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expert reaction to study looking at trees, carbon storage and climate change

A new study, published in Science, reports on the capacity for trees to reduce atmospheric carbon levels.

 

Dr James Borrell, Researcher at the Royal Botanic Gardens, Kew, said:

“Global forest restoration clearly has remarkable potential to address climate change, but restoring forest on this scale is a significant challenge in itself.

“If this kind of strategy is to work, we need to start now.  Restoring forests on this scale will take decades if we are to do it in the right way.

“One of the most startling findings is that more than half of forest restoration potential is in just six countries; Russia, the US, Canada, Australia, Brazil and China.  A huge responsibility rests on these countries to lead the way if we are to use forest restoration as a tool in the fight against climate change.

“Some strengths of the study stand out.  First, the authors have considered canopy cover on a continuous scale, acknowledging the fact that a sparse Mediterranean forest, for example, might be very different to an equatorial tropical forest.  In order to balance the relative contributions of these different types of forest, the authors account for this difference, improving the accuracy and relevance of their output.

“Second, a major criticism of many reforestation plans is that they might compete with other land uses, such as crop land.  Here, the authors are explicitly identifying areas for reforestation that exclude cropland and urban areas using a global land cover model.  There may however still be some level of conflict with areas used for grazing or livestock.”

 

Prof Myles Allen, Professor of Geosystem Science, University of Oxford, said:

“In the paper, the authors compare the extra CO2 in the atmosphere with the amount they assess could be stored in forests, but this is misleading, because it neglects the fact that only about 50% of the carbon we release into the atmosphere stays there, the rest being mixed into the near-surface oceans and the like.  To date, we have emitted 600 GtC and an additional 300 GtC (50%) of that CO2 remains in the atmosphere.  If we had emitted 600 GtC and removed 200 GtC, there would be 200 GtC (50% of 600-200) remaining in the atmosphere.  Comparing removal potentials with “atmospheric burden” exaggerates their importance by around a factor of two.

“Restoration of trees may be “among the most effective strategies”, but it is very far indeed from “the best climate change solution available,” and a long way behind reducing fossil fuel emissions to net zero.  The additional 200 billion tonnes of carbon the study highlights represents less than one third of human-induced carbon dioxide emissions to date, and less than 20 years of emissions at the current rate.  So, yes, heroic reforestation can help, but it is time to stop suggesting there is a “nature-based solution” to ongoing fossil fuel use.  There isn’t.  Sorry.”

 

Dr Phil Williamson, Honorary Reader, University of East Anglia, said:

“There is a clear message from the analysis of Jean-Francois Bastin and his colleagues: there is the land available, nearly a billion hectares, for a massive global tree-planting initiative that should help avoid climate catastrophe, without encroachment on existing arable land.  There is also a warning: this is a time-limited opportunity, since the scope for forest-based carbon removal is reduced in a warmer world.  What is not made explicit is that very rapid reductions in greenhouse gas emissions must also occur at the same time.  Whilst reforestation can undoubtedly assist in achieving net-zero, and subsequently net-negative emissions, it is potentially misleading for the authors to claim that “ecosystem restoration is the most effective solution at our disposal to mitigate climate change”.  The most effective solution remains as before: ending emissions, through the worldwide phase-out of fossil fuels within a few decades.  Unless that is also done, the newly-planted forests won’t survive for long enough to have the desired effect.”

 

Prof Simon Lewis, Professor of Global Change Science, UCL, said:

“Restoring ecosystems is good for the climate, especially reforestation, as new tree trunks store carbon taken from the atmosphere.  We need to plant new forests to meet our climate change obligations.  This paper assesses how much land could grow new forests, suggesting 900 million hectares could be returned to forest without impacting agriculture.  This may be a broad brush stroke reasonable global estimate, but it’s worth remembering that where to plant trees is constrained locally as landscapes need to fulfill many functions, not just carbon storage.

“The estimate that 900 million hectares restoration can store an addition 205 billion tones of carbon is too high and not supported by either previous studies or climate models (e.g. Lewis et al 2019 Science, Arora & Montenegro 2011, Nature Geoscience).  The authors forgot to subtract the carbon already on the land and in the soil that was there before any restoration happens.  Plus the biome specific carbon storage estimates are too high as they are the end points of hundreds or years of succession, not a couple of decades of forest growth.  But, the median estimate from the IPCC 1.5C report scenarios to meet the 1.5C target is 57 billion tones of carbon sequestered by new forests this century, which is certainly possible, if these new forests are adequately protected into the long-term.

“The idea that climate change risks losing areas of tropical forests before 2050 is also not consistent with the results from climate models or observational studies for today’s forests.  The authors make a critical error by ignoring the central role of carbon dioxide fertilisation, which counters much of the negative impacts of higher temperatures.  Today, the immediate major threat to tropical forests is deforestation by people and out of control fires, not the more subtle impacts of higher temperatures.

“To curb climate change means keeping fossil carbon out of the atmosphere.  That means ending the burning of fossil fuels and the dumping of carbon in the atmosphere.  New forests can play a role in mopping up some residual carbon emissions, but the only way to stabilise the climate is for greenhouse gas emissions to reach net zero, which means dramatic cuts in emissions from fossil fuels and deforestation.”

 

Prof Martin Lukac, Professor of Ecosystem Science, University of Reading, said:

“Yes in my back yard!  I love trees, we all do.  Planting trees to soak up two thirds of the entire anthropogenic carbon burden to date sounds too good to be true.  Probably because it is.  The paper uses latest advances in remote sensing and statistical modelling to give us the upper limit of global forest carbon capture potential.  This is certainly large – the number covers about 20 years’ worth of current global carbon emissions.

“The problem is people do not respect remote sensing and statistical models.  This far, humans have enhanced forest cover on a large scale only by shrinking their population size (Russia), increasing productivity of industrial agriculture (the West) or by direct order of an autocratic government (China).  None of these activities look remotely feasible or sustainable at global scale.

“Finally, the paper presents a risk assessment of future changes in potential tree cover.  This is the weakest part of the report, and beautifully illustrates the danger of extrapolation – as the authors admit themselves.  Looking at their map of the world in 2050, large parts of the Amazon have nearly 100% chance of losing continuous forest cover purely for climatic reasons.  We have examples of forest dieback driven by climate change, but I find it very hard to believe that this colossal scale of forest ecosystem disruption can happen in three decades from now.”

 

Prof Daniela Schmidt, Professor in Palaeobiology, University of Bristol, said:

“Planting trees in areas which have lost cover or in areas where they are now able to grow due to warming is a clear strategy to combat climate change suggested in the Paris Agreement.  Assessing if this approach is feasible is a complex question depending on the climate conditions at a given place and the needs of the trees.  This novel study assessed a vast dataset of information of tree growth and generated a model which allows the quantification if planting trees is feasible in areas large enough to allow a significant drawdown of CO2.  It is important to remember though that this action replaces existing ecosystems and agricultural land.  Planning the areas for reforestation needs to take into account the ecosystem which is currently there and the consequences replacing these with forests would have.”

 

* ‘The global tree restoration potential’ by Jean-Francois Bastin et al. will be published in Science at 19:00 UK time on Thursday 4 July 2019.

DOI: https://science.sciencemag.org/cgi/doi/10.1126/science.aax0848

 

Declared interests

Prof Myles Allen: “None.”

Dr Phil Williamson: “Dr Phil Williamson is an Honorary Reader at the University of East Anglia, employed by UK Research and Innovation (Natural Environment Research Council).  He is the Science Coordinator of the UK Greenhouse Gas Removal programme that is examining the feasibility of a range of option for removing carbon dioxide and other greenhouse gases from the atmosphere.”

Prof Simon Lewis: “No conflict of interests to declare.”

Prof Daniela Schmidt: “I do not have any interests.  I am declaring that I am the CLA for the IPCC WGII chapter on Europe but that my view is not part of the assessment.”

None others received.

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