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The Sustainability of Carbon Sequestration by Earth's Forests in a CO2-Accreting Atmosphere
Volume 6, Number 29: 16 July 2003

Originally hailed as a rational and effective means for significantly slowing the rate of rise of the air's carbon dioxide concentration, the planting of forests to remove CO2 from the atmosphere and sequester its carbon in vegetative tissues and soils has been under fierce attack for several years. Pearce (1999), for example, claimed that the planting of trees for this purpose is "based on a dangerous delusion," stating that IPCC scientists say "planned new forests, called 'carbon sinks,' will swiftly become saturated with carbon and begin returning most of their carbon to the atmosphere," similarly quoting Peter Cox of the British Meteteorological Office's Hadley Centre as stating that "this is not something that may or may not happen" but something that is "more or less inevitable."

In further support of this thesis, Pearce enlisted the purported thoughts of South Africa's Bob Scholes, who Pearce describes as "a leading light in the International Geosphere-Biosphere Programme's Global Carbon Project." According to Scholes -- according to Pearce -- because increasing CO2 concentrations have an ever-smaller effect on plant growth as they rise higher and higher, and because respiration increases with temperature (which Pearce assumes will rise as a consequence of increases in the air's CO2 content), CO2 fertilization rates "will flatten out while respiration rates soar," so that by 2050 "forests will have released much of what they have absorbed."

Based on these presumptions, Pearce went on to say that "the suggestion that planting trees means less atmospheric CO2 ignores simple logic." And in derisive denigration of both the concept and the scientists who embraced it, he mockingly asked: "How did researchers get it so wrong?"

Actually, they didn't, as a recently-published study of one of the world's premier Free-Air CO2 Enrichment (FACE) experiments clearly demonstrates (Lou et al., 2003). The subject of this study -- Duke Forest in North Carolina, USA -- is composed primarily of loblolly pines with sweetgum and yellow poplar trees as sub-dominants, together with numerous other trees, shrubs and vines that occupy still smaller niches. This ecosystem was established in 1983, following the clear-cutting of a regenerating forest in 1979; and in August of 1996, its three 30-meter-diameter FACE plots began to be enriched with CO2 to atmospheric concentrations 200 ppm above ambient, while three similar plots were maintained at the ambient CO2 concentration, as described by Hendrey et al. (1999).

A number of papers dealing with various aspects of this experiment were subsequently published. As recounted by Lou et al. (2003), these real-world studies revealed the existence of a CO2-induced "sustained photosynthetic stimulation at leaf and canopy levels [Myers et al., 1999; Ellsworth, 2000; Luo et al., 2001; Lai et al., 2002], which resulted in sustained stimulation of wood biomass increment [Hamilton et al., 2002] and a larger C accumulation in the forest floor at elevated CO2 than at ambient CO2 [Schlesinger and Lichter, 2001]."

Based upon these findings and what they implied about rates of carbon removal from the atmosphere and its different residence times in plant, litter and soil C pools, Luo et al. (2003) developed a model for studying the sustainability of forest carbon sequestration. Applying this model to a situation where the atmospheric CO2 concentration gradually rises from a value of 378 ppm in 2000 to a value of 710 ppm in 2100, they calculated that the carbon sequestration rate of the Duke Forest would rise from an initial value of 69 g m-2 yr-1 to a final value of 201 g m-2 yr-1.

This result is a far, far cry from the sad scenario promulgated by Pearce and the cadre of climate alarmists that have long sung the same song, wherein it is claimed that forests will have released much of the carbon they had previously absorbed as early as the year 2050. How did he and others get it so wrong? ? predicting not only no C absorption by 2050, but high rates of C loss?

The answer, of course, is that the climate alarmists were counting on rising temperatures to thwart this great biological powerhouse that is the planet's global forest. However, based on a large body of work that established the growth and decomposition responses of perennial ryegrass to atmospheric CO2 enrichment and increased temperature, Van Ginkel et al. (1999) showed that the combination of CO2-induced increases in plant growth rates and decreases in plant decomposition rates "are more than sufficient to counteract the positive feedback caused by the increase in temperature." Furthermore, in a three-year FACE study of cotton, Leavitt et al. (1994) found that about 10% of the organic carbon present in the soil below the CO2-enriched plants (which received an extra 180 ppm CO2) came from the extra CO2 supplied to the FACE plants; and some of this carbon had made its way into a recalcitrant soil carbon pool having an average residence time of fully 2200 years.

But what about trees, which are the subject of this editorial? Very briefly, Liski et al. (1999) found that carbon storage in soils of both high- and low-productivity boreal forests in Finland actually increased with increasing temperature, thereby putting to rest the idea that rising temperatures will spur carbon losses from forest soils. Also, King et al. (1999) showed that aspen seedlings increased their photosynthetic rates and biomass production as temperatures rose from 10 to 29C, putting to rest the idea that warming-induced increases in tree respiration rates would lead to large losses in biological carbon fixation. In addition, White et al. (1999) showed that rising temperatures increased the growing season by about 15 days in twelve different U.S. deciduous forest sites; and for each one-day increase in growing season length, net ecosystem production rose by 1.6%. Hence, rather than exerting a negative influence on forest carbon sequestration, if air temperatures were to rise in the future they would likely have a positive effect on this phenomenon.

In conclusion, the best real-world data currently at our disposal suggest that earth's forests will continue to expand their carbon sequestering prowess as the air's CO2 content continues to rise, even in the face of rising temperatures.

Sherwood, Keith and Craig Idso

References
Ellsworth, D.S. 2000. Seasonal CO2 assimilation and stomatal limitations in a Pinus taeda canopy with varying climate. Tree Physiology 20: 435-444.

Hamilton, J.G., DeLucia, E.H., George, K., Naidu, S.L., Finzi, A.C. and Schlesinger, W.H. 2002. Forest carbon balance under elevated CO2. Oecologia DOI 10.1007/s00442-002-0884-x.

Hendrey, G.R., Ellsworth, D.S., Lewin, K.F. and Nagy, J. 1999. A free-air enrichment system for exposing tall forest vegetation to elevated atmospheric CO2. Global Change Biology 5: 293-310.

King, J.S., Pregitzer, K.S. and Zak, D.R. 1999. Clonal variation in above- and below-ground responses of Populus tremuloides Michaux: Influence of soil warming and nutrient availability. Plant and Soil 217: 119-130.

Lai, C.T., Katul, G., Butnor, J., Ellsworth, D. and Oren, R. 2002. Modeling nighttime ecosystem respiration by a constrained source optimization method. Global Change Biology 8: 124-141.

Leavitt, S.W., Paul, E.A., Kimball, B.A., Hendrey, G.R., Mauney, J.R., Rauschkolb, R., Rogers, H., Lewin, K.F., Nagy, J., Pinter Jr., P.J. and Johnson, H.B. 1994. Carbon isotope dynamics of free-air CO2-enriched cotton and soils. Agricultural and Forest Meteorology 70: 87-101.

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.

Luo, Y., Medlyn, B., Hui, D., Ellsworth, D., Reynolds, J. and Katul, G. 2001. Gross primary productivity in the Duke Forest: Modeling synthesis of the free-air CO2 enrichment experiment and eddy-covariance measurements. Ecological Applications 11: 239-252.

Luo, Y., White, L.W., Canadell, J.G., DeLucia, E.H., Ellsworth, D.S., Finzi, A., Lichter, J. and Schlesinger, W.H. 2003. Sustainability of terrestrial carbon sequestration: A case study in Duke Forest with inversion approach. Global Biogeochemical Cycles 17: 10.1029/2002GB001923.

Myers, D.A., Thomas, R.B. and DeLucia, E.H. 1999. Photosynthetic capacity of loblolly pine (Pinus taeda L.) trees during the first year of carbon dioxide enrichment in a forest ecosystem. Plant, Cell and Environment 22: 473-481.

Pearce, F. 1999. That sinking feeling. New Scientist 164 (2209): 20-21.

Schlesinger, W.H. and Lichter, J. 2001. Limited carbon storage in soil and litter of experimental forest plots under increased atmospheric CO2. Nature 411: 466-469.

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.

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.