Ezra G's List: Global Warming Debate

• Any mainstream scientist present will trot out the standard response that water vapour is indeed an important greenhouse gas, it is included in all climate models, but it is a feedback and not a forcing.
• In terms of mass, water vapour is much more prevalent (about 0.3% of atmospheric mass, compared to about 0.06% for CO₂), and so is ~80% of all greenhouse gases by mass (~90% by volume). However, the radiative importance is less (since all molecules are not created equal). One way to quantify this is to take a radiation model and remove each long-wave absorber (principally the greenhouse gases, but also clouds and aerosols) and see what difference it makes to the amount of long-wave absorbed.
• The overlaps complicate things, but it's clear that water vapour is the single most important absorber (between 36% and 66% of the greenhouse effect), and together with clouds makes up between 66% and 85%. CO₂ alone makes up between 9 and 26%, while the O₃ and the other minor GHG absorbers consist of up to 7 and 8% of the effect, respectively.
• the maximum supportable number for the importance of water vapour alone is about 60-70% and for water plus clouds 80-90% of the present day greenhouse effect.
• So where does the oft quoted "98%" number come from? This proves to be a little difficult to track down. Richard Lindzen quoted it from the IPCC (1990) report in a 1991 QJRMS review* as being the effect of water vapour and stratiform clouds alone, with CO₂ being less than 2%. However, after some fruitless searching I cannot find anything in the report to justify that (anyone?). The calculations here (and from other investigators) do not support such a large number and I find it particularly odd that Lindzen's estimate does not appear to allow for any overlap.
• the issue that makes it a feedback (rather than a forcing) is the relatively short residence time for water in the atmosphere (around 10 days).
• Only the stratosphere is dry enough and with a long enough residence time (a few years) for the small anthropogenic inputs to be important. In this case (and in this case only) those additions can be considered a forcing. Oxidation of anthropogenic methane (which is a major source of stratospheric water) and, conceviably, direct deposition of water from increases in aircraft in the lower stratosphere, can increase stratospheric water and since that gives a radiative forcing effect, they do appear on the forcings bar chart (under "H₂O from CH₄").

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• Where does the myth come from? Naturally enough, there is a kernel of truth behind it all. Firstly, there was a trend of cooling from the 40's to the 70's (although that needs to be qualified, as hemispheric or global temperature datasets were only just beginning to be assembled then). But people were well aware that extrapolating such a short trend was a mistake (Mason, 1976) . Secondly, it was becoming clear that ice ages followed a regular pattern and that interglacials (such as we are now in) were much shorter that the full glacial periods in between. Somehow this seems to have morphed (perhaps more in the popular mind than elsewhere) into the idea that the next ice age was predicatable and imminent. Thirdly, there were concerns about the relative magnitudes of aerosol forcing (cooling) and CO2 forcing (warming), although this latter strand seems to have been short lived.
• he state of the science at the time (say, the mid 1970's), based on reading the papers is, in summary: "…we do not have a good quantitative understanding of our climate machine and what determines its course. Without the fundamental understanding, it does not seem possible to predict climate…" (which is taken directly from NAS, 1975).
• Will also quotes "a full-blown 10,000-year ice age" (Science, March 1, 1975). The quote is accurate, but the source isn't. The piece isn't from "Science"; it's from "Science News". There is a major difference: Science is (jointly with Nature) the most prestigous journal for natural science; Science News is not a peer-reviewed journal at all, though it is still respectable.
• • The cooling trend from the 40's to the 70's now looks more like a slight interruption of an upward trend (e.g. here). It turns out that the northern hemisphere cooling was larger than the southern (consistent with the nowadays accepted interpreation that the cooling was largely caused by sulphate aerosols); at first, only NH records were available. 
   
  • Sulphate aerosols have not increased as much as once feared (partly through efforts to combat acid rain); CO2 forcing is greater. Indeed IPCC projections of future temperature inceases went up from the 1995 SAR to the 2001 TAR because estimates of future sulphate aerosol levels were lowered (SPM). 
   
  • Interpretations of future changes in the Earth's orbit have changed somewhat. It now seems likely (Loutre and Berger, Climatic Change, 46: (1-2) 61-90 2000) that the current interglacial, based purely on natural forcing, would last for an exceptionally long time: perhaps 50,000 years.
• This episode shows the scientific press in a very good light; and a clear contrast to the lack of any such process in the popular press, then and now.

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• Several of my colleagues complained that a more significant error is Gore's use of the long ice core records of CO₂ and temperature (from oxygen isotope measurements) in Antarctic ice cores to illustrate the correlation between the two. The complaint is that the correlation is somewhat misleading, because a number of other climate forcings besides CO₂ contribute to the change in Antarctic temperature between glacial and interglacial climate. Simply extrapolating this correlation forward in time puts the temperature in 2100 A.D. somewhere upwards of 10 C warmer than present — rather at the extreme end of the vast majority of projections (as we have discussed here).
• The small errors don't detract from Gore's main point, which is that we in the United States have the technological and institutional ability to have a significant impact on the future trajectory of climate change. This is not entirely a scientific issue — indeed, Gore repeatedly makes the point that it is a moral issue — but Gore draws heavily on Pacala and Socolow's recent work to show that the technology is there (see Science 305, p. 968 Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies).