Science Questions

How many trees to combat climate change?

Mon, 14th Apr 2014

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Paul Jen asked:

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,




Helen - When it comes to climate change, yes, back of the envelope calculations, Ash Treethere'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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Think we need to consider plankton (around 50% of all oxygen as well as binding C02 in their shells) too. It's very tricky as any animal is defined by the availability of food and what type of climate, environment, it needs to survive. In a short term scenario assuming a increased availability of nutrients and more sun from thiner ice, plankton growth may be expected. That will give us more oxygen over some period of time, balancing other consequences of declining oxygen. But as the oceans continue to warm up I don't know how the planktons will fare. My guess is that the arctic plankton will decline, leaving us with less oxygen and more C02 as their shells seems to dissolve under a warming. It may be that it will be replaced by other plankton migrating, but I do not expect this to be able to replace the ones gone, or maybe it will? It depends on ones time frame I guess. Too many unknowns to know at this time. yor_on, Sun, 13th Apr 2014

Ah well. Look at this

As for what happens on land? Droughts and floods, none of them ideal for growing things I think.

You see I think we are in a tipping, from earths former balance to a new one. If it is so I don't expect it to really matter what we do? we won't be able to stop it anyhow, and once Earth finds a new (climate) balance it will be as hard changing as it was to produce it, all that man made CO2 we currently are living with I mean. We never noticed how hard we worked for it, did we? :) but think of all the man hours it took, and industrial facilities, etc etc, to create that CO2 under approximately 250 years.

The way we're going peeling away all the political bs, hiding those in reality fighting for a status quo, of such not willing to accept the cost, and loss of power that comes with it, is toward more CO2 every year. And it won't change the closest decades at least. Also, if I would deem from human short sightedness and self interests, we should have a major war waiting for us. Because a war is perfect for stopping uncomfortable truths, you will get so occupied with surviving that nothing else will matter. And afterwards? Depends on what type of war we will meet here. What we've had since 1945 is small arms war, not nuclear. We never had a nuclear war actually. That craves two participants to exist, as I see it. Yeah I know, doesn't really matter. Just what I expect.

Alternatively you can think of it in terms of small arms wars, constantly growing, as resources dwindle. From such a point of view it will be as fighting grass fires in Australia. You won't know where the next one flames up. And it will cost us all, in humanitarian terms as well as in resources and infra structure. The CO2 we made 250 years ago, up there today, should now start becoming deposited in oceans and land. But only the major part of it, depending on who you ask you will get different answers to how long CO2 stays in the atmosphere, but all agree on the 'tail' of each years production at least stays for millenniums, all as I understands it. And that tail adds constantly to the new CO2 being produced.

"Notice that the carbon dioxide lifetime is "hundreds of years", rather than a specific number. The IPCC ‘Third Assessment Report’ defines a gas's lifetime as the amount of the gas in the atmosphere divided by the rate at which it is removed from the atmosphere. That sounds simple enough, except that not all gases are removed by just one (or mainly one) process. Ironically, the gas that accounts for the greatest proportion of global warming, carbon dioxide (CO2), is the hardest to pin down. When CO2 is released into the atmosphere, about three-quarters of it dissolves into the ocean over a few decades (- Acidity -). The rest is neutralized by a variety of longer-term geological processes, which can take thousands of years.

From IPCC Fourth Assessment Report:  About 50% of a CO2 increase will be removed from the atmosphere within 30 years, and a further 30% will be removed within a few centuries. The remaining 20% may stay in the atmosphere for many thousands of years.

From U.S Greenhouse Gas Inventory Reports: (CO2) Atmospheric lifetime: 50-200 years. No single lifetime can be defined for CO2 because of the different rates of uptake by different removal processes.

From RealClimate: “My model indicates that about 7% of carbon released today will still be in the atmosphere in 100,000 years. I calculate a mean lifetime, from the sum of all the processes, of about 30,000 years. That's a deceptive number, because it is so strongly influenced by the immense longevity of that long tail. If one is forced to simplify reality into a single number for popular discussion, several hundred years is a sensible number to choose, because it tells three-quarters of the story, and the part of the story which applies to our own lifetimes.” ("How long will global warming last?")" From "Lisa Moore, Ph.D., scientist in the Climate and Air program at Environmental Defense."

Btw: this one belongs to "The Environment" I think? Especially after what I wrote :) yor_on, Sun, 13th Apr 2014

Paul Jen asked the Naked Scientists:

Are you aware if someone has done a back of the envelope calculation (or on a napkin if you prefer), 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 infeasible 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 envelope or napkin has to be large.

You all are continuing to do awesome work - kudos to you all!  Podcasts come and go but you have kept me listening for years now.

Thank you and warm regards,

What do you think? Paul Jen, Tue, 15th Apr 2014

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