Carbon cycle: better than we thought
Oct 14, 2014
Bishop Hill in Climate: carbon budget

A new paper in PNAS has been getting quite a bit of media play today, which is slightly surprising because the overall theme is "it's better than we thought". The original paper is here and there is a rather helpful "Significance" section alongside the abstract.

Understanding and accurately predicting how global terrestrial primary production responds to rising atmospheric CO2 concentrations is a prerequisite for reliably assessing the long-term climate impact of anthropogenic fossil CO2 emissions. Here we demonstrate that current carbon cycle models underestimate the long-term responsiveness of global terrestrial productivity to CO2 fertilization. This underestimation of CO2 fertilization is caused by an inherent model structural deficiency related to lack of explicit representation of CO2 diffusion inside leaves, which results in an overestimation of CO2 available at the carboxylation site. The magnitude of CO2 fertilization underestimation matches the long-term positive growth bias in the historical atmospheric CO2 predicted by Earth system models. Our study will lead to improved understanding and modeling of carbon–climate feedbacks.

The Telegraph's take is here and it's interesting to see who they have got hold of to belittle the results:

Dr Simon Lewis, Reader in Global Change Science at University College London, said: “This study shows, correctly in my view, that photosynthesis is highly responsive to carbon dioxide, but this is far from the only factor amongst many that will impact the forests of the 21st century and how much carbon they store.

Prof Peter Cox, Professor of Climate System Dynamics at the University of Exeter, said: “Avoiding two degrees of global warming is a huge challenge for humanity even if this effect is taken into account.”

Dr Chris Huntingford, Climate Modeller at the Centre for Ecology and Hydrology, said: “This new paper suggests plants are slightly better at capturing CO2 than we thought.

A little Googling shows that the diligent journalists at the Telegraph have simply pasted in these comments direct from the Science Media Centre press release, although it seems that Richard Betts' contribution didn't make the cut:

This is a very interesting paper adding to our understanding of plant physiology.  The authors remark on the potential importance of their results for global carbon cycle modelling, and this is indeed relevant, but as a priority for improving carbon cycle modelling there are other processes which current models treat either very simplistically or not at all. Fire disturbance, for example, is not included in some of the models examined here – its inclusion could be more important than any improvements in modelling CO2 fertilization, as it seems likely to be an important feedback on climate change.  Changes in global soil respiration at the global scale are also poorly understood.

“So while this is an interesting and useful contribution, it should be put into context with the bigger picture – disturbance mechanisms as well as physiological processes are important.”

Somewhat off message perhaps.

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