It is difficult these days to get a paper published in a mainstream climate journal if it emphasises the uncertainty associated with some basic aspect of global warming.
But occasionally it happens, and the research establishment then tends to get its knickers in a knot. Its actions can range from simply ignoring the paper’s existence to vengeful attempts at removal of the relevant journal editor. Of course the professional (and ethical) re-action should be to publish a rebuttal pointing out all the scientific problems with the original paper. This is fine in principle, except that the dice of the publication stakes are still loaded in favour of the politically correct, and the rebuttal can be fairly loose. The problem is that the tone and substance of the rebuttal is more-or-less immaterial in the grand scheme of things. The important thing is to have the last word in the formal literature. With the passage of time, loyalty to the establishment ensures that “Jones has raised doubts about Smith’s result on the grounds of……” becomes “Jones has shown Smith to be an idiot and his results are nonsense”. Its all a bit 1984-ish.
The fuss about Andy Dessler’s response to Roy Spencer and William Braswell’s paper on cloud feedback has a certain déjà vu to it. It seems to me (as a biased observer) that Dr Dessler is becoming more than a little industrious in his use of rebuttal papers to defend the concept of long-term positive feedback in the climate system.
A couple of years ago, three of us produced a paper pointing out that, if one were prepared to believe the NCEP re-analysis trend of middle-tropospheric humidity, one would also have to believe that the long-term feedback of atmospheric water-vapour is negative. Some time later, Dessler and Davis submitted a short paper to JGR effectively saying that nobody in his right mind would believe any such trend coming out of NCEP data. And of course they may have a point. However, I was one of the JGR reviewers of their paper, and for various reasons was not too happy or complimentary about it. In the end it was published without any real attention to the objections. Fair enough, editors have to make decisions like that.
Eventually however I got around to submitting to JGR a formal comment on Dessler and Davis’s paper. It was effectively a tidied-up version of the review comments. In due course it was knocked back by JGR on the grounds (mainly) that it had no new research to report. Which is a bit strange since it was only a comment, not a research paper.
Anyway, the comment ran as follows. It seemed reasonable at the time.
Comment on “Trends in tropospheric humidity from reanalysis systems” by Dessler and Davis
Dessler and Davis (2010) are of the opinion that the negative trends of specific humidity in the middle and upper troposphere which appear in the National Centers for Environmental Prediction (NCEP) re-analysis data are spurious – and therefore that overall water vapour feedback in the climate system is indeed positive and is an amplifier of global warming. The trends were reported and discussed by Paltridge, Arking and Pook (2009), hereafter referred to as P09. Dessler and Davis support their opinion with:
(a) Comparison of the NCEP trends with the trends in four other re-analysis data sets. The comparison indicates that the three more recent re-analyses all have positive trends in the middle and upper troposphere.
(b) Correlations in each of the re-analyses, including NCEP, between specific humidity q at various levels and surface temperature T. They all show that, at least for short-term change on time scales less than 10 years, the correlations are positive and thereby indicate positive feedback.
(c) Comment that the concept of long-term positive water-vapour feedback is in accord with virtually all the independent lines of evidence: models, observations, theory, and newer re-analyses.
RE (a): The NCEP re-analysis relies on balloon observations of water vapour, whereas all the other re-analyses use satellite data of one form or another. Some of the other re-analyses use balloon data as well as satellite data. The ECMWF ERA40 re-analysis includes balloon data, and has a tendency toward negative trends of mid-level (roughly 400 and 500 hPa) specific humidity. The Modern Era Retrospective-Analysis for Research and Applications makes use of balloon data, and if one excludes the obviously anomalous El Nino year of 1998, the tropical mid-level q of that re-analysis has no significant trend one way or the other.
Raw balloon data (e.g. tropical radiosonde data as discussed briefly in P09 for instance) seem to show a negative long-term trend of q at mid-levels. Raw satellite data (as mentioned in P09 and by Dessler and Davis for instance) seem to show a positive trend.
So the most likely and straightforward explanation of the difference between the outputs of the re-analysis schemes is that they (the schemes) are actually behaving as they were intended – namely, they are simply reflecting the behaviour of their different sources of input data. If so, then the issue is not whether some re-analyses are newer than others, but which source of input data (balloon or satellite) has the greater potential for error, and whether, in either case, those errors would or could lead to the trends that are observed. Any significant attempt to resolve such a question would have to consider not only the potential errors of both the balloon and the satellite information, but also the possibility that different sorts of satellite data have been introduced into the re-analysis schemes at different times over the 30-year period since 1979.
The point here is that it is distorting the situation somewhat to give the impression that the NCEP re-analysis is a single “outlier” pitched against a number of other independent re-analyses and can therefore probably be discarded. If one ignores the question as to which are the more reliable as input data (i.e. satellite or balloon data), the balance of ‘likelihood of verisimilitude’ between the NCEP re-analysis and the others has to be more like 50:50.
And therefore it is also distorting the situation somewhat to discuss the well- known problems associated with balloon measurements and make no reference to the many and various problems associated with satellite data. It is difficult enough to believe trends of total water vapour content of the atmosphere from past satellite measurements, let alone the trends of water vapour concentration at any particular level.
Re (b): Much of the Dessler and Davis discussion is devoted to the possibility flagged in PO9 that the long-term correlation between middle and upper level specific humidity q and surface temperature T might be negative even though the short-term correlation is positive.
Dessler and Davis say that there is no theory to explain such a difference in the sign of the correlations. Suffice it to say that P09 contains considerable discussion concerning two (admittedly only qualitative) theoretical suggestions as to how such an eventuality might occur. The suggestions concern possible causes of long-term increase in the stability of the lower atmosphere – an event which, according to the NCEP data, indeed seems to have occurred over the last few decades, and which, if real and continued, could confine a long-term increase of water vapour concentration to the convective boundary layer. One of the possible causes is the relatively large increase in radiative heating in the middle troposphere associated with increasing CO2. This is not to say that such theories are correct, but the absence of any reference to them by Dessler and Davis suggests a reluctance even to contemplate arguments on the other side of the fence.
Dessler and Davis make the point that there is poorer agreement among the re-analyses about the q vs T correlation on time scales longer than 10 years than there is about the correlation on time scales less than 10 years. They attribute this to “handling data inhomogeneities” having more impact on long-term trends than short-term trends. This is of course possible. But they could also have pointed out that, at least at face value, the slopes of the long-term correlations displayed in their diagram are generally a lot less than those of the short-term correlations, and that some of them are indeed negative at certain levels. The thing to remember here is that even a reduction of slope (if it were verified) would be very significant in the overall water-vapour feedback story.
Re (c): Superficially the statement is impressive. One wonders however what is this theory that is supposed to be an independent line of evidence – apart, that is, from the individual bits of theory so far built into the models. And in view of the discussion above about whether reference to the “newer re-analyses” is really germane to the issue, one wonders also about the significance of those analyses in the present context. And in view of the fact that the veracity of models relies (among many other things) upon the observations on which they are based, it is pushing things a bit far to say that models are truly “independent” evidence. Perhaps more to the point in the context of models, their long-term trends of q depend, among other things, on a correct simulation of vertical sub-grid-scale diffusion – the contribution of which is one of the most difficult characteristics of models to verify independently.
The bottom line here is that “virtually all the independent lines of evidence” probably boil down only to the observations. And at the moment, bearing in mind the discussion with regard to (a) above, there is still a lot of work to be done to establish just what the observations are telling us. The issue of the magnitude and sign of long-term water vapour feedback is far from resolved.
Dessler, A. E., and S. M. Davis (2010), Trends in tropospheric humidity from reanalysis systems, J. Geophys. Res., 115, D19127, doi:10.1029/2010JDO14192.
Paltridge, G. W., A. Arking, and M. Pook (2009), Trends in middle- and upper-level tropospheric humidity from NCEP reanalysis data, Theor. Appl. Climatol., 98, 351-359, doi:10.1007/s00704-009-0117-x.
Biosketch: Garth Paltridge was a Chief Research Scientist with the CSIRO Division of Atmospheric Research before his appointments as Director of the Institute of Antarctic and Southern Ocean Studies at the University of Tasmania and as CEO of the Antarctic Cooperative Research Centre. He retired in 2002, and is currently an Emeritus Professor with the University of Tasmania and a Visiting Fellow at the Australian National University.
Moderation note: this is a technical thread and comments will be moderated for relevance.