Reference
Pacala, S.W., Hurtt, G.C., Baker, D., Peylin, P., Houghton, R.A., Birdsey, R.A., Heath, L., Sundquist, E.T., Stallard, R.F., Ciais, P., Moorcroft, P., Caspersen, J.P., Shevliakova, E., Moore, B., Kohlmaier, G., Holland, E., Gloor, M., Harmon, M.E., Fan, S.-M., Sarmiento, J.L., Goodale, C.L., Schimel, D. and Field, C.B. 2001. Consistent land- and atmosphere-based U.S. carbon sink estimates. Science 292: 2316-2320.
What was done
By means of careful and detailed analyses, the authors derived separate land- and atmosphere-based estimates of the amount of carbon stored in the coterminous United States over the past two decades.
What was learned
For the period 1980-89, the authors land-based approach suggested the United States was a sink for carbon of magnitude somewhere in the range of 0.30 to 0.58 petagrams (Pg) of carbon (C) per year, where 1 Pg C = 1015 g C; but because additional carbon was exported from the country by rivers and commerce, the net flux of carbon from the atmosphere to the land was as high as 0.37 to 0.71 Pg C per year.
These estimates are considerably larger than earlier land-based estimates of 0.08 to 0.35 Pg C per year, due to "the inclusion of additional processes and revised estimates of some component fluxes." However, they fall in the mid-range of estimates the authors derived from independent atmospheric approaches to the problem, suggesting that the results of both techniques are robust, although the atmospheric approach has much greater uncertainty associated with it. Viewed in this light, the maximum land-based carbon sequestration value of 0.71 Pg C per year cannot be said to be significantly different from the values of 0.81 to 0.84 Pg C per year the authors calculate for the United States based on the earlier landmark atmospheric analysis of Fan et al. (1998).
What it means
The authors note that the large carbon sink they have documented to exist within the borders of the coterminous United States "stores between one-third and two-thirds of a billion tons of carbon annually," which Wofsy (2001) says is "equivalent to 20 to 40 percent of fossil fuel emissions worldwide." This statement, however, is not correct. It applies to the United States only, not the whole world. Nevertheless, it is clear the United States is offsetting much of its anthropogenic CO2 emissions by this means, if that is indeed a goal of any virtue, which, to be truthful, we seriously doubt. It also demonstrates the great potential of the biosphere to keep its own house in order. And that is why, as Wofsy also notes, "emission rates of CO2 from combustion of fossil fuel have increased almost 40 percent in the past 20 years, but the amount of CO2 accumulating in the atmosphere has stayed the same or even declined slightly." [Our italics.]
Yes, we are by no means headed for a runaway atmospheric CO2 greenhouse effect, or even a runaway atmospheric CO2 concentration. The biosphere is beginning to exert a powerful brake on the CO2-emitting side effects of the Industrial Revolution, as was accurately predicted by Idso (1991a,b) fully ten years ago.
ReferencesFan, S., Gloor, M., Mahlman, J., Pacala, S., Sarmiento, J., Takahashi, T. and Tans, P. 1998. A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data and models. Science 282: 442-446.
Idso, S.B. 1991a. The aerial fertilization effect of CO2 and its implications for global carbon cycling and maximum greenhouse warming. Bulletin of the American Meteorological Society 72: 962-965.
Idso, S.B. 1991b. Reply to comments of L.D. Danny Harvey, Bert Bolin, and P. Lehmann. Bulletin of the American Meteorological Society 72: 1910-1914.
Wofsy, S.C. 2001. Where has all the carbon gone? Science 292: 2261-2263.