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Feedback Factors (Diffuse Light) -- Summary
The initial impetus for the increase in surface air temperature in the negative feedback phenomenon we describe here focuses exclusively on the incremental enhancement of the atmosphere's greenhouse effect that is produced by an increase in the air's CO2 content; and from this starting point we identify a chain of events that ultimately counteracts this impetus for warming by the incremental enhancement of the planet's natural rate of CO2 removal from the air.

The first of the linkages in this negative feedback loop is the proven propensity for higher levels of atmospheric CO2 to enhance vegetative productivity (see Plant Growth in our Data Center and Water Use Efficiency in our Subject Index for verification), which phenomena are themselves powerful negative feedback mechanisms of the type we envision. Greater CO2-enhanced photosynthetic rates, for example, enable plants to remove considerably more CO2 from the air than they do under current conditions; while CO2-induced increases in plant water use efficiency allow plants to grow where it was previously too dry for them. This latter consequence of atmospheric CO2 enrichment establishes a potential for more CO2 to be removed from the atmosphere by increasing the abundance of earth's plants, whereas the former phenomenon does so by increasing their robustness.

The second of the linkages of the new feedback loop is the ability of plants to emit gases to the atmosphere that are ultimately converted into "biosols," i.e., aerosols that owe their existence to the biological activities of earth's vegetation, many of which function as cloud condensation nuclei. It takes little imagination to realize that since the existence of these atmospheric particles is dependent upon the physiological activities of plants and their associated soil biota, the CO2-induced presence of more and more-highly-productive plants will lead to the production of more of these cloud-mediating particles, which can then act to cool the planet. But this two-linkage-long negative feedback effect, like the one-linkage-long dual cooling mechanism described in the previous paragraph, is still not the endpoint of the new feedback loop we are describing.

The third linkage of the new scenario is the observed propensity for increases in aerosols and cloud particles to enhance the amount of diffuse solar radiation reaching the earth's surface. The fourth linkage is the ability of enhanced diffuse lighting to reduce the volume of shade within vegetative canopies. The fifth linkage is the tendency for less internal canopy shading to enhance whole-canopy photosynthesis, which finally produces the end result: a greater biological extraction of CO2 from the air and the subsequent sequestration of its carbon, compliments of the intensified diffuse-light-driven increase in total canopy photosynthesis and subsequent transfers of the extra fixed carbon to plant and soil storage reservoirs.

How significant is this multi-link process? Roderick et al. (2001) provide a good estimate based on the utilization of a unique "natural experiment," a technique that has been used extensively by Idso (1998) to evaluate the climatic sensitivity of the entire planet. Specifically, Roderick and his colleagues considered the volcanic eruption of Mt. Pinatubo in June of 1991, which ejected enough gases and fine materials into the atmosphere to produce sufficient aerosol particles to greatly increase the diffuse component of the solar radiation reaching the surface of the earth from that point in time through much of 1993, while only slightly reducing the receipt of total solar radiation. Based on a set of lengthy calculations, they concluded that the Mt. Pinatubo eruption may well have resulted in the removal of an extra 2.5 Gt of carbon from the atmosphere due to its diffuse-light-enhancing stimulation of terrestrial vegetation in the year following the eruption, which would have reduced the ongoing rise in the air's CO2 concentration that year by about 1.2 ppm.

Interestingly, this reduction is about the magnitude of the real-world perturbation that was actually observed (Sarmiento, 1993). What makes this observation even more impressive is the fact that the CO2 reduction was coincident with an El Niño event; because, in the words of Roderick et al., "previous and subsequent such events have been associated with increases in atmospheric CO2." In addition, the observed reduction in total solar radiation received at the earth's surface during this period would have had a tendency to reduce the amount of photosynthetically active radiation incident upon earth's plants, which would also have had a tendency to cause the air's CO2 content to rise, as it would tend to lessen global photosynthetic activity.

Significant support for the new negative feedback phenomenon was swift in coming, as the very next year a team of 33 researchers published the results of a comprehensive study (Law et al., 2002) that compared seasonal and annual values of CO2 and water vapor exchange across sites in forests, grasslands, crops and tundra -- which 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. As for their findings, the huge group of researchers reported that "net carbon uptake (net ecosystem exchange, the net of photosynthesis and respiration) was greater under diffuse than under direct radiation conditions," and in discussing this finding, which is the centerpiece of the negative feedback phenomenon we describe, they noted 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 reported 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 wrote 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]."

One year later, Gu et al. (2003) reported that they "used two independent and direct methods to examine the photosynthetic response of a northern hardwood forest (Harvard Forest, 42.5°N, 72.2°W) to changes in diffuse radiation caused by Mount Pinatubo's volcanic aerosols," finding that in the eruption year of 1991, "around noontime in the mid-growing 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. noted 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 [our italics] phenomenon."

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., 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. 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."

Although very impressive, in all of the situations discussed above the source of the enhanced atmospheric aerosol concentration was a singular significant event -- specifically, a massive volcanic eruption -- but what we really need to know is what happens under more normal conditions. This was the new and important question that was addressed the following year in the study of Niyogi et al. (2004): "Can we detect the effect of relatively routine aerosol variability on field measurements of CO2 fluxes, and if so, how does the variability in aerosol loading affect CO2 fluxes over different landscapes?"

To answer this question, the group of sixteen researchers used CO2 flux data from the AmeriFlux network (Baldocchi et al., 2001) together with cloud-free aerosol optical depth data from the NASA Robotic Network (AERONET; Holben et al., 2001) to assess the effect of aerosol loading on the net assimilation of CO2 by three types of vegetation: trees (broadleaf deciduous forest and mixed forest), crops (winter wheat, soybeans and corn) and grasslands. Their work revealed that an aerosol-induced increase in diffuse radiative-flux fraction [DRF = ratio of diffuse (Rd) to total or global (Rg) solar irradiance] increased the net CO2 assimilation of trees and crops, making them larger carbon sinks, but that it decreased the net CO2 assimilation of grasslands, making them smaller carbon sinks.

How significant were the effects observed by Niyogi et al.? For a summer mid-range Rg flux of 500 W m-2, going from the set of all DRF values between 0.0 and 0.4 to the set of all DRF values between 0.6 and 1.0 resulted in an approximate 50% increase in net CO2 assimilation by a broadleaf deciduous forest located in Tennessee, USA. Averaged over the entire daylight period, they further determined that the shift from the lower to the higher set of DRF values "enhances photosynthetic fluxes by about 30% at this study site." Similar results were obtained for the mixed forest and the conglomerate of crops studied. Hence, they concluded that natural variability among commonly-present aerosols can "routinely influence surface irradiance and hence the terrestrial CO2 flux and regional carbon cycle." And for these types of land-cover (forests and agricultural crops), that influence is to significantly increase the assimilation of CO2 from the atmosphere.

In the case of grasslands, however, the effect was found to be just the opposite, with greater aerosol loading of the atmosphere leading to less CO2 assimilation, due most likely, in the estimation of Niyogi et al., to grasslands' significantly different canopy architecture. With respect to the planet as a whole, however, the net effect is decidedly positive, as earth's trees are the primary planetary players in the sequestration of carbon. Post et al. (1990), for example, noted that woody plants account for approximately 75% of terrestrial photosynthesis, which comprises about 90% of the global total (Sellers and McCarthy, 1990); and those numbers make earth's trees and shrubs responsible for fully two thirds (0.75 x 90% = 67.5%) of the planet's net primary production.

What is especially exciting about these real-world observations is that much of the commonly-present aerosol burden of the atmosphere is plant-derived. Hence, it can be appreciated that earth's woody plants are themselves responsible for emitting to the air that which ultimately enhances their own photosynthetic prowess. In other words, earth's trees significantly control their own destiny, i.e., they alter the atmospheric environment in a way that directly enhances their opportunities for greater growth.

Man helps too, in this regard; for as he pumps ever more CO2 into the atmosphere, the globe's woody plants quickly respond to its aerial fertilization effect, becoming ever more productive, which leads to even more plant-derived aerosols being released to the atmosphere, which stimulates this positive feedback cycle to a still greater degree. Stated another way, earth's trees use some of the CO2 emitted to the atmosphere by man to alter the aerial environment so as to enable them to remove even more CO2 from the air. The end result is that earth's trees and humanity are working hand-in-hand to significantly increase the productivity of the biosphere; and it is happening in spite of all other insults to the environment that work in opposition to enhanced biological activity.

In light of these several observations, it should be obvious 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 fully and adequately 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 future catastrophic CO2-induced global warming may well be nothing more than catastrophic exaggerations.

Baldocchi, D., Falge, E., Gu, L.H., Olson, R., Hollinger, D., Running, S., Anthoni, P., Bernhofer, C., Davis, K., Evans, R., Fuentes, J., Goldstein, A., Katul, G., Law B., Lee, X.H., Malhi, Y., Meyers, T., Munger, W., Oechel, W., Paw U, K.T., Pilegaard, K., Schmid, H.P., Valentini, R., Verma, S., Vesala, T., Wilson, K. and Wofsy, S. 2001. FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor, and energy flux densities. Bulletin of the American Meteorological Society 82: 2415-2434.

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.

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.

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Last updated 9 January 2008