How does rising atmospheric CO2 affect marine organisms?

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Diffuse-Light-Enhanced Photosynthesis Revisited
Law, B.E., Falge, E., Gu,. L., Baldocchi, D.D., Bakwin, P., Berbigier, P., Davis, K., Dolman, A.J., Falk, M., Fuentes, J.D., Goldstein, A., Granier, A., Grelle, A., Hollinger, D., Janssens, I.A., Jarvis, P., Jensen, N.O., Katul, G., Mahli, Y., Matteucci, G., Meyers, T., Monson, R., Munger, W., Oechel, W., Olson, R., Pilegaard, K., Paw U, K.T., Thorgeirsson, H., Valentini, R., Verma, S., Vesala, T., Wilson, K. and Wofsy, S.  2002.  Environmental controls over carbon dioxide and water vapor exchange of terrestrial vegetation.  Agricultural and Forest Meteorology 113: 97-120.

In our Editorial of 10 October 2001, we describe a negative feedback mechanism that may help to protect the earth against excessive CO2-induced warming.  Very briefly, it begins with an increase in the air's CO2 content that increases both the amount and vitality of earth's plant life.  This augmented vegetative activity leads to an increase in the production of "biosols" that function as cloud condensation nuclei and thus enhance the degree of cloud cover and the amount of diffuse solar radiation reaching the earth's surface.  Penetrating deeper into plant canopies than does the direct solar beam, this extra diffuse radiation boosts overall rates of canopy net photosynthesis, leading to more CO2 being withdrawn from the atmosphere and completing the negative feedback loop.

What was done
The authors of this comprehensive study compared seasonal and annual values of CO2 and water vapor exchange across sites in forests, grasslands, crops and tundra that are part of an international network called FLUXNET, investigating the responses of these exchanges to variations in a number of environmental factors, including direct and diffuse solar radiation.

What was learned
In the words of the authors, "net carbon uptake (net ecosystem exchange, the net of photosynthesis and respiration) was greater under diffuse than under direct radiation conditions."  In discussing this finding, which is the centerpiece of the negative feedback phenomenon we describe, they note that "cloud-cover results in a greater proportion of diffuse radiation and constitutes a higher fraction of light penetrating to lower depths of the canopy (Oechel and Lawrence, 1985)."  More importantly, they also report that "Goulden et al. (1997), Fitzjarrald et al. (1995), and Sakai et al. (1996) showed that net carbon uptake was consistently higher during cloudy periods in a boreal coniferous forest than during sunny periods with the same PPFD [photosynthetic photon flux density]."  In fact, they say that "Hollinger et al. (1994) found that daily net CO2 uptake was greater on cloudy days, even though total PPFD was 21-45% lower on cloudy days than on clear days [our italics]."

What it means
The authors findings, as well as those of the other scientists they cite, provide strong support for the negative feedback phenomenon we outline in our Editorial of 10 October 2001, which describes but one of the many different ways in which earth's biosphere tempers the tendency for global warming produced by man's CO2 emissions.

Fitzjarrald, D.R., Moore, K.E., Sakai, R.K. and Freedman, J.M.  1995.  Assessing the impact of cloud cover on carbon uptake in the northern boreal forest.  In: Proceedings of the American Geophysical Union Meeting, Spring 1995, EOS Supplement, p. S125.

Goulden, M.L., Daube, B.C., Fan, S.-M., Sutton, D.J., Bazzaz, A., Munger, J.W. and Wofsy, S.C.  1997.  Physiological responses of a black spruce forest to weather.  Journal of Geophysical Research 102: 28,987-28,996.

Hollinger, D.Y., Kelliher, F.M., Byers, J.N. and Hunt, J.E.  1994.  Carbon dioxide exchange between an undisturbed old-growth temperate forest and the atmosphere.  Ecology 75: 134-150.

Oechel, W.C. and Lawrence, W.T.  1985.  Tiaga.  In: Chabot, B.F. and Mooney, H.A. (Eds.), Physiological Ecology of North American Plant Communities.  Chapman & Hall, New York, NY, pp. 66-94.

Sakai, R.K., Fitzjarrald, D.R., Moore, K.E. and Freedman, J.M.  1996.  How do forest surface fluxes depend on fluctuating light level?  In: Proceedings of the 22nd Conference on Agricultural and Forest Meteorology with Symposium on Fire and Forest Meteorology, Vol. 22, American Meteorological Society, pp. 90-93.

Reviewed 12 March 2003