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Re: Global warming: It's happening
by Kevin E. Trenberth

A Comment by Hugh Ellsaesser Note 1
A Reply from Kevin Trenberth

From: Hugh W. Ellsaesser
Date: Thu, 29 Jan 1998 15:52:08 -0800
Subject: Comments on Trenberth's naturalSCIENCE Article

Is human activity the cause of climate warming?

In the first paragraph of his article Global Warming: Its Happening, Trenberth (1) appears to state that global warming is occurring as a result of increases in atmospheric carbon dioxide concentration resulting from such human activities as fossil fuel combustion and deforestation. However, on close examination it will be seen that he has carefully avoided making this specific claim. Moreover the impression given is misleading, for as he later acknowledges:

It is one thing to identify changes in climate that deviate from past patterns. But it is much more difficult to demonstrate that such changes are the result of human activity… As a result [of the smallness of human-induced effects and natural climate variability], any anthropogenic signal in the climate record is hard to detect. (Comment in square brackets added by author.)

Nevertheless, Trenberth accepts the conclusion of the 1995 Scientific Assessment of the Intergovernmental Panel on Climate Change (IPCC95) (2); namely, that: "The balance of evidence suggests a discernible human influence on global climate," and by implication, the conclusion that anthropogenic carbon dioxide emissions are a significant contributor to global warming. However, this was not the view of all scientists who contributed to the draft material used in preparing IPCC95, and it is not the view held today by all scientists in the field. For example, in May 1997, Klaus Hasselmann (3) of the Max-Planck-Institute for Meteorology, wrote in Science magazine:

...the inherent statistical uncertainties in the detection of anthropogenic climate change can be expected to subside only gradually in the next few years while the predicted signal is still slowly emerging from the natural climate variability noise [i.e., the predicted signal has not yet emerged].

And in the same month, Richard A. Kerr (4), in a commentary for Science, reported that:

...many climate experts caution that it is not at all clear yet that human activities have begun to warm the planet--or how bad greenhouse warming will be when it arrives.

Such doubts were recognized by those responsible for the drafting of IPCC95. Thus Benjamin D. Santer (4), Convening Lead Author of Section 8 of IPCC95 is quoted by Richard Kerr (4) to have said:

...quite clearly few scientists would say the attribution issue [that warming is due to human-activity-induced greenhouse] is a done deal.

And John Houghton, Leading Editor of IPCC95 as well as the earlier reports IPCC90 and IPCC92, told me as recently as July 8, 1996 (personal communication):

No one to my knowledge who is informed is claiming certainty of detection or attribution [of anthropogenic influence on global climate]; certainly the IPCC is not...

From a letter in Nature by John T. Houghton (5) we also know that the statement that "The balance of evidence suggests that there is a discernible human influence on global climate," was adopted at the plenary session of the IPCC in Madrid in November 1995, which meeting, he says, constituted "the final part of the very comprehensive and thorough IPCC process of peer review." The 177 delegates to this plenary (from 96 countries and including representatives from 14 NGOs and 28 IPCC lead authors) subjected the Summary for Policymakers (SPM) to line-by-line approval and "accepted" the background scientific chapters [i.e., they satisfied themselves that these chapters present a "comprehensive, objective and balanced view" of the science].

However, acceptance of these chapters by the plenary was "subject to revision by the lead authors to take into account the guidance provided by the plenary meeting and in particular the need for overall consistency" [i.e., to make the chapters consistent with the SPM]. In particular, as Houghton (5), Santer (6) and others have made clear, the IPCC procedures required the scientific chapters to be made consistent with the SPM, rather than vice versa.

In fulfilling the Herculean task of revising Chapter 8, Convening Lead Author Benjamin Santer deleted several sections that were inconsistent with the SPM including this:

Finally we have come to the most difficult question of all: "When will the detection and unambiguous attribution of human-induced climate change occur?" In the light of the very large signal and noise uncertainties discussed in the Chapter, it is not surprising that the best answer to this question is, "We do not know."

In attempting to reconcile such diverging views with the SPM, the best wording that Santer and his collaborators could devise was that:

The body of statistical evidence in Chapter 8, when examined in the context of our physical understanding of the climate system, now points toward a discernible human influence on global climate.

Defending this formula in a letter to the Wall Street Journal (6), Santer argued that this is the same "bottomline" conclusion as that of the 9 October 1995 draft version of Chapter 8.

From the record, however, it is clear that this controversial conclusion was dictated by the Madrid plenary over the objection of some scientific contributors to the draft documents, and goes beyond what any leading atmospheric scientist has yet been willing to defend before the scientific community.

Conclusion: In considering the question of human activity and climate change it is essential to distinguish between global warming, which is a progressive increase in the annual mean global temperature, and human-activity-induced greenhouse warming, as may, for example, be caused by the release of greenhouse gases into the atmosphere as a result of fossil fuel combustion or deforestation.

Almost no meteorologist denies that the mean global temperature has increased by about 0.5° C over the century and a half for which we have enough observational data to estimate a global mean. Nevertheless, it remains true today, as it was concluded in the 1990 IPCC report (Reference 7, page 254), that: is not possible at this time to attribute all, or even a large part, of the observed global-mean warming to (an) enhanced greenhouse effect on the basis of the observational data currently available.


(1) Trenberth, Kevin E. 1997. Global warming: It's happening. naturalSCIENCE, Volume 1, Article 9,

(2) IPCC95. 1996. Climate Change 1995: The Science of climate change. Eds. J. T. Houghton, L. G. Meira Filho, B. A. Callander, N. Harris, A. Kattenberg and K. Maskell, Cambridge University Press, Cambridge, UK.

(3) Hasselmann, K. 1997. Are we seeing global warming? Science 276:914-915.

(4) Kerr, R.A. 1997. Greenhouse forecasting still cloudy. Science 276:1040-1042.

(5) Houghton, J.T. 1996. Justification of Chapter 8. Nature 382:665.

(6) Santer, B.D. 1996. Letter to the Editor, Wall Street Journal, 25 June 1996.

(7) IPCC90. 1990. Climate Change, the IPCC Scientific Assessment. Eds. J. T. Houghton, G. J. Jenkins and J. J. Ephraums. Cambridge University Press, Cambridge, UK.

Hugh W. Ellsaesser
HWE Atmospheric Consultant, 4293 Stanford Way, Livermore, CA 94550, USA

1 Hugh Ellsaesser holds a Ph.D. in Meteorology and is CEO of HWE Atmospheric Consultant. He retired as a Lt. Col. after serving twenty-one years as a Weather Officer in the US Air Force. He spent the following twenty-three years as a Physicist and ten years as a Participating Guest Scientist at the Lawrence Livermore National Laboratory. He is a member of the American Meteorological Society, The American Geophysical Union and of the American Association for the Advancement of Science.

From: Kevin E. Trenberth
Date: Wed, 4 Feb 1998 13:42:51 -0700 (MST)
Reply To:
Subject: Hugh Ellsaesser's Comment

Reply to Hugh Ellsaesser

It is surprising just how much attention the IPCC statement that "The balance of evidence suggests a discernible human influence on global climate" has attracted. I was one of the scientists in Madrid at the key plenary IPCC meeting and I participated in the debate to find exactly the right words in this statement. In the IPCC process, a role of the intergovernmental meeting and the reason for having government officials involved is to aid in communication. Often there are different ways that the same thing can be said, and politicians can help scientists choose the best form of words. Whatever is written must also be effectively communicated in multiple languages; sometimes this can be challenging even in countries that share the English language. Scientists participate vigorously to defend and clarify what can and can not be said. Each word in the IPCC statement was carefully chosen. "The balance of evidence" does not mean all evidence, "suggests" was chosen over "indicates" and other alternatives, and it was recognized that, although human influences were "appreciable" and "identifiable," "discernible" was a better word with which to describe them.

Even then the statement is misinterpreted. I would assert that most of the comments given by Ellsaesser are fully compatible with the IPCC statement. The same is true of Hasslemann's article as a whole, Kerr's comments, Santer's reported comments, and Houghton's comments. The requirement that the IPCC chapters be made consistent with the summary was not to change what was said but simply to clarify it. Clarification was certainly required (scientists are often ill-equipped to communicate effectively with non-scientists); although the substance of the chapter was not changed by the revisions.

Ellsaesser is quite wrong in interpreting global warming as "a progressive increase in annual mean global temperature." Global warming refers to the heating caused by increased atmospheric concentrations of greenhouse gases, and global temperature increases are just one manifestation of that. Global temperature changes, however, have a distinct structure related to latitude and height above ground, and include cooling of the stratosphere. It is the distinctive patterns of change, as much as anything else, that provided the basis for the IPCC statement. Indeed a number of publications have made this evidence stronger than it was at that time (especially by clarifying the role of decreases in stratospheric ozone). Thus the IPCC statement does not refer to global mean temperature alone. Currently, our best understanding is that natural internal variations of annual mean temperature may account for plus or minus about 0.2 C, and solar variations may account for perhaps 0.2 C of the Twentieth Century warming. So, at a stretch, it is possible that a substantial portion of the observed 0.6 C warming might have been caused by non-human agency. Possible, but unlikely. The most likely explanation for the continued warming of the Twentieth Century is that it has arisen from the human-caused changes in atmospheric composition. It is also possible that the warming might have been even larger if natural variability has temporarily acted to counter the warming. Some warming has probably been masked by increased particulate air pollution. So I, for one, will stand by the IPCC statement that "The balance of evidence suggests a discernible human influence on global climate."


Kevin E. Trenberth
e-mail: Climate Analysis Section, NCAR, ML (303) 497 1318 P. O. Box 3000, [1850 Table Mesa Drive] (303) 497 1333 (Fax) Boulder, CO 80307 USA

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