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Carbon Sequestration (CO2 x Temperature Interaction) -- Summary
Agricultural and forage plants grown at elevated atmospheric CO2 concentrations nearly always exhibit increased photosynthetic rates and biomass production.  Due to this productivity enhancement, more plant material is typically added to soils from root growth, turnover, and exudation, as well as from leaves and stems following their abscission during senescence.  In addition, rising CO2 levels tend to maintain (Henning et al., 1996) or decrease (Torbert et al., 1998; Nitschelm et al., 1997) CO2 fluxes from agricultural soils.  Consequently, these phenomena tend to increase the carbon contents of most soils in CO2-enriched atmospheres [see our Summary: Agricultrual Lands and Carbon Sequestration].  However, it is sometimes suggested that rising air temperatures, which can accelerate the breakdown of soil organic matter and increase biological respiration rates, could negate this CO2-induced enhancement of carbon sequestration, possibly leading to an even greater release of carbon back to the atmosphere.  Thus, we turn to the scientific literature for added light on this issue.

Casella and Soussana (1997) grew perennial ryegrass in controlled environments receiving ambient and elevated (700 ppm) atmospheric CO2 concentrations, two levels of soil nitrogen, and ambient and elevated (+3C) air temperatures for a period of two years, finding that "a relatively large part of the additional photosynthetic carbon is stored below-ground during the two first growing seasons after exposure to elevated CO2, thereby increasing significantly the below-ground carbon pool."  At the low and high levels of soil nitrogen supply, for example, the elevated CO2 increased soil carbon storage by 32 and 96%, respectively, "with no significant increased temperature effect."  The authors thus concluded that in spite of predicted increases in temperature, "this stimulation of the below-ground carbon sequestration in temperate grassland soils could exert a negative feed-back on the current rise of the atmospheric CO2 concentration."

Much the same conclusion was reached by Van Ginkel et al. (1999).  After reviewing prior experimental work that established the growth and decomposition responses of perennial ryegrass to both atmospheric CO2 enrichment and increased temperature, these researchers concluded that, at both low and high soil nitrogen contents, CO2-induced increases in plant growth and CO2-induced decreases in plant decomposition rate are more than sufficient to counteract any enhanced soil respiration rate that might be caused by an increase in air temperature.  In addition, after reconstructing carbon storage in the terrestrial vegetation of Northern Eurasia as far back as 125,000 years ago, Velichko et al. (1999) determined that plants in this part of the world were more productive and efficient in sequestering carbon at higher temperatures than they were at lower temperatures.  Similarly, Allen et al., (1999) used sediment cores from a lake in southern Italy and from the Mediterranean Sea to conclude that, over the past 102,000 years, warmer climates have been better for vegetative productivity and carbon sequestration than have cooler climates.

In our own day, Liski et al. (1999) studied soil carbon storage across a temperature gradient in a Finnish boreal forest, determining that carbon sequestration in the soil of this forest increased with temperature.  In deciduous forests of the eastern United States, White et al. (1999) also determined that persistent increases in growing season length (due to rising air temperatures) may lead to long-term increases in carbon storage, which, of course, tends to counterbalance - indeed, overpower - the effects of increasing air temperature on respiration rates.  In fact, a data-driven analysis by Fan et al. (1998) suggests that the carbon sequestering abilities of North America's forests between 15 and 51N latitude are so robust that they can yearly remove from the atmosphere all of the CO2 annually released to it by fossil fuel consumption in both the United States and Canada (and this calculation was done during a time touted as having the warmest temperatures on record).  Moreover, Phillips et al. (1998) have shown that carbon sequestration in tropical forests has increased substantially over the past 42 years, in spite of any temperature increases that may have occurred during that time.

In conclusion, research conducted to date strongly suggests that the CO2-induced enhancement of vegetative carbon sequestration will not be reduced by any future rise in air temperature, regardless of its cause.  In fact, it is likely that in a CO2-eriched atmosphere, any increase in temperature will actually enhance biological carbon sequestration.

References
Allen, J.R.M., Brandt, U., Brauer, A., Hubberten, H.-W., Huntley, B., Keller, J., Kraml, M., Mackensen, A., Mingram, J., Negendank, J.F.W., Nowaczyk, N.R., Oberhansli, H., Watts, W.A., Wulf, S. and Zolitschka, B.  1999.  Rapid environmental changes in southern Europe during the last glacial period.  Nature 400: 740-743.

Casella, E. and Soussana, J-F.  1997.  Long-term effects of CO2 enrichment and temperature increase on the carbon balance of a temperate grass sward.  Journal of Experimental Botany 48: 1309-1321.

Fan, S., Gloor, M., Mahlman, J., Pacala, S., Sarmiento, J., Takahashi, T. and Tans, P.  1998.  A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data and models.  Science 282: 442-446.

Henning, F.P., Wood, C.W., Rogers, H.H., Runion, G.B. and Prior, S.A.  1996.  Composition and decomposition of soybean and sorghum tissues grown under elevated atmospheric carbon dioxide.  Journal of Environmental Quality 25: 822-827.

Liski, J., Ilvesniemi, H., Makela, A. and Westman, C.J.  1999.  CO2 emissions from soil in response to climatic warming are overestimated - The decomposition of old soil organic matter is tolerant of temperature.  Ambio 28: 171-174.

Nitschelm, J.J., Luscher, A., Hatrwig, U.A. and van Kessel, C.  1997.  Using stable isotopes to determine soil carbon input differences under ambient and elevated atmospheric CO2 conditions.  Global Change Biology 3: 411-416.

Phillips, O.L., Malhi, Y., Higuchi, N., Laurance, W.F., Nunez, P.V., Vasquez, R.M., Laurance, S.G., Ferreira, L.V., Stern, M., Brown, S. and Grace, J.  1998.  Changes in the carbon balance of tropical forests: Evidence from long-term plots.  Science 282: 439-442.

Torbert, H.A., Prior, S.A., Rogers, H.H. and Runion, G.B.  1998.  Crop residue decomposition as affected by growth under elevated atmospheric CO2Soil Science 163: 412-419.

Van Ginkel, J.H., Whitmore, A.P. and Gorissen, A.  1999.  Lolium perenne grasslands may function as a sink for atmospheric carbon dioxide.  Journal of Environmental Quality 28: 1580-1584.

Velichko, A.A., Zelikson, E.M. and Borisova, O.K.  1999.  Vegetation, phytomass and carbon storage in Northern Eurasia during the last glacial-interglacial cycle and the Holocene.  Chemical Geology 159: 191-204.

White, M.A., Running, S.W. and Thornton, P.E.  1999.  The impact of growing-season length variability on carbon assimilation and evapotranspiration over 88 years in the eastern US deciduous forest.  International Journal of Biometeorology 42: 139-145.