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Global Climate Change Digest

A Guide to Information on Greenhouse Gases and Ozone Depletion
Published July 1988 through June 1999


Feature of the Month: North America as a Carbon Sink

Item #d98oct44

Just before the negotiators were to meet in Buenos Aires to deliberate such topics as emissions trading, credits for reforestation, emission offsets, and certification of emission reductions, a team of highly regarded researchers published an article in Science (see Fan et al. article in Prof. Pubs/Of General Interest in this issue) indicating that the United States, which is widely regarded as a major producer of greenhouse gases, is indeed a net sink for CO2. The report immediately raised questions about whether the United States (and Canada, too, for that matter) had any emissions of carbon dioxide to negotiate about. As The Boston Globe (Oct. 16) put it, concerns were being voiced that “groups opposed to the 1997 Kyoto climate treaty will use the findings to argue that the United States does not need to reduce its emissions of so-called greenhouse gases.” After all, fitting a series of computer models to hard data that had been collected at 63 aerometric stations around the world indicated that North America absorbed an annual mean of 1.7 ± 0.5 PgC/y between 1988 and 1992 as the winds crossed the continent from west to east. This value represents about a third of the CO2 released to the atmosphere each year in the United States and about the same amount produced by the burning of fossil fuels in the United States and Canada.

The magnitude of the estimated uptake startled the climate-change research community. In the very issue of Science in which the article was published, colleagues and competitors of the researchers commented on the results [“Possibly Vast Greenhouse Gas Sponge Ignites Controversy,” Science 282 (5388), 386-387]. David Schimel said there was “a huge amount of skepticism about the result.” (He was also quoted in the Globe article as saying, “I don’t believe this result.”) Richard Houghton, an expert on carbon uptake by land covers, said “It’s hard for me to know where that much carbon could be accumulating in North America.” And Inez Fung, a coauthor with several of the research team members on previous publications, noted that, if the two models used to gauge carbon flux were “off by just a little bit, ... you get a very different conclusion.”

Indeed, as the article itself points out, if the data from one more measuring station were included in the calculations, the amount of carbon absorbed in North America would be decreased by almost 30%. The station referred to is Sable Island in Nova Scotia, which was excluded because of questions raised about the data’s reliability. That one change would bring the estimate much closer to the value of maximum possible uptake calculated from the ground up (i.e., by estimating forest and plant uptake on a per-hectare basis and extrapolating to larger areas).

Questions about the timing of the observation period (1988 to 1992) were also raised. In the press release issued by one of the researchers’ institutions (“North America Absorbing Carbon Dioxide at Surprisingly High Rate, Team Report,” Columbia University News, Oct. 15), other studies are noted that indicate that the size of a sink (the magnitude of CO2 absorption) can vary by a factor of 5 from year to year. Still unknown is how representative are the data that were used. Also unknown is the meaningfulness of the underlying assumption that air masses traverse the United States from west to east instead of varying markedly in direction from season to season and even from air mass to air mass.

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