The various links in this complex chain of events have all been thoroughly investigated and determined to be sound by literally hundreds of experimental studies.  Consequently, we here proceed in similar fashion - based upon a number of other well-established biophysical phenomena - to describe a second temperature-stabilizing feedback loop that appears to be equally well supported by a solid base of evidence.  Whereas almost anything could serve as a stimulus for warming in the scenario elucidated by Charlson et al., however, the initial impetus for the increase in surface air temperature in the negative feedback loop 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 of this negative feedback loop is the proven propensity for higher levels of atmospheric CO2 to enhance vegetative productivity (see Plant Growth Data on our sidebar for verification) and plant water use efficiency (see 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.  Both limbs of this one-linkage-long double-barreled negative feedback loop are extremely powerful, as Idso (1991a,b) has demonstrated how just the first of them may be capable of stabilizing the air's CO2 concentration at less than a doubling of its pre-industrial value.  Nevertheless, these tremendous "side-effects" comprise but the first link of the more extended negative feedback loop that is the subject of this essay.

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 (Duce et al., 1983; Mooney et al., 1987), many of which function as cloud condensation nuclei (Went, 1966; Meszaros, 1988; Kavouras et al., 1998; Hopke et al., 1999).  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 (Idso, 1990), 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 as described by Charlson et al. 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 (Suraqui et al., 1974; Abakumova et al., 1996).  The fourth linkage is the ability of enhanced diffuse lighting to reduce the volume of shade within vegetative canopies (Roderick et al., 2001).  The fifth linkage is the tendency for less internal canopy shading to enhance whole-canopy photosynthesis (Healey et al., 1998), which finally produces the end result: a greater biological extraction of CO2 from the air and 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 the process?  Roderick et al. provide a good estimate based on one of our favorite approaches to questions of this type: 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 consider the volcanic eruption of Mt. Pinatubo in June of 1991.  This event ejected enough gases and fine materials into the atmosphere that it produced 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, Roderick et al. conclude 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 Niņo event; because, in the words of the authors, "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.

Clearly, as we probe ever deeper into the secrets of nature, the story that emerges is one of amazingly complex cross-linkages between the physical and biological worlds.  This web of interconnectedness wields great power to maintain the climate of the earth within bounds that are suitable for the continued existence of life in all its variety.  We would do well to better learn the lessons these subtle but powerful negative feedback loops have to tell us before we lunge forward in our hubris and make a massive misstep in regulating anthropogenic CO2 emissions.  Although deemed to be good by many conscientious and concerned people, this course might ultimately prove our undoing ... and that of the rest of the biosphere as well (see our Editorial of 4 July 2001).

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