Free Air CO2 Enrichment (FACE) methodology has for decades been the standard for investigating the impacts of elevated atmospheric CO2 levels on terrestrial ecosystems. But this was not easily accomplished, and in order to assess the challenge we face we need to look back at the course of terrestrial ecosystem research over the last 20 years. At that time the terrestrial community faced a similar problem. It was widely believed that elevated atmospheric CO2 levels would have a positive effect on growth of land plants by facilitating entry of CO2 into the leaf stoma while minimizing water loss. But concerns over the validity of enclosed greenhouse experiments arose, and in 1985 a major review was held (US DoE, 1985) to evaluate which experimental techniques would yield the correct result for natural ecosystems.
The debate was fierce; it was estimated in one paper that for a simple forest experiment release of some 383 tons CO2 per day would be required! Yet the march of technology proceeded, designs and control systems were refined, and far more efficient studies emerged. The end result was that a large international network of experiments was created, with CO2 enrichment techniques being applied at many sites around the world through the creative endeavors of a large number of scientists. These sites do require releases of the order of hundreds of tons CO2 per year and this has been accepted as a necessary cost of uncovering the truth and predicting the future. When the results were finally synthesized 20 years after the experiments were first proposed (Long et al., 2006; Schimel, 2006) a far lower CO2 fertilization effect than first anticipated was found. Enclosed, and open, experiments can thus yield quite different results. Some 20 years later the ocean sciences face the same problem.
Two examples of FACE experimental systems
FACE experiments come in many different sizes as the two examples illustrate the extremes here. On the left is an example of a Mini-FACE experiment: a 1 meter diameter ring with an array of emitters for controlling the CO2 concentration in a very small area. On the right is an example of one of the larger FACE systems showing an array of tower rings enclosing much larger areas which include many trees per each enclosure. The use of multiple enclosures allows for replication of the experiment as well as running simultaneous controls. For the development of the FOCE systems proposed by MBARI, beginning with a smaller size array offers the best route forward, but the goal of adopting a larger size framework for a true "open" system remains despite the fact that the challenges will be large.
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