Planting almost a billion hectares of trees worldwide is the “biggest and cheapest tool” for tackling climate change, according to a new study. The researchers claimed that reforestation could remove 205 gigatons of carbon from the atmosphere — equivalent to about 20 years’ worth of the world’s current emissions. This has been criticized as an exaggeration. It could actually be dangerous.
Although the paper itself included no costings, the researchers suggested a best-case estimate of just $300 billion to plant trees on 0.9 billion hectares. That’s just 40 US cents per ton of carbon dioxide (CO₂) removed. For more detailed studies on the costs of carbon removal through reforestation put the figure closer to $20-50 per ton — and even this may be optimistic at such large scales.
Our research suggests that the promises implied in such studies could actually set back meaningful action on climate change. This is because of what we call “mitigation prevention” — promises of cheap and easy CO₂ removal in the future make it less likely that time and money will be invested in reducing emissions now.
Can trees stop climate change?
The answer of the question is more complicated than you might think. Trees use CO2 as part of the process of photosynthesis – with the carbon ending up in the branches, trunk and roots. But at the same time they rely on respiration, which releases some CO2. That’s why, over the years, people have described trees as “breathing” – inhaling and exhaling a flow of gases. And it turns out that understanding exactly how that flow works is so hard.
Prof Rob MacKenzie, of the University of Birmingham, is honest about the lack of knowledge. “There are lots of things we don’t know about the precise movement of carbon.”
We’re in a hi-tech outdoor laboratory that he is running in a forest in Staffordshire.
The materials are mounted on tree trunks and on the ground to measure every aspect of how the trees are functioning. Research so far has shown that every square metre draws in about 1,700g of CO2 every year – while also releasing up to 1,200g.
And as a forest gets older, those flows are likely to become more balanced. Prof MacKenzie says it would be a “disaster” if governments and companies rely on forests to “clear up the mess” of carbon pollution.
And he paints a terrible picture of what could go wrong. “We plant lot of trees, we think we’ve done the job, we forget about them, and what we’re left with is a really desolate dying diseased landscape that no one cares about.”
So what is the solution?
Partly, they involve choosing the right trees and partly this will benefit the local people. In the sprawling forest of Thetford, in Norfolk, much of it planted in a rush after the First World War, Eleanor tew has researched the best options.
In the 1960s and 1970s, a government-encouraged rash of planting ended up with regimented rows of the same species of conifers – which meant they were susceptible to the same pests and diseases.
Without a plan, planting trees can end up with “more harm than good”. For Eleanor, it is important to ensure that future forests are more resilient. “It’s like making sure you don’t put all your eggs in one basket,” she says. “It’s may seem that the obvious thing is to plant one species that’s really good for timber or another species that’s good for carbon but if they don’t cope with a disease, then the whole forest fails.” For Nathalie Sidon, a professor of biodiversity at the University of Oxford, it is important that the forestry projects, especially those in developing countries, not be imposed on them. She points to a project in the humbo region of Ethiopia where farmers were encouraged to regenerate woodland by being given legal rights over the trees and also by getting training in forest management. By contrast, a forestry scheme in northwest China successfully protected people living there from dust storms – a positive development – but the growth of the trees then led to water shortages in villages downstream.
She says: “There is an idea that you can just buy land and plant it, but it is very simple – there is a risk of doing more harm than good.”
How many trees should be planted to save the planet?
The International Panel on Climate Change in its 2018 special report says that we need to plant around about 2.5 billion acres (or 1 billion hectares) of trees—in addition to cutting our emissions—to stop the planet from warming beyond 1.5 ºC. Several programs have sprouted up to commit countries to restoring their forests and hopefully upping their carbon-storing capacities.
Scientists studied satellite images of tree cover throughout natural preserves and considered how the soil and climate of a given area influenced tree growth. The survey reported that even if we avoid land that’s currently developed or used for agriculture, there’s still 0.9 billion hectares of area available for tree-planting—an area about the size of the United States. This includes degraded forest lands, logged areas, and grasslands. Half of these lands are found in six countries: Russia, the United States, Canada, Australia, Brazil, and China.