Within the complex entity we call climate, nearly everything affects everything else in such a way that the original "affector" ends up being affected as a consequence of its own initial action. We call this phenomenon feedback. Furthermore, if the thing doing the original affecting is thereby driven further in the direction of its initial perturbation, we call the phenomenon positive feedback; while if it is driven in the opposite direction, we call it negative feedback. Fortunately, we can conclude from the very fact that we exist that most feedbacks of a climatic nature are negative; for if they were positive, life would have long ago disappeared from the planet, as positive feedbacks would have driven the earth into either a global hothouse or icehouse condition, manifestly incompatible with the functioning of biological processes.
Because feedback factors typically link a number of diverse phenomena, many of the papers described in other sections of our Subject Index are additionally classified as Feedback Factor papers, such as Barber et al. (1999), Driscoll and Haug (1998), Haug and Tiedemann (1998), and Hunt and Malin (1998), which also appear in the Deep Water Formation section. These latter papers all suggest that any warming of the earth will intensify its hydrologic cycle, which should enhance precipitation, leading to more freshwater runoff to the oceans and the lowering of surface salinity levels in certain critical areas of deep water formation. This latter phenomenon, in turn, is postulated to have the capacity to slow the global thermohaline circulation, which would be expected to reduce heat transport to high northern latitudes via the Gulf Stream, thereby cooling that portion of the planet and acting to thwart the initial impetus for warming. And as the latter effect is in opposition to the initial perturbation that started the series of events, this scenario is a clear example of negative feedback.
A number of other negative feedback processes are also evident in earth's climate system. In a study of land-based observations of cloud cover in the southwest, northeast and southern plains sections of the United States, Croke et al. (1999) determined that the mean cloud cover rose from an initial value of 35% to a final value of 47% as mean global air temperature rose by 0.5°C from 1900 to 1987, which, of course, is a negative feedback, for the presence of more clouds tends to counteract the initial impetus for warming, as it reflects more incoming solar radiation back to space and thereby tends to cool the planet.
Sud et al. (1999) studied a related phenomenon over the tropical oceans, where they found that as sea surface temperature (SST) warms to 28-29°C, the cloud-base air is charged with the moist static energy needed for the tops of the clouds to reach the upper troposphere. At this point, the cloud cover reduces the amount of solar radiation received at the surface of the sea; and cool, dry downdrafts promote ocean surface cooling by increasing sensible and latent heat fluxes that cause temperatures there to decline. This "thermostat-like control," as the authors put it, tends to "ventilate the tropical ocean efficiently and help contain the SST between 28-30°C."
Living things also play a role in orchestrating some of the negative feedbacks that keep earth's climate within equable bounds. Kavouras et al. (1998), for example, found that certain hydrocarbons emitted by vegetation in a Eucalyptus forest in Portugal were transformed into particles that can function as cloud condensation nuclei, which tend to promote cloud formation and the reflection back to space of incoming solar radiation. Even in the northernmost manned research site in the world - Alert, Northwest Territories, Canada - have the effects of this and related phenomena been observed. Hopke et al. (1999), for example, identified biogenic sulfur (a precursor of cloud condensation nuclei) in the atmosphere; and they found the signal to be strongly correlated with the average temperature of the Northern Hemisphere. Hence, in the words of the authors, "this result suggests that as the temperature rises, there is increased biogenic production of the reduced sulfur precursor compounds that are oxidized in the atmosphere to sulfate and methane sulfonate [sources of cloud condensation nuclei] and could be evidence of a negative feedback mechanism in the global climate system."
The oceans are also good sources of biogenic precursors of cloud condensation nuclei. Literally hundreds of studies have shown that warming tends to stimulate the growth of marine phytoplankton, leading to the production of more copious quantities of dimethylsulphoniopropionate, which leads in short order to more dimethyl sulphide, which is oxidized to form acidic aerosols that function as cloud condensation nuclei, which then produce more and brighter clouds, completing the negative feedback circle by reflecting more solar radiation back to space and thereby cooling the planet. In addition, Simo and Pedros-Alio (1999) recently demonstrated that warming-induced changes in mixing-layer depth promote this same chain of events via a number of phenomena not previously elucidated.
Still other negative feedbacks help to maintain earth's climate in a state that is suitable for the continued existence of life. Broecker and Sanyal (1998), for example, describe how atmospheric CO2 enrichment enhances plant growth and development, leading to an increase in the chemical weathering of soil, which consumes CO2 and reduces its atmospheric concentration, thereby curtailing the potential for further CO2-induced global warming. Walsh and Pittock (1998), on the other hand, describe how evaporative feedbacks in a warming world may stabilize SSTs in the central regions of tropical storms, thus minimizing any increase in intensity that such storms might otherwise experience. Finally, Schrope et al. (1999) report the results of two years of experiments with rice, where elevated levels of atmospheric CO2 and temperature significantly reduced methane emission rates from the soil in which the plants grew. This phenomenon also has a negative feedback to climate, as methane is an even more powerful greenhouse gas - molecule for molecule - than is carbon dioxide.
Because of these several phenomena, and others not mentioned, we state in our Position Paper - Carbon Dioxide and Global Warming: Where We Stand on the Issue - that "strong negative climatic feedbacks prohibit catastrophic warming." And so we ask in our editorial of 15 July 1999, "why worry about CO2?" Indeed, with all of its many beneficial impacts on plant growth and development, and in light of the self-regulating nature of earth's complex climate system - which is built upon an intricate conglomerate of interconnected negative feedbacks that involve both the living and inanimate components of the biosphere - the ongoing rise in the air's CO2 content would appear to be something to be desired, not abhorred. Perhaps a better question to be asked would thus be "why mess with a good thing?" Indeed, to do so is truly illogical.
References
Barber, D.C., Dyke, A., Hillaire-Marcel, C., Jennings, A.E., Andrews, J.T., Kerwin, M.W., Bilodeau, G., McNeely, R., Southon, J., Morehead, M.D. and Gagnon, J.-M. 1999. Forcing of the cold event of 8,200 years ago by catastrophic drainage of Laurentide lakes. Nature 400: 344-348.
Broecker, W.S. and Sanyal, A. 1998. Does atmospheric CO2 police the rate of chemical weathering? Global Biogeochemical Cycles 12: 403-408.
Croke, M.S., Cess, R.D. and Hameed, S. 1999. Regional cloud cover change associated with global climate change: Case studies for three regions of the United States. Journal of Climate 12: 2128-2134.
Driscoll, N.W. and Haug, G.H. 1998. A short circuit in thermohaline circulation: A cause for Northern Hemisphere glaciation? Science 282: 436-438.
Haug, G.H. and Tiedemann, R. 1998. Effect of the formation of the Isthmus of Panama on Atlantic Ocean thermohaline circulation. Nature 393: 673-676.
Hopke, P.K., Xie, Y. and Paatero, P. 1999. Mixed multiway analysis of airborne particle composition data. Journal of Chemometrics 13: 343-352.
Hunt, A.G. and Malin, P.E. 1998. Possible triggering of Heinrich events by ice-load-induced earthquakes. Nature 393: 155-158.
Kavouras, I.G., Mihalopoulos, N. and Stephanou, E.G. 1998. Formation of atmospheric particles from organic acids produced by forests. Nature 395: 683-686.
Schrope, M.K., Chanton, J.P., Allen, L.H. and Baker, J.T. 1999. Effect of CO2 enrichment and elevated temperature on methane emissions from rice, Oryza sativa. Global Change Biology 5: 587-599.
Simo, R. and Pedros-Alio, C. 1999. Role of vertical mixing in controlling the oceanic production of dimethyl sulphide. Nature 402: 396-399.
Sud, Y.C., Walker, G.K. and Lau, K.-M. 1999. Mechanisms regulating sea-surface temperatures and deep convection in the tropics. Geophysical Research Letters 26: 1019-1022.
Walsh K. and Pittock, A.B. 1998. Potential changes in tropical storms, hurricanes, and extreme rainfall events as a result of climate change. Climatic Change 39: 199-213.