Full article: https://science.sciencemag.org/content/365/6448/76 (paywalled; SciHub link: https://sci-hub.tw/10.1126/science.aax0848)

Pop-Sci Guardian article: https://www.theguardian.com/environment/2019/jul/04/planting-billions-trees-best-tackle-climate-crisis-scientists-canopy-emissions

Key quote: "The most effective projects are doing restoration for 30 US cents a tree. That means we could restore the 1tn trees for $300bn [£240bn], though obviously that means immense efficiency and effectiveness. But it is by far the cheapest solution that has ever been proposed."

Forest-related charities (CoolEarth et al.) have already received a good bit of attention in the past, but if these new findings are true, then perhaps we have been underestimating the benefit of reforestation, and should be directing more funds towards them?





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I have not looked at this deeply, so take all of this with a grain of salt, but a quick scan makes me believe that this is very hyped.

Having skimmed the paper it seems the only thing the authors do and what is peer-reviewed is estimate earth's theoretical maximum capacity for reforestation.

There are few problems:

1) Feasibility: "200 GtC is a technical potential assuming every hectare of forestland on earth is increased to 100% forest cover"


"An assessment of the biophysical capacity for restoring global tree cover provides a necessary but insufficient foundation for evaluating where tree cover can be feasibly increased. The kinds of trees as well as how and where they are grown determine how and which people benefit. In some contexts, increasing tree cover can elevate fire risk, decrease water supplies, and cause crop damage by wildlife. Reforestation programs often favor single-species tree plantations over restoring native forest ecosystems. This approach can generate negative consequences for biodiversity and carbon storage (5), threaten food and land security, and exacerbate social inequities. How restored lands are governed determines how reforestation costs and benefits are distributed."


There are no dollar signs or economic feasibility analysis in that paper. So the people who wrote it just cited some number that is not peer-reviewed and even if loosely based on previous numbers might not work at the scale.


2) There seems to be no scientific consensus how much (or even if!) new trees are a GHG sink on net. Some legitimate papers even suggests that new trees could be a net GHG source. (the paper does not make any new contributions to this question and I think just assumes that trees absorb GHG).

For instance, see this recent paper in Nature communications:

a) "Limited capacity of tree growth to mitigate the global greenhouse effect under predicted warming" https://www.nature.com/articles/s41467-019-10174-4

b) this recent paper in PNAS (top journal as well)

"On-going carbon uptake due to forest demography is large, but much smaller than previous influential estimates have suggested. Contrary to previous findings, these latest data sources indicate that the sink is predominantly in mid-high latitude, rather than tropical, forests."


2) is great stuff, thanks for posting it.

Have some disagreements with 1 because there is no possible intervention that can't be made infeasible with enough impact report requirements.

The amount of carbon that they say could be captured by restoring these trees is 205 GtC, which for $300bn to restore comes to ~70¢/ton of CO2 ~40¢/ton of CO2. Founders Pledge estimates that, on the margin, Coalition for Rainforest Nations averts a ton of CO2e for 12¢ (range: factor of 6) and the Clean Air Task Force averts a ton of CO2e for 100¢ (range: order of magnitude). So those numbers do check out.

I did not look at the details, but it appears that neither of these estimates take into account opportunity costs. Typical farming profit is around $200 per hectare per year, so if instead you sequester 5 tCO2e per hectare per year, that would cost ~$40 per tCO2e, ~2 orders of magnitude more expensive. By the way, I believe $300 billion divided by 205 billion tons carbon = 750 billion tons CO2 would be $0.40 per ton CO2.

Some companies are trying to make reforestation cheaper using drones:


Working at a company like this could be high-impact, and could also be a good way to build career capital in AI/robotics/machine learning.

I saw a few of those articles or summaries. There are some disputes over exactly how much tree planting can 'offset' (or reverse) climate change, but the magnitude even in those disputes looks to be similar to cutting back on meat based and industrial agriculture diets . The main problem is political---clearing forests for meat, soy and palm oil, corn and cotton, is a big business.

there are even some people who say 'habitat preservation' (eg rainforests) is a bad idea because then you have alot of suffering animals living in them, who would be happier not being born.

i am in contact with a few groups in Africa who apparently quite busy planting trees (since I haven't been there i can't verify this, except from what they send me). My view is 'precautionary' or 'prevention' principles---avoid as much as possible deforestation and consumption of products that rely on it.

Great to see some momentum in the space. I looked into this about a year ago and concluded that I could *personally* pull off about $2 a tree if I really went at it just with commodity resources. Then based on looking at how things scaled in other domains guessed that you could get down to maybe $1 a tree with scaling + an effective org around you. That plus the idea that if smart people spent some serious time analysing cost bottlenecks harder costs would go down even more multiplied out by the relative carbon sequestration cost compared to next best geoengineering soltuions I concluded that this likely wouldn't remain neglected for very long and stopped paying attention. I still think climate shifts are going to be terrible in lots of minor costly ways and we may still fail at the coordination needed to get it *right*, but I've mostly stopped paying attention to the doom and gloom estimates.

Caveat: Tree-planting programs can backfire if they disrupt local ecosystems such as wetlands and arid areas. For example, when Canada planted spruce trees in swamps, this actually made them more susceptible to forest fires.


The current #TeamTrees program ensures that the trees planted are well suited to their local ecosystems. They're aiming to raise $20M to plant 20 million trees around the world. I suggest donating to this campaign if you're interested in reforestation.