Glasgow cathedral

Trees are 50% carbon

Wouldn’t it be nice if there was a single, simple solution to the climate crisis that all political parties agreed to support.

By the end of the 19th century, Glasgow was known as the “Second City of the Empire”. It produced more than half Britain’s tonnage of shipping and a quarter of all locomotives globally, but it held the COP26 UN climate change conference even with this history. At the meeting, there was a lot of talk about trees. 

India promised forests across a third of its land area and the UAE said they would plant 100 million mangroves by 2030, and more than 100 countries pledged to halt and reverse deforestation by 2030, including Brazil.

Elsewhere Amazon founder, Jeff Bezos, announced a $1bn fund to plant trees, “revitalise” grasslands in Africa, and restore landscapes across the US. 

The sudden appearance of trees on the climate talks agenda is because trees are carbon sinks.

Here is why. 

Roughly 35% of the green mass of a tree is water leaving 65% as solid biomass, and half of this dry mass is carbon. As well as the visible trunk, branches and leaves, trees also have roots that are also 50% carbon by dry mass, and in most tree species, roots are around 30% of the total biomass of the tree.

Image source: Jason Leung on Unsplash

Even better than all this woody material, the conversion of this carbon mass to the equivalent amount of carbon dioxide or tCO2e, the currency of climate accounting, the carbon figure is multiplied by a factor of 3.67 

10 tonnes of carbon becomes 36.7 tCO2e

So if a tree is planted where there wasn’t a tree before, all the carbon captured as the tree grows comes from carbon dioxide in the atmosphere — and with classic human ingenuity, that carbon can be converted to tCO2e and accounted as a carbon credit.

It is a logical step to assume that if enough new trees are planted and grow well, it could draw down a lot of the carbon emissions, perhaps even all the previous emissions from fossil fuel use.

Here is human irony at its best. Recall that coal was once trees.

How many trees do we need to perform this climate miracle that bails us out of our fossil fuel burning hole? One estimate for how many trees to do this would require around 1.6 billion hectares of land, the size of Brazil and Australia combined.

Ah, a slight snag.

Given the FAO tells us that the land area used for agriculture is approximately 5 billion hectares, or 38% of the global land surface, with about 1.6 billion ha of this area used as cropland, humanity has a problem with this climate change solution.

In its latest State of the Worlds Forests report, the FAO also estimates that between 2015 and 2020, the rate of deforestation was estimated at 10 million hectares per year, down from 16 million hectares per year in the 1990s. The area of primary forest worldwide has decreased by over 80 million hectares since 1990.

Humans cut trees down.

10 million hectares worth a year.

But to grow enough trees to sequester the carbon dioxide from all the current and historical burning of fossil fuels 1,600 million hectares of new planting are needed—one and a half times the area of the U.S.

Political tree planting has become a lovely idea. In the US it is one of the very few policy options with bipartisan support, with a Pew Foundation survey reporting that 90% of Americans favour planting about a trillion trees around the world to absorb carbon emissions. Only not presumably on their cornfields.

If only it was a simple case of planting a few trees, and we could all go back to normal coal burning and gas-guzzling.

Tragically there is no such panacea.

There simply isn’t room on a planet populated with 7.8 billion ravenous humans.

Hero image from photo by Craig McKay on Unsplash


Mark is an ecology nerd who was cursed with an entrepreneurial gene and a big picture view making him a rare beast, uncomfortable in the ivory towers and the disconnected silos of the public service. Despite this he has made it through a 40+ year career as a scientist and for some unknown reason still likes to read scientific papers.

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