Plants capture more CO2 than expected – experts respond

A study published in the journal Proceedings of the National Academy of Sciences has suggested that current climate models have underestimated how much carbon dioxide plants absorb, therefore overstating how much of the gas is actually in the atmosphere.

NZ forestThe scientists say current forecasts don’t account for the slow diffusion of atmospheric carbon dioxide inside plant leaves, and that 16 per cent more of the gas was absorbed between 1901 and 2010 than previously thought.

However, this is unlikely to make a difference to overall global warming predictions.

Our colleagues at the Australian and UK SMCs have collected the following expert commentary. Feel free to use these quotes in your reporting. If you would like to contact a New Zealand expert, please contact the SMC (04 499 5476;

Professor Peter Rayner, professorial fellow in the School of Earth Sciences at the University of Melbourne, comments:

“The paper studies the mechanism by which plants absorb carbon dioxide from the atmosphere. It provides one explanation for a puzzle: Why isn’t CO2 in the atmosphere rising even faster? We already knew nature was working hard to hold CO2 concentrations in check, this gives one explanation of how. Unfortunately it doesn’t change the basic story.

“The total effect over the 20th century is about 130 billion tonnes of CO2. That sounds like a lot but is less than 4 years of fossil fuel emissions. When it comes to controlling changes in CO2 in the atmosphere, nature is interesting but humans are in control: It’s our problem and our choice.”

Dr Pep Canadell, Executive Director of the Global Carbon Project at CSIRO Oceans and Atmosphere Flagship, comments:

“The paper provides great new insights into how the very intricacies of leaf structure and function can have a planetary scale impact. It provides a potential explanation for why global earth system models cannot fully reproduce the observed atmospheric CO2 growth over the past 100 years and suggests that vegetation might be able to uptake more carbon dioxide in the future than is currently modelled. Having more carbon taken up by plants would slow down climate change but there are many other processes which lay in between this work and the ultimate capacity of terrestrial ecosystems to remove carbon dioxide and store it for long enough to make a difference to atmospheric CO2 trends.

“Additional research will show the net effect on atmospheric CO2, which undoubtedly, will still fall within the already well-established large uncertainty of future projections of the land carbon sink due to other processes, such as responses to nutrient limitation, fires, and the thawing of permafrost.”

Dr Roger Dargaville, research fellow and leader of the MEI Energy Futures Group at the University of Melbourne, comments:

“Sun et al.’s PNAS paper suggests the CO2 fertilisation effect may be underestimated and that as a result the projected carbon uptake of terrestrial ecosystems may be too small. This is important research into a complex field that has significant uncertainty. The research is suggesting that plant uptake may be larger than expected, but does not cover the issue that a larger carbon turnover does not necessarily mean higher net storage, in much the same way that larger cash turnover in a business does not necessarily result in larger profits. Warmer temperatures are likely to drive an increase in respiration rates (return of carbon to the atmosphere via decomposition), and shifts in precipitation will also likely have long term detrimental effects on carbon uptake.

“As the authors note, the complexities of CO2 fertilisation interaction with nutrient and water availability make this a complex issue. Overall the CO2 fertilisation effect has a modest impact on the global carbon budget, and does not remove the necessity to dramatically reduce fossil carbon emissions over the next 40 years to avoid dangerous climate change.”

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

“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, comments:

“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, comments:

“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, comments:

“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.”