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Atmospheric CO2 Enrichment and Plants of Warm and Dry Regions
Donohue, R.J., Roderick, M.L., McVicar, T.R. and Farquhar, G.D. 2013. Impact of CO2 fertilization on maximum foliage cover across the globe's warm, arid environments. Geophysical Research Letters 40: 3031-3035.

The authors say that "increases in atmospheric CO2 concentrations (Ca) are expected to lead to a CO2 fertilization effect where photosynthesis is enhanced with the rise in CO2 (Farquhar, 1997)." However, they note that "while a land-based carbon sink has been observed (Ballantyne et al., 2012; Canadell et al., 2007) and satellites reveal long-term, global greening trends (Beck et al., 2011; Fensholt et al., 2012; Nemani et al., 2003), it has proven difficult to isolate the direct biochemical role of Ca in these trends from variations in other key resources (such as light, water and nutrients) and from socioeconomic factors such as land use changes (Houghton, 2003)."

What was done
Noting that "this complexity can be reduced by focusing on warm, arid environments, where water plays the dominant role in primary production and where foliage cover, plant water use, and photosynthesis are all tightly coupled," Donohue et al. say "it is in these warm and arid environments where the CO2 fertilization effect on cover should be most clearly expressed." And so it was that they went on to use gas exchange theory to predict that the 14% increase in atmospheric CO2 experienced between 1982 and 2010 should have led to a 5-10% increase in green foliage cover under these conditions, after which they went looking for evidence of this prognosticated sign of real-world CO2-induced plant growth stimulation.

What was learned
The four Australian researchers report that satellite observations, which were analyzed to remove the effect of variations in precipitation, showed that cover across these environments had indeed increased, and by approximately 11%.

What it means
As for what it all means, Donohue et al. write in the concluding sentence of their paper's abstract that their results "confirm that the anticipated CO2 fertilization effect is occurring alongside ongoing anthropogenic perturbations to the carbon cycle and that the fertilization effect is now a significant land surface process." Or as they write in the final sentence of the body of their paper, "our results confirm that the direct biochemical impact of the rapid increase in Ca over the last 30 years on terrestrial vegetation is an influential and observable land surface process." Or as we would more directly put it, the greening of the earth continues, courtesy of the ongoing rise in the air's CO2 content.

Ballantyne, A.P., Alden, C.B., Miller, J.B., Tans, P.P. and White, J.W. 2012. Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years. Nature 488: 70-72.

Beck, H.E., McVicar, T.R., van Dijk, A.I.J.M., Schellekens, J., de Jeu, R.A.M. and Bruijnzeel, L.A. 2011. Global evaluation of four AVHRR-NDVI data sets: Intercomparison and assessment against Landsat imagery. Remote Sensing of Environment 115: 2547-2563.

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 USA 104: 18,866-18,870.

Farquhar, G.D. 1997. Carbon dioxide and vegetation. Science 278: 1411.

Fensholt, R., Langanke, T., Rasmussen, K., Reenberg, A., Prince, S., Tucker, C., Scholes, R., Le, Q., Bondeau, A., Eastman, R., Epstein, H., Gaughan, A., Hellden, U., Mbow, C., Olsson, L., Paruelo, J., Schweitze,r C., Seaquist, J. and Wessels, K. 2012. Greenness in semi-arid areas across the globe 1981-2007 - An Earth Observing Satellite based analysis of trends and drivers. Remote Sensing of Environment 121: 144-158.

Houghton, R.A. 2003. Revised estimates of the annual net flux of carbon to the atmosphere from changes in land use and land management 1850-2000. Tellus Series B 55: 378-390.

Nemani, R.R., Keeling, C.D., Hashimoto, H., Jolly, W.M., Piper, S.C., Tucker, C.J., Myneni, R.B. and Running, S.W. 2003. Climate-driven increases in global terrestrial net primary production from 1982 to 1999. Science 300: 1560-1563.

Reviewed 13 November 2013