Weeds grow well in high CO2. Crops?

The top card in the denialist deck is that global warming is good for plants. There are three reasons for this.

• Lengthened growing seasons.
• Increased CO2 decreases evaporation from leaves, increasing drought resistance
• Increased atmospheric CO2 makes more CO2 available in C3 plants (rice, wheat, etc. ) but not in C4 plants (maize, aka corn).

More on the details below. Recently Eli pointed to a study which showed that poison ivy was REALLY fertilized at high CO2 levels. Not good. Now comes Long, Ainsworth, Leakey, Noesberger and Ort in Science 312 (2006) 1918, to offer us " Food for thought: Lower than expected crop yield stimulation with rising CO2 concentrations"

This is a meta (study of studies) study of experiments which monitored plant growth in high CO2 concentrations.

Model projections suggest that although increased temperature and decreased soil moisture will act to reduce global crop yields by 2050, the direct fertilization effect of rising carbon dioxide concentration will offset these losses. The CO2 fertilization factors used in models to project future yields were derived from enclosure studies conducted approximately 20 years ago. Free-air concentration enrichment (FACE) technology has now facilitated large-scale trials of the major grain crops at elevated [CO2] under fully open-air field conditions. In those trials, elevated [CO2] enhanced yield by 50% less than in enclosure studies. This casts serious doubt on projections that rising [CO2] will fully offset losses due to climate change

This is a very well written summary accessible to non-experits and Prof. Rabett would encourage all with access to read it and Schimel's perspective on page 1889 of the same issue. The results, if they hold up will be important to climate change policy (remember tho according to the usual and unusual suspects, everything published in Science is crap - something that yr humble hare considers crap, but what do you expect from the internet).

The bottom line is

• There is no CO2 fertilization effect for C4 crops although increased drought resistance may be significant.
• FACE studies show that current ag models significantly overestimate CO2 fertilization for crops
• C3 crop CO2 fertilization saturates somewhere between 600 and 800 ppm CO2
  
• Crop breeders should work on developing strains that can benefit from higher CO2

The paper has a concise paragraph which describes the biological origin of CO2 fertiliztion which is worth reproducing

Crops sense and respond directly to rising [CO2] through photosynthesis and stomatal conductance, and this is the basis for the fertilization effect on yield . In C3 plants, mesophyll cells containing ribulose-1,5-bisphosphate carboxylase - oxygenase (RuBisCO) are in direct contact with the intercellular air space that is connected to the atmosphere via stomatal pores in the epidermis. Hence, in C3 crops, rising CO3 increases net photosynthetic CO2 uptake because RuBisCO is not CO2 -saturated in today's atmosphere and because CO2 inhibits the competing oxygenation reaction leading to photorespiration. RuBisCO is highly conserved across terrestrial plants, so instantaneous responses to increased [CO2] may be generalized across C3 plants, including rice, soybeans, and wheat. In theory, at 25°C, an increase in [CO2] from the present-day value of 380 ppm to that of 550 ppm, projected for the year 2050, would increase C3 photosynthesis by 38%. In contrast, in C4 crops such as maize and sorghum, RuBisCO is localized to bundle sheath cells in which CO2 is concentrated to three to six times atmospheric [CO2]. This concentration is sufficient to saturate RuBisCO and in theory would prevent any increase in CO2 uptake with rising [CO2]. Although C4 crops may not show a direct response in photosynthetic activity, an indirect increase in the efficiency of water use via reduction in stomatal conductance may still increase yield.