Global cooling myth

Mai 7, 2009 at 3:48 pm Lasă un comentariu

The global cooling myth

Every now and again, the myth that „we shouldn’t believe global warming predictions now, because in the 1970’s they were predicting an ice age and/or cooling” surfaces. Recently, George Will mentioned it in his column (see Will-full ignorance) and the egregious Crichton manages to say „in the 1970’s all the climate scientists believed an ice age was coming” (see Michael Crichton’s State of Confusion ). You can find it in various other places too [here, mildly here, etc]. But its not an argument used by respectable and knowledgeable skeptics, because it crumbles under analysis. That doesn’t stop it repeatedly cropping up in newsgroups though.



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 (

The 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). In a bit more detail, people were aware of various forcing mechanisms – the ice age cycle; CO2 warming; aerosol cooling – but didn’t know which would be dominant in the near future. By the end of the 1970’s, though, it had become clear that CO2 warming would probably be dominant; that conclusion has subsequently strengthened.

George Will asserts that Science magazine (Dec. 10, 1976) warned about „extensive Northern Hemisphere glaciation.”. The quote is from Hays et al. But the quote is taken grossly out of context. Here, in full, is the small section dealing with prediction:


One approach to forecasting the natural long-term climate trend is to estimate the time constants of response necessary to explain the observed phase relationships between orbital variation and climatic change, and then to use those time constants in the exponential-response model. When such a model is applied to Vernekar’s (39) astronomical projections, the results indicate that the long-term trend over the next 20,000 years is towards extensive Northern Hemisphere glaciation and cooler climate (80).

The point about timescales is worth noticing: predicting an ice age (even in the absence of human forcing) is almost impossible within a timescale that you could call „imminent” (perhaps a century: comparable to the scales typically used in global warming projections) because ice ages are slow, when caused by orbital forcing type mechanisms.

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. In this case, its process went a bit wrong: the desire for a good story overwhelmed its reading of the NAS report which was presumably too boring to present directly.

The Hays paper above is the most notable example of the „ice age” strand. Indeed, its a very important paper in the history of climate, linking observed cycles in ocean sediment cores to orbital forcing periodicities. Of the other strand, aerosol cooling, Rasool and Schneider, Science, July 1971, p 138, „Atmospheric Carbon Dioxide and Aerosols: Effects of Large Increases on Global Climate” is the best exemplar. This contains the quote that quadrupling aerosols could decrease the mean surface temperature (of Earth) by as much as 3.5 degrees K. If sustained over a period of several years, such a temperature decrease could be sufficient to trigger an ice age!. But even this paper qualifies its predictions (whether or not aerosols would so increase was unknown) and speculates that nuclear power may have largely replaced fossil fuels as a means of energy production (thereby, presumably, removing the aerosol problem). There are, incidentally, other scientific problems with the paper: notably that the model used was only suitable for small perturbations but the results are for rather large perturbations; and that the estimate of CO2 sensitivity was too low by a factor of about 3.

Probably the best summary of the time was the 1975 NAS/NRC report. This is a serious sober assessment of what was known at the time, and their conclusion was that they didn’t know enough to make predictions. From the „Summary of principal conclusions and recommendations”, we find that they said we should:

  1. Establish National climatic research program
  3. Develope Climatic index monitoring program
  5. Adoption and development of International climatic research program
  6. Development of International Palaeoclimatic data network

Which is to say, they recommended more research, not action. Which was entirely appropriate to the state of the science at the time. In the last 30 years, of course, enormous progress has been made in the field of climate science.

Most of this post has been about the science of 30 years ago. From the point of view of todays science, and with extra data available:

  1. 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.
  2. 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).
  3. 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.

Finally, its clear that there were concerns, perhaps quite strong, in the minds of a number of scientists of the time. And yet, the papers of the time present a clear consensus that future climate change could not be predicted with the knowledge then available. Apparently, the peer review and editing process involved in scientific publication was sufficient to provide a sober view. 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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