In light of some statements in articles and on other blogs about the likely effects of increasing atmospheric concentrations of CO₂ on plant growth, I thought it might be interesting to write something up about one of the more fascinating (I think) ongoing experiments in the global climate field: Free Atmospheric CO₂ Enrichment (FACE).

Duke University's Free Atmospheric CO₂ Enrichment (FACE) experiment site. Photo by Will Owens.

But first, a little on why this is interesting…

During photosynthesis (which you may recall from grade school science) plants use atmospheric carbon dioxide and water drawn from the soil to make glucose – which they use for development – and oxygen – which they release back into the atmosphere. Both the intake of CO₂ and the output of O₂ occurs across small openings in on the underside of leaves, called stomata. These little openings don’t stay open all the time though, since plants also lose water through them. Instead they open to take in CO₂ when the sun rises and the day’s photosynthesis can begin, but close in the evening to avoid unnecessary water loss.

Here’s where things get complicated – we know that the opening and closing of stomata are regulated by the atmospheric CO₂ concentration. High CO₂ concentrations cause the stomata to close slightly. This makes sense since a plant can then take in more carbon dioxide and make more glucose while giving up less water to the atmosphere. The implications of this simple fact are profound. Will carbon sequestration increase with increasing atmospheric CO₂? Will plants grow faster, or more efficiently? Climate change denialists perceive this last question as the figurative feather in their collective cap.

Denialist claims, however, don’t pan out.

…any elevation of productivity is likely to be short-lived and is unlikely to significantly offset any gradual, long-term increases in co2 due to human activity. This is due in part to the fact that other conditions (e.g. availability of nutrients such as Nitrogen and Phosphorus) appear to quickly become limiting, even when carbon availability is removed as a constraint on plant growth when ambient CO₂ concentrations are sufficiently increased.

Even claims of using forests to sequester carbon don’t hold much water, since forests only truly sequester carbon when they remain standing, retaining the carbon that fueled their growth. This doesn’t jive with current trends in worldwide forest loss.

The real utility of these experiments is examining changes in forest ecosystems due to changing fluxes of carbon through plants and soil. This sounds much less grandiose than potential silver linings or even solutions to global warming, but don’t be fooled. At Duke the sites are being used to experiment on potential limiting nutrients other than just carbon; in grassland ecosystems FACE has been used to study changes in nitrogen fixation; without FACE technology there would be no way to examine changes in microclimates due to increasing CO₂ concentrations. If you’re desperate for this technology to help mitigate climate change, consider that some FACE sites are examining the potential of increased CO₂ on sequestration in soil (a much more efficient and longer lasting carbon sink than forests). This is good science, but most importantly, it’s wicked cool.

Brookhaven National Laboratory maintains several FACE facilities, including the one in Duke University’s experimental forest. Here’s a link for the engineering nerds in the audience that explains how Brookhaven’s enrichment structures actually work.