How much CO2 could be removed by planting trees?

Credit: NASA

This article in the guardian points to some recent research.  The article begins:

Planting billions of trees across the world is one of the biggest and cheapest ways of taking CO2 out of the atmosphere to tackle the climate crisis, according to scientists, who have made the first calculation of how many more trees could be planted without encroaching on crop land or urban areas.

As trees grow, they absorb and store the carbon dioxide emissions that are driving global heating. New research estimates that a worldwide planting programme could remove two-thirds of all the emissions from human activities that remain in the atmosphere today, a figure the scientists describe as “mind-blowing”.

and goes on to quote the lead scientist from ETH Zürich saying

"This new quantitative evaluation shows [forest] restoration isn’t just one of our climate change solutions, it is overwhelmingly the top one"

Let's test this claim with two tools everyone has at their disposal: Google and arithmetic.

• Global CO2 emissions in 2019 were ~37 GTon according to carbonbrief
• That's $37 \times \frac{14}{14 + 2\times16} \approx 11$ GTon of carbon atoms
  
• The total mass of carbon atoms in all plants is ~450GTon according to vox
• Therefore we add the equivalent of all the carbon in all plants to our atmosphere every $\frac{450}{11} \approx 41$ years
• The article says the study found that 11% of all land is available for planting and could host 1.2 trillion saplings
• According to the BBC there are currently 3 trillion trees on Earth
• So 1.2 trillion new trees could eventually absorb $\frac{1.2}{3}\times 41 \approx 16$ years worth of human emissions

This tells us that if we use all available space to plant trees, plant only super-fast growing trees which get to maturity in 16 years (ignoring the ecological consequences thereof), and despite previous experience, all planted saplings survive, then all we will have achieved is to create a temporary plateau in the rising atmospheric CO2 levels.

Even with these incredibly optimistic assumptions, the tree planting programme cannot possibly "remove two-thirds of all the emissions from human activities that remain in the atmosphere today".  In fact all it could do is delay the necessity to zero our emissions by a few years.

Why is it important we challenge stories like this whenever they appear in the media - using a bit of Google and a bit of mental arithmetic?  I think it's important because all of these stories - whether they're about tree planting or CCS - send us the same message: it's okay, the boffins are working on a solution.  And specifically, that solution is something - anything - other than the one we know would work, but really don't want to think about, namely rapidly zero-ing our emissions.

Let's not be fooled by any more conjuring tricks.

Addendum

How does this calculation change if you take into account soil carbon?

Trees create leaf litter which turns into soil, some of which doesn't decompose and is therefore locked away.  So, unlike with the tree mass there is additional drawdown from the atmosphere year on year which the above calculation ignores.  Does it change things if we include it?  The short answer is, a bit, but not much.

According to Carbon Brief a new analysis

"finds that, if techniques to improve soil carbon were rolled out at the maximum assumed level worldwide, they could remove up to 5.5bn tonnes of CO2e a year."

Of course, the same caveats about the scale and feasibility of this project apply as do to the project to add the maximum number of trees possible.  However, that 5.5 GTonCO2 per year is only 10% of current emissions.  So the conclusion is that even maximum tree planting and maximum soil improvement would only buy a few years, before requiring us to reduce by 90% our current level of emissions.  This is not to say that tree planting or soil improvements are a bad idea - they're great ideas - only that anyone trying to sell this as the solution without mentioning the need to drastically cut  almost all of our emissions is hiding the majority of the story.