October 13, 2014

expert reaction to new study on plants and CO2

A study published in the journal Proceedings of the National Academy of Sciences has suggested that differences in carbon dioxide concentrations inside plants may account for errors in estimations of their capacity for carbon storage.

Dr Simon Lewis, Reader in Global Change Science at University College London, said:

“Earth’s vegetation currently removes about one quarter of all human emissions of carbon dioxide. This new analysis suggests that some modelling studies slightly underestimated the size of this major free subsidy from nature over the past 100 years.

“But what does this mean for the future? This is hard to tell from the new study as it does not model the future. Looking forward 100 years the amount of uptake of carbon dioxide by the world’s vegetation is uncertain. Many scientists think climate models are too optimistic about how much carbon dioxide forests can take up. Few think trees will grow ever-bigger as they are fertilized by ever-higher amounts of carbon dioxide in the atmosphere. Something else, such as nutrients, water or extremely high temperatures may well limit growth in the future. This study, considering only one aspect of photosynthesis 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.

“The level and speed of greenhouse gas emissions cuts needed to avoid dangerous levels of climate change are not altered by this new study.”

Prof Richard Betts, Head of Climate Impacts Research at the Met Office and Chair in Climate Impacts at the University of Exeter, said:

“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.”

Prof Peter Cox, Professor of Climate System Dynamics at the University of Exeter, said:

“We are usually told that CO2-fertilization is over-estimated in climate models, mainly because we neglect the limitations nutrients can have on plant growth. So this paper goes against that flow by suggesting that the models might actually under-estimate the effect of CO2 on plant growth. However, results from Free-Air CO2 Enrichment Experiments (FACE) still tend to support the prevailing view that models most likely over-estimate CO2 fertilization on the century timescale.

“In any case, the effect discussed in this paper is relatively small compared to the overall uncertainties in the future land carbon sink.  Avoiding 2 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:

“Atmospheric concentrations of carbon dioxide can be compared to how much is produced by burning fossil fuels.  Approximately 50% of the CO2 we put into the atmosphere is drawn down into the land or oceans, partly offsetting global warming. To understand how much the planet will warm over the next decades depends on how much the earth continues to pull CO2 from the atmosphere. This new paper suggests plants are slightly better at capturing CO2 than we thought.

“This new research implies it will be slightly easier to fulfil the target of keeping global warming below two degrees – but with a big emphasis on ‘slightly’. Overall, the cuts in CO2 emissions over the next few decades will still have to be very large if we want to keep warming below 2 degrees.”

‘Impact of mesophyll diffusion on estimated global land CO2 fertilization’ by Ying Sun et al. published in Proceedings of the National Academy of Sciences on Monday 13 October 2014.

Declared interests

None declared