8th January 2020

Planting trees is vital, but it won’t stop climate change. We must reduce fossil fuel use.

By Rupesh Paudyal

2019 saw a significant shift in media reporting and dialogue around climate change and biodiversity loss. Several comprehensive and influential studies gave a damning verdict on the current fragile state of the global ecosystems and rising greenhouse gas emissions. Climate change movements such as the School Strike for Climate and Extinction Rebellion went global with millions of people engaging protests. Even scientists urged on their colleagues to join civil disobedience movements.

According to Google, Greta Thunberg was the 7th most searched person in 2019 — she was also the TIME person of the year. Earlier in the last summer, climate change and Greta Thunberg dominated headlines around the world when she left Europe to go to the Americas to attend the 2019 UN Climate Action Summit.

In 2019 summer, at the height of the climate change debate, a scientific study suggested that an additional 900 million hectares — roughly the size of the United States — of land on Earth can undergo reforestation. The research predicted that once these trees mature, they could store over 200 billion tonnes of carbon, which is two-thirds of the total carbon released by humans since the Industrial Revolution. The study proved to be highly influential and was reported by hundreds of media outlets all over the world with headlines such as: “Best way to fight climate change? Plant a trillion trees.”

This study by Jean-Francois Bastin and colleagues (Bastin 2019) also drew the attention of scientists all around the world, who have reanalysed the results. Reanalyses found that the original research significantly overestimated the potential of global reforestation to capture carbon and mitigate climate change. “Contradictory results” are posted in bioRxiv preprint, and in several “Technical Comments” are published in Science (here, here, here and here).

Tree planting is important in our fight against climate change, but to say that it’s “the answer” to climate change is wrong.

Scientists say that the key assumptions in Bastin 2019’s analysis were incorrect, resulting in an overestimation of the potential of global reforestation. New contradictory results show that curbing climate change isn’t as simple as worldwide reforestation and would require a move away from fossil fuels towards sustainable energy sources.

The initial investigation by Bastin 2019 calculated the full carbon-store potential for the 900 million hectare land. However, it failed to consider the carbon already stored by the vegetation in the “restored land”. A comment posted in Science magazine says that Bastin 2019 overestimated the carbon-capture potential of restored land by a factor of five, estimating that reforestation can capture 40 billion tonnes of carbon, which is still a substantial amount of carbon to take away from the atmosphere.

Scientists comment that “although ecological restoration — if carefully implemented — can have a role in mitigating climate change, it is no substitute for the fact that most fossil fuel emissions will need to stop to meet the targets of the Paris Agreement. Such action should be accompanied by policies that prioritise the conservation of intact, biodiverse ecosystems, irrespective of whether they contain a lot of trees.”

Another group of scientists commented here that recapturing over 200 billion tonnes of carbon, as suggested by Bastin 2019, would actually require 2.9 billion (2,900 million) hectare land, rather than 900 million hectares that Bastin estimated. Authors question the feasibility of reforesting of this magnitude. They note that 2.9 billion hectares “is approximately equivalent to the area of Africa, 3 times that of the United States, or 115 times that of the United Kingdom.”

A separate reanalysis of Bastin 2019’s figures were also published in Science. Here, scientists found that the carbon-capture potential of global tree replantation is only half of Bastin’s estimation of 200 billion tonnes carbon. Scientists say that — in the 900 million hectares of the proposed restorable land — the Bastin 2019 study incorrectly assumes that, before tree restoration: (1) soil carbon stocks are zero; and (2) live stem, leaf, root and biomass from dead organisms are all zero. Bastin 2019 also (incorrectly) assumes that future environmental change will have no impact on forest carbon stocks, say the technical comments.

These reanalyses were also verified by the study posted in bioRxiv, where scientists found that the carbon storage potential of reforesting an additional 900 million hectare land could capture 75–100 billion tonnes of carbon, less than half of the value estimated by the original study. Researchers conclude that their result “demonstrates that the original Bastin 2019 estimate was clearly overestimated.”

Authors of the study say that a more accurate carbon capture estimation of reforestation requires factoring in the complex carbon dynamics, potential feedback loops, societal costs, and carbon saturation as forests mature. Additionally, estimating the reforesting potential also requires an “estimation of the timescale of global forest regrowth, which requires local studies using more nuanced analysis of carbon uptake,” argue the scientists, who reanalysed the Bastin 2019’s model.

Scientists have warned that “the claim [by Bastin 2019] that global tree restoration is our most effective climate change solution is simply incorrect scientifically and dangerously misleading.” Warning from experts is clear that only planting trees won’t solve the climate crisis, and we need to implement meaningful policy changes, including moving away from fossil fuel energy.

Even though reanalyses of Bastin 2019 shows overestimation of the carbon-capture potential of trees replantation, it shouldn’t take away from the fact that tree-planting is, and will still be vital to mitigate the effects of climate change.

Another technical comments, also published in Science, says that the research from Bastin 2019 “is to be welcomed for drawing fresh attention to the potential for expanding forest and tree cover to mitigate global climate change by sequestering CO2 from the atmosphere.”

Read the original study by Bastin and Colleagues: “ The global tree restoration potential” http://doi.org/10.1126/science.aax0848

Contradictory studies, which say that Bastin 2019 overestimated global carbon-capture potential of reforestation of 900 million hectare land are as follows: