I was alerted to an interesting paper from 2011 that documents why the addition of CO2 into the atmosphere favors some vegetation, at the expense of others. This biogeochemical risk (rather than its radiative effect) due to the addition of CO2 into the atmosphere is , in my view, the more important concern, as we really do not understand all of the consequences.
This paper is
Morgan et al, 2011: C4 grasses prosper as carbon dioxide eliminates desiccation in warmed semi-arid grassland Nature 476,202–205(11 August 2011)doi:10.1038/nature10274Received 17 December 2010 Accepted 07 June 2011 Published online 03 August 2011
The abstract reads [highlight added]
Global warming is predicted to induce desiccation in many world regions through increases in evaporative demand. Rising CO2 may counter that trend by improving plant water-use efficiency. However, it is not clear how important this CO2-enhanced water use efficiency might be in offsetting warming-induced desiccation because higher CO2 also leads to higher plant biomass, and therefore greater transpirational surface. Furthermore, although warming is predicted to favour warm season, C4 grasses, rising CO2 should favour C3, or cool-season plants8. Here we show in a semi-arid grassland that elevated CO2 can completely reverse the desiccating effects of moderate warming. Although enrichment of air to 600 p.p.m.v. CO2 increased soil water content (SWC), 1.5/3.0 C day/night warming resulted in desiccation, such that combined CO2 enrichment and warming had no effect on SWC relative to control plots. As predicted, elevated CO2 favoured C3 grasses and enhanced stand productivity, whereas warming favoured C4 grasses. Combined warming and CO2 enrichment stimulated above-ground growth of C4 grasses in 2 of 3 years when soil moisture most limited plant productivity. The results indicate that in a warmer, CO2-enriched world, both SWC and productivity in semi-arid grasslands may be higher than previously expected. Grass-dominated, dry rangelands account for over 30% of Earth’s terrestrial surface and provide most of the forage for the world’s domestic livestock. Among the most important of these include the vast Central Asian steppes, North American prairies, Australian rangelands, plus extensive grazing lands of Africa, South Americaand the Mediterranean.
Climate Science: Roger Pielke Sr. 
