Greening of the Earth: The Globe

From ClimateWiki

Periodically, even in some of the world’s most prestigious scientific journals, it is said the natural sinks of Earth’s carbon cycle are becoming decreasingly effective in removing from the atmosphere the CO2 released to it as a result of humanity’s energy-intensive activities (Canadell et al., 2007; LeQuere et al., 2007). This myth, however, has been put to rest once and for all by a new analysis of real-world data.

In a study published in the December 2009 issue of Oceanography that considered several related topics, Pieter Tans of the National Oceanic and Atmospheric Administration (NOAA) employed measurements of atmospheric and oceanic carbon contents, along with reasonably constrained estimates of global anthropogenic CO2 emissions, to calculate the residual fluxes of carbon (in the form of CO2) from the terrestrial biosphere to the atmosphere (+) or from the atmosphere to the terrestrial biosphere (-), obtaining the results depicted in Figure 7.11.1. As the figure illustrates, Earth’s land surfaces were a net source of CO2-carbon to the atmosphere until about 1940, primarily due to the felling of forests and the plowing of grasslands to make way for expanded agricultural activities. From 1940 onward, however, the terrestrial biosphere has become, in the mean, an increasingly greater sink for CO2-carbon, and it has done so even in the face of massive global deforestation, for which it has more than compensated. These findings do “not depend on models” but “only on the observed atmospheric increase and estimates of fossil fuel emissions,” Tans notes.

Tans concludes, “suggestions that the carbon cycle is becoming less effective in removing CO2 from the atmosphere (e.g., LeQuere et al., 2007; Canadell et al., 2007) can perhaps be true locally, but they do not apply globally, not over the 50-year atmospheric record, and not in recent years.” In fact, he adds, “to the contrary” and “despite global fossil fuel emissions increasing from 6.57 GtC in 1999 to 8.23 in 2006, the five-year smoothed global atmospheric growth rate has not increased during that time, which requires more effective uptake [of CO2] either by the ocean or by the terrestrial biosphere, or both, to satisfy atmospheric observations.” Tans’ results, depicted in Figure 6.11.1, clearly indicate this “more effective uptake” of CO2-carbon has occurred primarily over land.

This observation-based analysis of real-world data provides strong evidence for both the reality and the tremendous strength of the CO2-induced greening of the Earth phenomenon, which has been observed in numerous independent studies conducted throughout the world. In addition, it refutes the unfounded arguments that various environmental stresses and resource limitations will not allow the full potential of the aerial fertilization effect of atmospheric CO2 enrichment to be manifest in nature. This phenomenon is itself a “force of nature” that can be neither hindered nor halted.

Most recently, Lin et al. (2010) noted “most models predict that climate warming will increase the release of carbon dioxide from the terrestrial biosphere into the atmosphere, thus triggering positive climate-terrestrial carbon feedback which leads to a warmer climate.” However, they state the “stimulation of biomass accumulation and net primary productivity of terrestrial ecosystems under rising temperature (Rustad et al., 2001; Melillo et al., 2002; Luo et al., 2009) may enhance carbon sequestration and attenuate the positive feedback between climate warming and the terrestrial biosphere.”

In an effort to find out which view is correct, Lin et al. conducted a meta-analysis of pertinent data from 127 individual studies published before June 2009, in order to ascertain whether the overall impact of a substantial increase in the air’s CO2 concentration on terrestrial biomass production would likely be positive or negative. The three scientists determined that for the totality of terrestrial plants included in their analysis, “warming significantly increased biomass by 12.3%,” and there was a “significantly greater stimulation of woody (+26.7%) than warming effects on plant biomass production “did not change with mean annual precipitation or experimental duration,” and “other treatments, including CO2 enrichment, nitrogen addition, drought and water addition, did not alter warming responses of plant biomass.”

The Chinese researchers thus conclude “results in this and previous meta-analyses (Arft et al., 1999; Rustad et al., 2001; Dormann and Woodin, 2002; Walker et al., 2006) have revealed that warming generally increases terrestrial plant biomass, indicating enhanced terrestrial carbon uptake via plant growth and net primary productivity.” Thus, we can logically expect that the ongoing rise in the air’s CO2 content will soften its tendency to increase global temperatures while causing greater growth rates and biomass production of terrestrial vegetation.

References

Arft, A.M., Walker, M.D., Gurevitch, J., Alatalo, J.M., Bret-Harte, M.S., Dale, M., Diemer, M., Gugerli, F., Henry, G.H.R., Jones, M.H., Hollister, R.D., Jonsdottir, I.S., Laine, K., Levesque, E., Marion, G.M., Molau, U., Molgaard, P., Nordenhall, U., Raszhivin, V., Robinson, C.H., Starr, G., Stenstrom, A., Stenstrom, M., Totland, O., Turner, P.L., Walker, L.J., Webber, P.J., Welker, J.M., and Wookey, P.A. 1999. Responses of tundra plants to experimental warming: meta-analysis of the international tundra experiment. Ecological Monographs 69: 491–511.

Canadell, J.G., LeQuere, C., Raupach, M.R., Field, C.B., Buitenhuis, E.T,., Ciais, P., Conway, T.J., Gillett, N.P., Houghton, R.A., and Marland, G. 2007. Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks. Proceedings of the National Academy of Sciences of the United States of America 104: 18,866–18,870.

Dormann, C.F. and Woodin, S.J. 2002. Climate change in the arctic: using plant functional types in a meta-analysis of field experiments. Functional Ecology 16: 4–17.

LeQuere, C., Rodenbeck, C., Buitenhuis, E.T., Conway, T.J., Langenfelds, R., Gomez, A., Labuschagne, C., Ramonet, M., Nakazawa, T., Metzl, N., Gillett, N., and Heimann, M. 2007. Saturation of the Southern Ocean CO2 sink due to recent climate change. Science 316: 1735–1738.

Lin, D., Xia, J., and Wan, S. 2010. Climate warming and biomass accumulation of terrestrial plants: a meta-analysis. New Phytologist 188: 187–198.

Luo, Y.Q., Sherry, R., Zhou, X.H., and Wan, S.Q. 2009. Terrestrial carbon-cycle feedback to climate warming: experimental evidence on plant regulation and impacts of biofuel feedstock harvest. Global Change Biology Bioenergy 1: 62–74.

Melillo, J.M., Steudler, P.A., Aber, J.D., Newkirk, K., Lux, H., Bowles, F.P., Catricala, C., Magill, A., Ahrens, T., and Morrisseau, S. 2002. Soil warming and carbon-cycle feedbacks to the climate system. Science 298: 2173–2176.

Rustad, L.E., Campbell, J.L., Marion, G.M., Norby, R.J., Mitchell, M.J., Hartley, A.E., Cornelissen, J.H.C., Gurevitch, J., and GCTE-NEWS. 2001. A meta-analysis of the response of soil respiration, net nitrogen mineralization, and aboveground plant growth to experimental ecosystem warming. Oecologia 126: 543–562.

Tans, P. 2009. An accounting of the observed increase in oceanic and atmospheric CO2 and an outlook for the future. Oceanography 22: 26–35.

Walker, M.D., Wahren, C.H., Hollister, R.D., Henry, G.H.R., Ahlquist, L.E., Alatalo, J.M., Bret-Harte, M.S., Calef, M.P., Callaghan, T.V., Carroll, A.B., Epstein, H.E., Jonsdottir, I.S., Klein, J.A., Magnusson, B., Molaug, U., Oberbauer, S.F., Rewan, S.P., Robinson, C.H., Shaver, G.R., Suding, K.N., Thompson, C.C., Tolvanen, A., Totland, O., Turner, P.L., Tweedie, C.E., Webber, P.J., and Wookey, P.A. 2006. Plant community responses to experimental warming across the tundra biome. Proceedings of the National Academy of Sciences USA 103: 1342–1346.