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Claims for tree planting by Zurich scientists were withdrawn

Summary

  • A group of scientists in Zurich published a report in July 2019 claiming that global tree restoration is "our most effective climate change solution".
  • The claims attracted a lot of media interest.
  • But following much criticism from fellow researchers, the key claim for the place of global tree restoration was withdrawn and replaced by a much lesser claim: "one of the most effective carbon drawdown solutions" - and an amended version of the report was published in October 2019.
  • This withdrawal was given little media coverage, and planting trees remains a much-discussed "solution" to the climate crisis - but the scientific consensus remains that it has only a small role.
  • This is another example of mankind's poor collective decision-making.

The original report

The original research report [1] was published in the scientific journal Science in July 2019. It was written by a group of researchers at the Swiss university, ETH-Zurich.

The report was publicized by a PR company (Greenhouse PR) with the result that "700 pieces of media coverage were published, including more than 100 national titles, spanning over 100 countries" [2] - and there were headlines such as "Tree planting 'has mind-blowing potential' to tackle climate crisis" in the Guardian [3].

The claims in detail

The report estimated that the area of the earth that could be covered by trees is 0.9 billion hectares and that this could store 205 gigatonnes of carbon. The report originally claimed that this made global tree restoration "our most effective climate change solution".

Criticisms of the claims

The original report was criticized by other scientists on several grounds, the most important of which is that absorption of CO2 would be too slow to help the current crisis. Tom Crowther, the head of the department at ETH-Zurich that carries out the research has acknowledged that absorption of the CO2 would take 200 years [4].

This means that even if there was reforestation of 0.9 billion hectares (an area the size of the USA), the rate of carbon absorbed would be only 1 gigatonne per year on average - only a small fraction of the annual 34 gigatonnes CO2 currently released by burning fossil fuels globally [5].

The updated report

Following the extensive criticism, the original report was replaced by an updated version [6] in October 2019, and the claim for global tree restoration in the report summary was changed from
"our most effective climate change solution" to
"one of the most effective carbon drawdown solutions"
- meaning that of the methods we have for taking back CO2 from the atmosphere, planting trees is one of the most effective. This is not a strong claim, as the amount of CO2 being taken back from the atmosphere is very much smaller than the rate at which we are adding CO2 to it.

Conclusions

Following the publication of the report, and its amendment, the scientific consensus remains that tree planting has only a small role in tackling the climate crisis.

This episode is another example of mankind's poor collective decision making. It is not unusual for a scientific report to be challenged, and for the authors to modify their conclusions - it is a part of the scientific process. However media reporting is biased towards selecting sensational stories such as "scientists announce the mind-blowing potential of planting trees" rathan than the less interesting "scientists withdraw claims for tree planting".

And unscrupulous politicians are biased towards policy decisions that are easy (a commitment to tree planting requires no immediate action) and won't attract opposition - rather than taking the hard choices such as closing airports and a steadily increasing carbon tax that might actually save our "civilization".


Notes and references

[1]Original paper: Bastin et al, Science 365 76-79 (2019) The global tree restoration potential https://www.scribd.com/document/420473078/The-global-tree-restoration-potential
[2]Greenhouse PR website https://www.greenhousepr.co.uk/trees-potential/
[3]Tree planting 'has mind-blowing potential' to tackle climate crisis (Jul 2019) The Guardian https://www.theguardian.com/environment/2019/jul/04/planting-billions-trees-best-tackle-climate-crisis-scientists-canopy-emissions
[4]Remember the headlines: Tree planting is our “most effective climate change solution”? Well, it would take 200 years, and we can only plant 10% of the area, says Tom Crowther of ETH-Zürich (Oct 2019) https://redd-monitor.org/2019/10/05/remember-the-headlines/ - the statement by Tom Crowther is at 3:25.
[5]https://www.carbonindependent.org/67.html
[6]Updated paper: Bastin et al, Science 365 76-79 (2019) The global tree restoration potential https://science.sciencemag.org/content/365/6448/76



First published: Jan 2020
Last updated: 29 Jan 2020