How many trees to combat climate change?

15 April 2014

Question

Are you aware if someone has done a back of the envelop calculation of the number of trees needed to stop the rise of Carbon Dioxide concentration?

There are probably better technologies to "suck up" CO2 from the atmosphere, but, nonetheless, some of us can plant trees while waiting for these technologies to mature. Are we talking about 1 million, billion or some un-feasible number of trees? I guess the calculation would have to account for the type of tree, the CO2 generated in the effort, the amount of land required and available, natural deforestation as an offset, etc. I guess the envelop or napkin has to be large.

Thank you and warm regards,

Paul

Answer

Helen - When it comes to climate change, yes, back of the envelope calculations, there's lots of variables when it comes to figuring out how much carbon is stored in the short, medium and long term in forests, with the general idea being that you've got enormous trees, the wood and the carbon inside that is carbon that would be otherwise be in the atmosphere. A kind of very basic idea to bear in mind is that it's thought that about 50% of the dry biomass, the dry weight, if you take the water out of a tree, out of plants, about half of that is carbon. And you can translate a tonne of carbon stored inside a tree to about 3.6 tons of carbon dioxide in the atmosphere roughly and then if you want to do a back of the envelope, I believe that last year, 36 billion tonnes of carbon dioxide were put into the atmosphere just from burning fossil fuels. So, it's a lot of trees. I did find one study which I thought really kind of nailed home just how many trees we would need if we were going to do this and this was based on UK emissions only. This was a few years ago, but I think we can get a good idea from this. Just cars, if we wanted to offset just the emissions from cars in the UK, we would need to plant between 3 and 12 million hectares of woodland, depending on which species because that matters. Currently, there's only 2.5 million hectares of forest in the UK and 23 million hectares of land. If we wanted to offset all our emissions in the UK, we would basically need 2 Britains. We don't have enough space. We would need to plant about 50 million hectares of conifers in plantations to fix that carbon. So, it's a lot of trees.

Chris - That is an awesome number of trees.

Helen - It is.

Chris - I wouldn't have thought it was as big as that.

Helen - It is huge and then there are a lot of trees in the world and a lot of carbon is fixed inside those forests which is another important point, which is apart from just planting new trees, we also have to try and stop cutting down the ones we've got because that also releases a huge amount of carbon dioxide into the atmosphere. And also, trees that grow at the edge of the oceans - mangroves are very important stores of carbon - natural carbon stores. So, we should be thinking about replanting those areas of habitat as well.

Chris - What about your favourite places though Helen, the oceans because don't they account for a disproportionate amount of the CO2 pulldown in the form of marine plants - algae?

Helen - Yes, absolutely. It's about half actually. Half of the productivity if you like, the photosynthesis that grabs carbon dioxide from the atmosphere and makes it into organic molecules. Half of that happens in the ocean. Half of that happens on land in forests and grasslands and so on. So, the oceans are enormously important. And also, carbon dioxide just dissolves in the ocean. If that hadn't happened, and if the oceans weren't there, climate change would already be enormously worse because there just would be much more carbon dioxide already in the atmosphere.

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