Background
After deseasonalizing and detrending long-term monthly time series of the atmosphere's CO2 concentration, a time series of residuals remains that is believed to be driven by temporal variations in the uptake and/or release of carbon by the world's oceans and the terrestrial biosphere.

What was done
The individual influences of ENSO, volcanic eruptions and the North Atlantic Oscillation (NAO) on the 1958-1994 time series of anomalous CO2 fluxes were investigated with the help of the High Resolution Biosphere Model of Esser et al. (1994) and real-world CO2 measurements.

What was learned
Although the authors could conclude nothing about the NAO, other than to say "the influence of the NAO remain[s] unclear," they were able to determine that periods of anomalous rising atmospheric CO2 concentrations coincided with El Niņo periods, while periods of anomalous declining atmospheric CO2 concentrations coincided with periods of significant volcanism.

Enlarging on these observations, they say "the globally averaged effect of [the El Niņo] circulation pattern on the terrestrial biosphere is a net release of carbon," in agreement with the results of earlier investigations of the subject (Bacastow, 1976; Bacastow et al., 1980; Keeling et al., 1989), which in turn "confirms earlier findings that the terrestrial biosphere is mainly responsible for atmospheric CO2 variations on the ENSO timescale (Keeling et al., 1995; Lee et al., 1998; Feely et al., 1999; Gerard et al., 1999; Rayner and Law, 1999; Battle et al., 2000; Bousquet et al., 2000; Houghton, 2000; Knorr, 2000, Le Quere et al., 2000; Langenfelds et al., 2002)."

At the other end of the spectrum, Reichenau and Esser report that "volcanic eruptions with considerable aerosol production may create disturbances of the (biospheric) carbon cycle by increasing the photosynthetic carbon uptake due to the enhanced diffuse fraction of the incoming [solar] radiation," which accords with the findings of Roderick et al. (2001), Cohan et al. (2002), Law et al. (2002) and Gu et al. (2002, 2003).

What it means
The many published studies of anomalous CO2 fluxes between earth's surface and its atmosphere clearly indicate that warm El Niņo conditions tend to reduce biospheric productivity, while cool volcanic conditions tend to enhance biospheric productivity.  Does this observation imply that "cool is good" and "warm is bad" for the planet's plants?

No, it does not.  For one thing, the productivity-enhancing effect of volcanic eruptions arises not from their cooling influence, but from their increasing the amount of diffuse solar radiation received at the earth's surface, which allows for an enhanced penetration of solar radiation deeper into plant canopies, which reduces within-canopy shade and boosts rates of canopy net photosynthesis.  As for the productivity-reducing effect of El Niņos, it could well be more a consequence of changes in global precipitation patterns than of the direct effect of an increase in temperature.  We know from the work of Indermuhle et al. (1999), for example, that the pattern of biospheric productivity over the last 7,000 years of the Holocene was essentially that of a slow monotonic decline from the peak growth conditions of the interglacial's Climatic Optimum, which productivity decline, in their words, was "due to a change from the warmer and wetter mid-Holocene climate to colder and drier conditions."

Viewed from this perspective, it can be appreciated that long-term global warming tends to substantially increase biospheric productivity.  Short-term deviations from this basic relationship that are evident in anomalous variations of CO2 fluxes between the earth's surface and atmosphere are just that: short-lived and anomalous.

References
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Bacastow, R.B., Adams, J.A., Keeling, C.D., Moss, D.J., Whorf, T.P. and Wong, C.S.  1980.  Atmospheric carbon dioxide, the Southern Oscillation and the weak 1975 El Niņo.  Science 210: 66-68.

Battle, M., Bender, M.L., Tans, P.P., White, J.W.C., Ellis, J.T., Conway, T. and Francey, R.J.  2000.  Global carbon sinks and their variability inferred from atmospheric O2 and ¹³C.  Science 287: 2467-2470.

Bousquet, P., Ciais, P., Monfray, P., Balkanski, Y., Ramonet, M. and Tans, P.  1996.  Influence of two atmospheric transport models on inferring sources and sinks of atmospheric CO2.  Tellus Series B 48: 568-582.

Cohan, D.S., Xu, J., Greenwald, R., Bergin, M.H. and Chameides, W.L.  2002.  Impact of atmospheric aerosol light scattering and absorption on terrestrial net primary productivity.  Global Biogeochemical Cycles 16: 10.1029/2001GB001441.

Esser, G., Hoffstadt, J., Mack, F. and Wittenberg, U.  1994.  High Resolution Biosphere Model-Documentation of Model Version 3.00.00.  Institute fur Pflanzenokologie der Justus-Liebig-Universitat, Giessen.

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Gu, L., Baldocchi, D.D., Wofsy, S.C., Munger, J.W., 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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