Saturday, July 14, 2007

Can Increasing Carbon Dioxide Cause Climate Change?

The following paper by Richard S. Lindzen, Professor and Climate Scientist at MIT quite clearly states that he sees no connection between increasing atmospheric carbon dioxide levels and global warming. The article, although now ten years old, still reflects his current position on the subject.

Prof. Lindzen is denounced as a global warming "skeptic", but it is difficult to argue with his credentials and his experience.

Proc. Natl. Acad. Sci. USA
Vol. 94, pp. 8335–8342, August 1997
Colloquium Paper
This paper was presented at a colloquium entitled “Carbon Dioxide and Climate Change,” organized by Charles D.
Keeling, held November 13–15, 1995, at the National Academy of Sciences, Irvine, CA.

Can increasing carbon dioxide cause climate change?
Building 54, Room 1720, Massachusetts Institute of Technology, Cambridge, MA 02139

The realistic physical functioning of the greenhouse effect is reviewed, and the role of dynamic transport and water vapor is identified. Model errors and uncertainties are quantitatively compared with the forcing due to doubling CO2, and they are shown to be too large for reliable
model evaluations of climate sensitivities.
The possibility of directly measuring climate sensitivity is reviewed.
A direct approach using satellite data to relate changes in globally averaged radiative flux changes at the top of the atmosphere to naturally occurring changes in global mean temperature is described. Indirect approaches to evaluating climate sensitivity involving the response to volcanic eruptions and Eocene climate change are also described. Finally, it is explained how,
in principle, a climate that is insensitive to gross radiative forcing as produced by doubling CO2 might still be able to undergo major changes of the sort associated with ice ages and
equable climates.

The title suggested for this paper (by Dave Keeling) is tantalizing for its ambiguity. At some level, the answer is philosophically trivial. After all, our knowledge is rarely so perfect
that we can say anything is absolutely impossible. In connection with this question we can go a bit further, and state that increasing CO2 is likely to cause some climate change, and that
the resulting change will involve average warming of the earth. However, this answer is almost as trivial as the first.
The climate is always undergoing change, and if the changes due to increasing CO2 are smaller than the natural variability, then these changes will be of only modest concern except as an
exercise in weak signal detection. The more serious question then is do we expect increasing CO2 to produce sufficiently large changes in climate so as to be clearly discernible and of
consequence for the affairs of humans and the ecosystem of which we are part. This is the question I propose to approach in this paper.
I will first consider the question of whether current model predictions are likely to be credible. We will see why this is unlikely at best. I will then show how we might estimate and bound climate sensitivity both directly and indirectly from existing data. Finally, I will consider the relationship of changes in mean temperature to changes in the structure of climate. It has been suggested that small changes in mean temperature are important because major changes in
past climate were associated with major changes in the equator-to-pole temperature difference, but only small changes in the mean temperature. I will argue that the changes in mean
temperature may be only residuals of the changes in the meridional temperature distribution rather than the cause.

The brief conclusion of this paper is that current GCMs are inadequate for the purpose of convincingly determining whether the small changes in TOA flux associated with an
increase in CO2 are capable of producing significant climate change.
However, we may not be dependent on uncertain models to ascertain climate sensitivity. Observations can potentially
directly and indirectly be used to evaluate climate sensitivity to forcing of the sort produced by increasing CO2 even without improved GCMs.
The observations needed for direct assessment are, indeed, observations that we are currently capable of making, and it is possible that the necessary observations may already be in hand, though the accuracy requirements may be greater than current data provide. Still,the importance of the question suggests that such avenues be adequately explored. Since the feedbacks involved in climate sensitivity are atmospheric, they are associated with short time scales. Oceanic delays are irrelevant, since observed surface temperatures are forcing the flux changes we are concerned with. The needed length of record must be determined empirically.
Indirect estimates, based on response to volcanos, suggest sensitivity may be as small as 0.3–0.5°C for a doubling of CO2, which is well within the range of natural variability. This is not to suggest that such change cannot be detected; rather, it is a statement that the anticipated change is well within the range of what the earth regularly deals with. It is further noted that the common assertion that even small changes in mean temperature can lead to major changes in climate distribution is ill-founded and, likely, wrong.

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