A recent study published by researchers at the Federal Institute of Technology in Zurich suggests that planting at least one trillion trees around the world could capture two-thirds of the carbon pumped into the atmosphere due to human activity.


We speak to Bob Ward, the policy and communications director for the Grantham Research Institute on Climate Change and the Environment.

Why are trees so vital for the Earth’s atmosphere?

They’re vital because trees, like all plants, absorb carbon dioxide from the atmosphere and produce oxygen. So, given that the problem that we have is the growth in the concentration of carbon dioxide and other greenhouse gases in the atmosphere, trees are a vital part of it.

The loss of trees that we’ve experienced over the past few decades and centuries has been a major contributor to creating the problem we have. So planting new vegetation and new trees is an essential part of the solution, and we certainly have to stop destroying forest areas. The main message is that planting forests is an important contribution to effort against global warming – but we have to be careful not to exaggerate what we can do, and it’s certainly not a ‘get out of jail free’ card. We will still have to stop emitting carbon dioxide. We can’t just continue as we are and plant lots of trees to make up for it – that’s not going to work.

What sort of impact has human activity had on carbon levels?

The concentration of carbon dioxide, which is the main greenhouse gas, before industrialisation started was around 280 parts per million. It is now over 410 parts per million.

How do we calculate that figure?

Well, the main way is that you can measure the isotopes in carbon dioxide, and you can see that the increase is due to the presence of carbon dioxide with carbon isotopes in it that can only have occurred from the burning of fossil fuels. Naturally occurring carbon dioxide and the carbon dioxide you produce from burning fossil fuel have different isotopes, so that’s how we know.

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And indeed, if you look at where all this extra carbon dioxide could have come from, the only way you would be able to produce it is by burning fossil fuels anyway. There’s no other major source. Some people have suggested that you also get carbon dioxide from volcanoes, but we know that just doesn’t happen at a scale that could explain the huge increase in carbon dioxide levels that we’ve seen in the atmosphere.

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And we measure the pre-industrial carbon dioxide levels from analysing the bubbles in ice cores. Where ice forms in the major ice caps in Antarctica and Greenland, you can drill down and, essentially, you can calculate when each level of ice formed. When ice forms, it traps little bubbles of air, the air at that time in it. So if you go back in now and extract these ice cores, you can then analyse the air in these bubbles and calculate what the concentration of what each gas is, and that’s how we know it used to be about 280 parts per million.

In order to calculate the areas of land masses that could be used to plant trees, the Zurich study makes an estimate of how much carbon dioxide a given tree can absorb, and multiplies them. That seems a bit simplistic?

Yes, they’ve over-simplified the calculation and ended up with an inaccurate estimate because they’ve assumed that if you started planting more trees, then that would simply remove carbon dioxide from the atmosphere.

But we know that carbon dioxide is also absorbed by seawater and soils, and that the concentrations are in equilibrium with each other. So when we emit carbon dioxide by the burning of fossil fuels, some of it goes into the atmosphere, some goes into the ocean, and some goes into the land.

Equally, if you start extracting it from the atmosphere, you will get a flow of some carbon dioxide coming out of the oceans and the land into the atmosphere. Not all of the carbon dioxide you remove from the atmosphere will result in a straightforward reduction of concentrations.

So with that estimate you could essentially compensate for two-thirds of the carbon dioxide that’s already been emitted, they only counted the carbon dioxide that had gone into the atmosphere – they either forgot or overlooked the carbon dioxide that’s been emitted and gone into the oceans and the land, and that would go back into the atmosphere if you started removing it.

How about the areas of land that they have identified?

Large parts of the areas that they’re suggesting could be forested probably aren’t suitable for growing forests in, often because soils are not right. There are also large areas in Northern Canada and Siberia where if you plant trees you end up with, ironically, with an impact that might make climate change worse, because one of the other things that’s causing the Earth to warm is something called albedo.

What happens is that when you have white surfaces, like ice, a lot of sunlight is reflected back out into the atmosphere, preventing warming. Dark surfaces tend to absorb more of the sunlight and cause more warming. And so, if you start planting trees, they’re relatively dark compared to the ice that’s there now, and you might end up causing a bit more of a warming effect by doing it, which would not be helpful.


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Jason Goodyer
Jason GoodyerCommissioning editor, BBC Science Focus

Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Instant Genius Podcast.