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Diffuse Light: A Powerful Promoter of Photosynthesis
Volume 6, Number 32: 6 August 2003

In our Editorial of 10 October 2001 -- Yet Another Biophysical Feedback Mechanism That May Help to Protect the Planet Against Deleterious CO2-Induced Global Warming -- we suggested that an increase in the air's CO2 concentration would (1) enhance the photosynthetic rates of earth's plants (thereby enabling them to withdraw more CO2 from the air) and (2) enhance the water use efficiencies of earth's plants (thereby enabling them to grow where it was previously too dry for them and, therefore, enabling them to remove still more CO2 from the air), which would thus complete a "double-barreled" negative feedback loop that would slow the rate of rise of the air's CO2 content and help reduce both the rate and ultimate magnitude of global warming. None of these ideas, of course, was new, the rising-CO2-induced reverse-desertification concept having been suggested nearly two decades earlier by Idso and Quinn (1983) and the rising-CO2-induced increase in carbon sequestration having been described and roughly quantified by Idso (1991a, b).

Next, we suggested that a largely-ignored biological phenomenon could well serve as a significant amplifier of this negative feedback cycle. Specifically, we proposed that the increased amount of vegetation in a CO2-enriched world, together with its greater robustness or enhanced activity, would (1) boost the atmosphere's concentration of biosols, i.e., aerosols that owe their existence to the biological activities of earth's vegetation, which would (2) lead to the creation of more cloud condensation nuclei, which would (3) act to ultimately cool the planet by creating more clouds of a more highly reflective nature, as originally proposed by Charlson et al. (1987) within the context of their "oceanic phytoplankton, atmospheric sulfur, cloud albedo and climate" scenario.

Although this idea was modestly original, we added a new dimension that made it even more so, proposing that in addition to reducing the amount of total (direct plus diffuse) solar radiation reaching the surface of the earth, more biosols in the atmosphere, together with the extra cloud particles they created, would actually (1) enhance the amount of surface-available diffuse solar radiation, which would (2) reduce the volume of shade within plant canopies, which would (3) increase whole-canopy photosynthesis, which would (4) lead to even more CO2 being removed from the atmosphere. Again, however, none of these individual linkages was original with us. In fact, Roderick et al. (2001) had actually suggested all of the linkages described in this paragraph, with the slight difference that in the place of biosols they used the more inclusive term aerosols.

But Roderick et al. did much more than merely suggest this latter set of linkages. Noting that the volcanic eruption of Mount Pinatubo in June of 1991 had ejected enough sulfur gases and fine particulate matter into the atmosphere to produce sufficient aerosol particles to greatly increase the diffuse component of solar radiation reaching the surface of the earth, they carried out a set of lengthy calculations, which indicated that the Mount Pinatubo eruption may have led to the removal of an extra 2.5 Gt of carbon from the atmosphere due to its aerosol-enhanced diffuse-light stimulation of terrestrial photosynthesis in the year following the eruption, which would have reduced the magnitude of the long-term rise in the air's CO2 concentration that year by about 1.2 ppm, which is about the size of the real-world perturbation that was actually observed (Sarmiento, 1993).

So why this lengthy recap of our Editorial of 10 October 2001? What's new? Actually, something pretty important: a real-world experimental observation of the significant ability of enhanced diffuse light to increase the photosynthetic prowess of earth's plants and their capacity to withdraw greater amounts of CO2 from the atmosphere, as recently described by Gu et al. (2003) and discussed by Farquhar and Roderick (2003).

Specifically, Gu et al. report that they "used two independent and direct methods to examine the photosynthetic response of a northern hardwood forest (Harvard Forest, 42.5N, 72.2W) to changes in diffuse radiation caused by Mount Pinatubo's volcanic aerosols [our italics]," finding that "around noontime in the midgrowing season, the gross photosynthetic rate under the perturbed cloudless [our italics] solar radiation regime was 23, 8, and 4% higher than that under the normal cloudless [our italics] solar radiation regime in 1992, 1993, and 1994, respectively," and that "integrated over a day, the enhancement for canopy gross photosynthesis by the volcanic aerosols [our italics] was 21% in 1992, 6% in 1993 and 3% in 1994." Commenting on the significance of these observations, Gu et al. note that "because of substantial increases in diffuse radiation world-wide after the eruption and strong positive effects of diffuse radiation for a variety of vegetation types, it is likely that our findings at Harvard Forest represent a global phenomenon [our italics]."

In the preceding paragraph, we have highlighted the fact that the diffuse-light-induced photosynthetic enhancement observed by Gu et al., in addition to likely being global in scope, was caused by volcanic aerosols under acting under cloudless conditions. Our reason for calling attention to these two italicized words is to clearly distinguish this phenomenon from a closely related one that is also described by Gu et al. (2003), i.e., the propensity for the extra diffuse light created by increased cloud cover to further enhance photosynthesis, even though the total flux of solar radiation received at the earth's surface may be reduced under such conditions (Stanhill and Cohen, 2001). Based on still more real-world data, for example, Gu et al. note that "Harvard Forest photosynthesis also increases with cloud cover, with a peak at about 50% cloud cover," citing as evidence their Supporting Online Material.

In light of these several observations, it should be painfully obvious, as we originally suggested in our Editorial of 10 October 2001, that the historical and still-ongoing CO2-induced increase in atmospheric biosols should have had, and should be continuing to have, a significant cooling effect on the planet that exerts itself by both slowing the rate of rise of the air's CO2 content and reducing the receipt of solar radiation at the earth's surface, neither of which effects is included in any general circulation model of the atmosphere of which we are aware. Hence, it should be equally obvious that climate-alarmist predictions of catastrophic CO2-induced global warming are simply catastrophic exaggerations.

Sherwood, Keith and Craig Idso

Charlson, R.J., Lovelock, J.E., Andrea, M.O. and Warren, S.G. 1987. Oceanic phytoplankton, atmospheric sulfur, cloud albedo and climate. Nature 326: 655-661.

Farquhar, G.D. and Roderick, M.L. 2003. Pinatubo, diffuse light, and the carbon cycle. Science 299: 1997-1998.

Gu, L., Baldocchi, D.D., Wofsy, S.C., Munger, J.W., Michalsky, J.J., Urbanski, S.P. and Boden, T.A. 2003. Response of a deciduous forest to the Mount Pinatubo eruption: Enhanced photosynthesis. Science 299: 2035-2038.

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.

Idso, S.B. and Quinn, J.A. 1983. Vegetational Redistribution in Arizona and New Mexico in Response to a Doubling of the Atmospheric CO2 Concentration. Climatological Publications Scientific Paper No. 17. Laboratory of Climatology, Arizona State University, Tempe, AZ, USA.

Roderick, M.L., Farquhar, G.D., Berry, S.L. and Noble, I.R. 2001. On the direct effect of clouds and atmospheric particles on the productivity and structure of vegetation. Oecologia 129: 21-30.

Sarmiento, J.L. 1993. Atmospheric CO2 stalled. Nature 365: 697-698.

Stanhill, G. and Cohen, S. 2001. Global dimming: a review of the evidence for a widespread and significant reduction in global radiation with discussion of its probable causes and possible agricultural consequences. Agricultural and Forest Meteorology 107: 255-278.