Will plant growth from carbon emissions cool the planet?

One of the arguments put forward by climate sceptics and deniers is that CO2 = life. That is, more carbon dioxide in the Earth’s atmosphere will result in increased plant growth, which will in turn lead to a relative cooling of the atmosphere, thereby mitigating if not reversing climate change.

Do the sceptics have anything to go on when making this statement? Not really, as we do not yet understand in detail the mechanisms involved, and there are thus no hard numbers with which to back up the cooling feedback claim.

A team of researchers led by Lund University bioscientist Almut Arneth has now given us a first estimate of the effect of atmosphere-biosphere feedbacks, and the numbers do not look good for those who see in the process a natural means of mitigating anthropogenic climate change.

In a paper published recently in the journal Nature Geoscience, Arneth and her colleagues show that, by the end of the 21st century, the positive radiative forcing arising from feedbacks between the terrestrial biosphere and atmosphere could be as much as 1.5 watts per square metre of the Earth’s surface, per degree of temperature change.

What does this mean in practice? If Arneth’s estimate is correct, the biogeochemical feedback mechanisms described could substantially reduce or even eliminate the cooling effect of carbon dioxide fertilisation in the terrestrial biosphere. The overall magnitude of the feedbacks could, say the researchers, be similar to that of feedbacks in the physical climate system. It all depends on the synergies between individual feedback mechanisms, and the extent to which interactions with the nitrogen cycle stimulate or restrict carbon sequestration.

The researchers say that an improved knowledge of interactions between the Earth’s biosphere and atmosphere should be a priority for the scientific community. Such an understanding will facilitate atmosphere and ecosystem management in the light of post-Kyoto negotiations, and the development of climate and air pollution control strategies.

Further reading: Arneth et al., “Terrestrial biogeochemical feedbacks in the climate system”, Nature Geoscience 3, 525 (2010)