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CO2 Fertilization

Filed under: — mike @ 28 November 2004

It has sometimes been argued that the earth’s biosphere (in large part, the terrestrial biosphere) may have the capacity to sequestor much of the increased carbon dioxide (CO2) in the atmosphere associated with human fossil fuel burning. This effect is known as “CO2 fertilization” because, in the envisioned scenario, higher ambient CO2 concentrations in the atmosphere literally “fertilize” plant growth. Because plants in turn, in the process of photosynthesis, convert CO2 into oxygen, it is thus sometimes argued that such “co2 fertilization” could potentially provide a strong negative feedback on changing CO2 concentrations.

Recent experiments and model calculations, however, suggest that this is unlikely to be the case. A set of controlled experiments known as FACE (“Free Air CO2 Enrichment”) experiments have been performed in which ambient CO2 levels are elevated in forest stands and changes in various measures of productivity are made over several years. Experiments of this sort that have been done at Duke Forest indicate (in agreement with models), that 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.

A few simple calculations indicate that any hypothesized co2 fertilization response is unlikely to offset a significant fraction of projected increases in atmospheric co2 concentration over the next century. At present, about 600 billion tons of carbon are tied up in the above-ground vegetation. About 2-3 times this much is tied up in roots and below ground carbon, which is a more difficult carbon pool to augment. By comparison, scenarios for fossil fuel emissions for the 21st century range from about 600 billion tons (if we can keep total global emissions at current levels) to over 2500 billion tons if the world increases its reliance on combustion of coal as economic growth and population increase dramatically. These numbers clearly indicate that sequestering a significant fraction of projected emissions in vegetation is likely to be very difficult, especially as forests are cleared to make way for agriculture and communities. While there are possibilities of storage in wells and deep in the ocean, stabilizing the atmospheric CO2 concentration would require gathering up the equivalent of 1 to 2 times the world’s existing above ground vegetation and putting it down abandoned oil wells or deep in the ocean. While CO2 fertilization could help to increase above ground vegetation a bit, storing more than a few tens of percent of the existing carbon would be quite surprising, and this is likely to be more like a few percent of global carbon emissions projected for the 21st century.

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