{"id":504,"date":"2007-12-18T09:44:01","date_gmt":"2007-12-18T14:44:01","guid":{"rendered":"http:\/\/www.realclimate.org\/index.php\/archives\/2007\/12\/les-chevaliers-de-l%e2%80%99ordre-de-la-terre-plate-part-ii-courtillots-geomagnetic-excursion\/"},"modified":"2011-12-13T15:14:16","modified_gmt":"2011-12-13T20:14:16","slug":"les-chevaliers-de-lordre-de-la-terre-plate-part-ii-courtillots-geomagnetic-excursion","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2007\/12\/les-chevaliers-de-lordre-de-la-terre-plate-part-ii-courtillots-geomagnetic-excursion\/","title":{"rendered":"Les Chevaliers de l\u2019Ordre de la Terre Plate, Part II: Courtillot’s Geomagnetic Excursion"},"content":{"rendered":"
\n

\t\tThis article continues the critique of writings on climate change by Allègre and Courtillot, started in Part I<\/a> . If you would like to read either post in French, please click on the flag icon beside the post title above.<\/i>
\n Cet article poursuit la critique des écrits sur le climat d’Allègre et Courtillot, commencée dans la Partie I.<\/a><\/i><\/lang_fr><\/p>\n

Prelude: It’s the physics<\/i>, stupid<\/font><\/h3>\n

…which of course is a paraphrase of Bill Clinton’s famous quote regarding the economy. We put the last word in small letters since we’ve learned that it is not a good debating technique<\/a> to imply (even inadvertently) that those who are having trouble seeing the force of our arguments might be stupid. What we wish to emphasize by this paraphrase is the simple fact that the expectation of a causal link between increasing long-lived greenhouse gases (like CO2) and increasing temperature does not rest on some vague, unexplained correlation between 20th century temperature and 20th century greenhouse gas concentration. <\/p>\n

The anticipated increase in temperature was predicted long before it was detectable in the atmosphere, indeed long before it was known that atmospheric CO2 really was increasing; it was first predicted by Arrhenius in 1896 using extremely simple radiation balance ideas, and was reproduced using modern radiation physics by Manabe and co-workers in the 1960’s. Neither of these predictions rests on general circulation models, which came in during subsequent decades and made more detailed forecasts possible. <\/p>\n

Still, the basic prediction of warming is founded on very fundamental physical principles relating to infrared absorption by greenhouse gases, theory of blackbody radiation, and atmospheric moist thermodynamics. All these individual elements have been verified to high accuracy in laboratory experiments and field observations. For a time, there was some remaining uncertainty about whether water vapor feedback would amplify warming in the way hypothesized in the early energy balance models, but a decade or two of additional observational and theoretical work has shown that there is no real reason to doubt the way in which general circulation models calculate the feedback. When modified by inclusion of the cooling effect of anthropogenic aerosols, the theory gives a satisfactory account of the pattern of 20th and 21st century temperature change. <\/p>\n

No other theory based on quantified physical principles has been able to do the same. If somebody comes along and has the bright idea that, say, global warming is caused by phlogiston raining down from the Moon, that does not make everything we know about thermodynamics, infrared absorption, energy balance, and temperature suddenly go away. Rather, it is the job of the phlogiston advocate to quantify the effects of phlogiston on energy balance, and incorporate them in a consistent way beside the existing climate forcings. Virtually all of the attempts to poke holes in the anthropogenic greenhouse theory lose sight of this simple and unassailable principle.<\/p>\n

In a paper entitled "Are there connections between the Earth’s magnetic field and climate?<\/a>" published recently in Earth and Planetary Science Letters<\/i>, Courtillot and co-authors attempt to cast doubt on carbon dioxide as a primary driver of recent (and presumably future) climate change; he argues instead that fluctuations in the Earth’s magnetic field (partly driven by solar variability) have an important and neglected role. Like most work of this genre, it is carried out in an intellectual void — as if everything we know currently about physics of climate had to be set aside in order to make way for one new (or in fact not-so-new) idea. But the problems don’t end there. With the help of a Comment published by Bard and Delaygue (available here at<\/a> <\/a>EPSL<\/a> or here as pdf<\/a>) , we’ll expose a pattern of suspicious errors and omissions that pervades Courtillot’s paper. Sloppiness and ignorance is by far the most charitable<\/i> interpretation that can be placed on this pattern.<\/p>\n

\t\t<\/p>\n

Let’s set the stage by noting that, as a significant competitor to anthropogenic greenhouse forcing of recent climate change, the direct radiative forcing by solar irradiance variations is dead on arrival. The solar output has been monitored by accurate satellite instruments since 1978. Measured peak to trough over the 11 year solar cycle, averaging over the Earth’s surface and allowing for albedo, the radiative forcing amplitude is under 0.2 W\/m2<\/sup>. The trend left after averaging over the solar cycle is even smaller. This pales by comparison with over 2 W\/m2<\/sup> of radiative forcing arising from long-lived greenhouse gases that have accumulated in the atmosphere since 1750; it pales yet more by comparison with the forcing to come in the future if action is not taken to control emissions. There is nothing in climate physics to suggest that the sensitivity of climate to solar irradiance variation differs substantially from the sensitivity to infrared radiative forcing arising from greenhouse gas changes. As far as the climate cares, a Watt is (for the most part) a Watt, regardless of whether it comes from changes in the incoming solar energy or greenhouse-induced changes in the infrared radiation loss.<\/p>\n

To get a bigger bang out of solar variability, one needs to invoke something else about the way the Sun affects climate. Something exotic, like magnetic field variations. Since there is no quantified<\/i> physical mechanism linking field variations to climate, Courtillot must fall back on showing us a few supposed correlations between temperature variations and magnetic field variations. To make matters worse, Courtillot can’t always make up his mind even about whether an increasing field index should warm the climate or cool it, making it unclear just what correlations one is looking for. The lack of a physical model makes it impossible to treat the various forcings on an equal footing and make a reliable attribution of causes. This is particularly fatal when the various forcings are strongly correlated with each other. For example, on time scales of years to centuries, the magnetic field variability, cosmic rays and solar irradiance vary nearly in lock-step, so if there is a correlation with temperature (or cloud cover) one cannot tell whether it means that climate is responding with high sensitivity directly to luminosity changes, or whether something more exotic is going on. Over a period when temperature, greenhouse gas forcing, and some magnetic field index are all going up, a statistical attribution technique which ignores greenhouse gases and considers only the magnetic field index will of course find that the magnetic field "explains." the signal. If we knew nothing about how CO2 affects climate, this would put the magnetic field on an equal footing with CO2 as a candidate explanation but this is not the case. We know a great deal about how CO2 affects climate and no amount of additional fiddling with cosmic rays or magnetic fields can make this physics go away<\/i>. One can get even more confused by forgetting about the important role of anthropogenic aerosols in the past century, as Courtillot all too often does.<\/p>\n

The confidence with which Courtillot casts doubt on the generally accepted role of anthropogenic forcing in climate change of the past century is surprising, in view of the essential limitations of any argument from correlation alone. But it’s worse than just that: as Bard and Delaygue show, most of the correlations upon which Courtillot et al.<\/i> rest their flimsy case are in fact bogus.<\/p>\n

Solar variability and climate: The Good, the Bad, and the Ugly<\/h3>\n

Work on the influence of solar variability (and on its close cousin, the influence of the Earth’s magnetic field) tends to fall into one of three categories. There is the Good, in which careful scientists do their objective best to unravel a complex and probably small (but nonetheless important) signal. As examples of work in this category, I would mention Judith Lean’s tireless efforts on relating luminosity to sunspot number, the work of Bard and colleagues on developing isotopic solar proxies like 10<\/sup>Be, Shindell’s work on response to solar ultraviolet variability, and the work of Foukal et al<\/a> on factors governing solar irradiance variations. I would also include the recent work by Camp and Tung<\/a> diagnosing the amplitude of the solar cycle in temperature in the "Good" category; that it is an easy paper for greenhouse skeptics to misquote takes away nothing from the quality of the science. In fact, I’d say most work on climate and solar variability falls into the Good category. That’s rather nice. In fact, scientists have long recognized the importance of solar variability as one of the factors governing climate (see the very scholarly review of the subject by Bard and Frank, available here at EPSL<\/a> or here as pdf)<\/a> An understanding of solar variability needs to be (and is) taken into account in attribution of climate change of the past century, and in attempts to estimate climate sensitivity from recent climate variations. Further, the Little Ice Age demands an explanation, and solar variability at present provides the only viable possibility. (It’s less clear that the Medieval Warm period is a sufficiently coherent phenomenon to require an explanation).<\/p>\n

Then, there is the Bad, exemplified by two papers by Scafetta and West that have been discussed on RealClimate here<\/a> and here<\/a>. This is just normally bad science, in the sense that there is something wrong in the approach taken by the authors which leads to erroneous conclusions. Perhaps some of this work should never have made it through peer review, but as long as the methods are well documented and honestly described, subsequent investigators will be able to identify the errors and either salvage or discard the results.<\/p>\n

And then … there is the Ugly. These papers cross the line from the merely erroneous into the actively deceptive. Papers in this category commit what Damon and Laut<\/a> judiciously call a "Pattern of strange errors.". Papers in this category often use questionable (and often hidden and undocumented) data manipulations to manufacture correlations where none exist. The work by the Danish solar boosters, discussed extensively by Damon and Laut, typifies the Ugly category. We’ll leave it to the reader to decide, after the discussion to follow, whether Courtillot’s paper is merely Bad, or has crossed over into the Ugly.<\/p>\n

Spin vs. Scholarship<\/h3>\n

The general style of discourse in Courtillot et al.<\/i> has more in common with the kind of one-sided polemic one finds in Lomborg or the Robinson et al.<\/i> fake PNAS<\/i> article<\/a> distributed with the original Global Warming Petition Project<\/a> than it does with scholarship whose intent is to get at the truth. It quotes papers uncritically and selectively if they can be made to appear to support the authors’ thesis (e.g. the uncritical use of the aforementioned single-factor Scafetta and West paper to support a large attribution of twentieth century climate change to solar variability). There is also a lot of general spin here; for example, greenhouse gases are listed last in a laundry-list of things that can affect climate, without any indication as to the relative magnitudes of the various forcings. Other problems include the following:<\/p>\n