How does rising atmospheric CO2 affect marine organisms?

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The Future of Mediterranean Forests
Keenan, T., Serra, J.M., Lloret, F., Ninyerola, M. and Sabate, S. 2011. Predicting the future of forests in the Mediterranean under climate change, with niche- and process-based models: CO2 matters! Global Change Biology 17: 565-579.

The authors write that climate models "consistently project significant increases in temperature and decreases in precipitation in the Mediterranean basin," and they say that these changes may have a large impact on current Mediterranean forests and the related ecosystem services they provide. In addition, they note that niche-based models -- also known as bioclimatic envelope models or habitat models -- are by far the most commonly used method for predicting potential species distribution responses to future climatic changes. And these models typically predict significant negative consequences for terrestrial plants and animals in the face of continued increases in atmospheric CO2 concentrations.

Keenan et al., on the other hand, prefer process-based models, which describe eco-physiological processes ranging from purely empirical relationships to mechanistic descriptions based on physical laws. And they write that these models -- supported by experiments and growth and yield surveys -- "suggest that global warming will have a positive impact on forest productivity (van der Meer et al., 2002; Nigh et al., 2004; Norby and Luo, 2004; Briceņo-Elizondo et al., 2006; Gaucharel et al., 2008), due to the direct fertilization effect of increased CO2 and indirect effects such as lengthening of the growing period."

What was done
To elucidate the difference in results obtained by employing these two approaches to divining the future, the five researchers assessed and compared the projections of each of them when applied to stands of three forest species (Quercus ilex, Pinus halepensis and Pinus sylvestris) with widely contrasting distributions in continental Spain.

What was learned
Keenan et al. report that "CO2 fertilization through projected increased atmospheric CO2 concentrations is shown to increase forest productivity in the mechanistic process-based model (despite increased drought stress) by up to three times that of the non-CO2 fertilization scenario by the period 2050-2080, which is in stark contrast to projections of reduced habitat suitability from the niche-based models by the same period."

What it means
The Spanish and U.S. scientists write that their results show that "previous reports of species decline in continental Spain (e.g. Benito-Garzon et al., 2008) may be overestimated due to two reasons: the use of only one predictive niche-based model, and the failure to account for possible effects of CO2 fertilization." And they add that "similar studies in other regions, which do not consider these two aspects, are also potentially overestimating species decline due to climate change." Furthermore, they suggest that "niche-based model results also likely overestimate the decline in [habitat] suitability," and they therefore conclude that "an organism's niche must be modeled mechanistically if we are to fully explain distribution limits," additionally citing the work of Kearney (2006) in this regard.

Benito-Garzon, M., Sanchez de Dios, R. and Sainz Ollero, H. 2008. Effects of climate change on the distribution of Iberian tree species. Applied Vegetation Science 11: 169-178.

Briceņo-Elizondo, R., Garcia-Gonzalo, J., Peltola, H., Matala, J. and Kellomaki, S. 2006. Sensitivity of growth of Scots pine, Norway spruce and silver birch to climate change and forest management in boreal conditions. Forest Ecology and Management 232: 152-167.

Gaucharel, C., Guiot, J. and Misson, L. 2008. Changes of the potential distribution area of French Mediterranean forests under global warming. Biogeosciences 5: 1493-1503.

Kearney, M. 2006. Habitat, environment and niche: what are we modeling? Oikos 115: 186-191.

Nigh, G.D., Ying, C.C. and Qian, H. 2004. Climate and productivity of major conifer species in the interior of British Columbia, Canada. Forest Science 50: 659-671.

Norby, R.J. and Luo, Y. 2004. Evaluating ecosystem responses to rising atmospheric CO2 and global warming in a multi-factor world. New Phytologist 162: 281-293.

van der Meer, P.J., Jorritsma, I.T.M. and Kramer, J.K. 2002. Assessing climate change effects on long-term forest development: adjusting growth, phenology and seed production in a gap model. Forest Ecology and Management 162: 39-52.

Reviewed 23 February 2011