painted pane is already essentially opaque. It should be stressed that we are interested in climate forcing, and not simply levels of CO2; the two are most certainly not linearly proportional. Essential to alarm is the fact that most current climate models predict a response to a doubling of CO2 of about 4C (which is much larger than what one expects the simple doubling of CO2 to produce: ie, about 1C).
vapor and clouds) by about. 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. In some respects, these three pillars of consensus are relatively trivial. Remaining completely open is the question of whether there is any reason to consider this basic agreement as being alarming. Relatedly, is there any objective basis for considering the approximate.6C increase in global mean surface temperature to be large or small regardless of its cause? The answer to both questions depends on whether.6C is larger or smaller than what we might have expected on the basis of models which have led to our concern. These models are generally called General Circulation Models (GCMs).
They fail to note that there are many sources of climate change, and that profound climate change occurred many times both before and after man appeared on earth; given the ubiquity of climate change, it is paperless implausible that all change is for the worse. Moreover, the coincidence of increasing CO2 and the small warming over the past century hardly establishes causality. Nevertheless, for the most part I do not personally disagree with the consensus (though the absence of any quantitative considerations should be disturbing). Indeed, i know of no serious split, and suspect that the claim that there is opposition to this consensus amounts to no more than setting up a straw man to scoff. However, i believe that people are being led astray by the suggestion this agreement constitutes support for alarm. Let us view the components that comprise this consensus a little more precisely while recognizing that there is, indeed, some legitimate controversy connected with specific aspects of even these items. 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.6.15 degrees Centigrade (C).
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For the sensitive reader or listener, the language used in connection with the issue of Global Warming must frequently sound strange. Weather and climate catastrophes of all sorts are claimed to be what one expects from global warming, and global warming is uniquely associated with mans activities. The reality of the threat of global warming is frequently attested to by reference to a scientific consensus: Tony Blair: The overwhelming view of experts is that climate change, to a greater or lesser extent, is man-made, and, without action, will get worse. Elizabeth Kolbert in, new Yorker 1: All that the theory of global warming says report is that if you increase the concentration of greenhouse gases in the atmosphere, you will also increase the earths average temperature. Its indisputable that we have increased greenhouse-gas concentrations in the air as a result of human activity, and its also indisputable that over the last few decades average global temperatures have gone. Given the alarm that surrounds the issue, such statements seem peculiarly inconclusive and irrelevant to the catastrophes cited. To be sure, these references are one-sided.