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14 Jan 2005
The global cooling myth
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— william @ 5:31 am - (fr flag)
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.
I should clarify that I'm talking about predictions in the scientific press. There were some regrettable things published in the popular press (e.g. Newsweek; though National Geographic did better). But we're only responsible for the scientific press. If you want to look at an analysis of various papers that mention the subject, then try http://www.wmconnolley.org.uk/sci/iceage/.
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.
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:
Future climate. Having presented evidence that major changes in past climate were associated with variations in the geometry of the earth's orbit, we should be able to predict the trend of future climate. Such forecasts must be qualified in two ways. First, they apply only to the natural component of future climatic trends - and not to anthropogenic effects such as those due to the burning of fossil fuels. Second, they describe only the long-term trends, because they are linked to orbital variations with periods of 20,000 years and longer. Climatic oscillations at higher frequencies are not predicted.
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
2. Establish Climatic data analysis program, and new facilities, and studies of impact of climate on man
3. Develope Climatic index monitoring program
4. Establish Climatic modelling and applications program, and exploration of possible future climates using coupled GCMs
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.
Further Reading:
Imbrie & Imbrie "Ice Ages: solving the mystery" (1979) is an interesting general book on the discovery of the ice ages and their mechanisms; chapter 16 deals with "The coming ice age".
Spencer Weart's History of Global Warming has a chapter on Past Cycles: Ice Age Speculations.
An analysis of various papers that mention the subject is at www.wmconnolley.org.uk/sci/iceage/.
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OP-ED SCIENCE A MYTH:
GLOBAL WARMING IS HAPPENING
THOMAS R. KARL,1 KEVIN TRENBERTH2 and JAMES HANSEN3
1 National Oceanic and Atmospheric Administration, National Climatic Data Center, Asheville, NC, USA
2 Climate Analysis Section, National Center for Atmospheric Research, Boulder, CO, USA
3 NASA/Goddard Institute for Space Studies, Greenbelt, MD, USA
The recent article by the chemists, Robinson and Robinson, appearing in The Wall Street Journal's Op-ed section on Thursday, December 4, 1997 "Science Has Spoken: Global Warming is a Myth" claims that, "there is not a shred of persuasive evidence that humans have been responsible for increasing global temperatures." The title of the article and the article itself contain many factual errors, unsubstantiated claims, and misleading statements. We enumerate some of these:
* Robinson and Robinson state, "The rise in [carbon dioxide] probably results from human burning of coal, oil, and natural gas, although this is not certain." On the contrary, there is no doubt that the atmospheric carbon dioxide increase is due to human activities as well as the increase in atmospheric concentrations of other greenhouse gases, such as methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, ozone in the troposphere, and sulfur hexafluoride. What's more, scientific evidence shows that greenhouse gases, such as carbon dioxide, can remain in the atmosphere for centuries.
* Robinson and Robinson state that the global warming hypothesis is no longer tenable, and that scientists have been able to test it carefully and it no longer holds up. A review of the scientific literature reveals this simply is not true. First, there is no question that adding greenhouse gases will change the climate. There is a greenhouse effect. Second, man-made causes play a role. Most climate change detection studies find a strong likelihood, a 95% chance, that the pattern and magnitude of global warming are related to man-made causes.
* Robinson and Robinson state that the highest temperatures occurred about 1940. They further state that during the past 20 years, satellite measurements of atmospheric temperatures have actually tended to decrease. Unfortunately, the Robinsons' discussion is misleading. There are several salient facts to consider. (1) Near-surface temperatures where we live, work, and grow our food, reveal that through 1996, 1995 was the warmest year. (2) Seven of the ten warmest years on record (since 1853, when instrumental records begin) have all occurred in the past ten years. (3) During 1997, temperatures over the land are at near-record high levels as computed by both the National Oceanic and Atmospheric Administration and the National Aeronautics and Space Administration. Moreover, the United Kingdom's Meteorological Office projects 1997 to be the warmest since 1853 when both land and ocean data are combined. (4) Near-surface temperatures have increased by about 1 degree Fahrenheit (0.625 Celcius) over the past century. (5) High above the surface of the planet, one satellite temperature record (made up of contributions from nine different satellites) that averages data between about 5,000 and 30,000 feet in altitude shows some slight cooling since 1979, but the same satellite, averaging over a greater layer of the atmosphere between about 10,000 and 50,000 feet, shows warming over the same period. Moreover, weather balloons with a longer history show significant warming when they are analyzed further back in time, e.g., after 1957. (6) Lastly, extensive analysis of paleoclimatic data from tree rings, ice cores, and other sources suggest that Twentieth Century temperatures are definitely higher than any century since 1400 A.D. Prior to this time, global coverage is incomplete.
The scientific community has established an extensive peer-review process where well-qualified experts can assess and test the veracity of scientific claims. In atmospheric science, there are dozens of high-quality scientific journals with a long history of profound articles. An incorrect major scientific claim would be extremely difficult to maintain because scientists earn their reputations by developing better explanations of observed phenomena. We know of no major scientific articles since the 1996 Intergovernmental Panel on Climate Change statement that "The balance of evidence suggests a discernible human influence on global climate," that directly challenges this conclusion. On the contrary, improved analyses, data sets, and models continue to support global warming as a real phenomenon.
We may agree with Robinson and Robinson on one point. There may be more serious issues than global warming that threaten continued advances in the quality of life for humankind. One of these is the attempt by Robinson and Robinson to misinform the general public about the scientific process and what is known and unknown in the frontiers of science.
December 18, 1997
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