April 12, 2020

Will trees save the world?

Everyone seems to agree that trees are a major solution to climate change, but they can’t be the only fix, says Adam Vaughan

Tree planting doesn’t usually feature in US presidents’ speeches, UK general election battles or the business pitches of oil companies. Yet in the past year, pledges to embark on reforestation efforts have become a popular way to show you are committed to fighting climate change. There are several initiatives to plant or protect a trillion trees,to add to the 3 trillion we have today. So how did we get here, with humble tree planting taking centre stage among the tools to stave off extreme warming? Can we really plant the numbers needed to lock up enough carbon to make a difference? Perhaps most importantly, is all this talk of trees just a big distraction?

“Suddenly, this last year there’s been an explosion of interest,” says Fred Stolle at Global Forest Watch, a US initiative from the University of Maryland and other groups. Rising public concerns seem to be making governments and corporations realise they need to do more on climate action, or at least be seen to do more.

“I think there’s massive concern about climate change now and people genuinely want to do something about it. I think they are reaching for what are easy solutions,” says Joanna House, a lead author of a UN Intergovernmental Panel on Climate Change (IPCC) report on land use published last year. Planting trees is popular, usually uncontroversial and brings benefits beyond storing carbon, from our mental well-being to habitats for wildlife. “People love trees,” says House.

The spotlight on tree planting may have its roots in the 2015 Paris Agreement, in which governments committed to try to hold global temperature rises to 1.5°C, rather than the 2°C many had expected.

This led to a 2018 IPCC report, which made it clear that to hit 1.5°C, global greenhouse gas emissions need to fall to net zero by 2050. There was much debate about “negative emissions” technology, such as machines to capture carbon dioxide from the air. But with these in their infancy, the focus fell on trees as the only proven option.

“I think a lot of the talk around the new ambition for 1.5°C was one of the biggest driving forces for putting negative emissions – and particularly nature-based negative emissions – on the stage,” says Stephanie Roe at the University of Virginia.

But tree mania accelerated last year, when Tom Crowther at ETH Zurich in Switzerland and his colleagues published a paper that said Earth has room for nearly a billion hectares of extra trees, which could lock up several years’ worth of humanity’s carbon emissions. The research has been criticised as an overestimate, but was influential and made global headlines. “I think that played a key role in relegitimising reforestation,” says Mark Hirons at the University of Oxford.

A few months later, political parties campaigning in the run up to the UK general election competed on who promised to plant the most trees. Last month marked peak tree planting fever, when the World Economic Forum launched 1t.org, a plan to plant a trillion trees (other plans launched three years ago). Even US president Donald Trump,who has withdrawn the US from the Paris agreement, backed the initiative.

But even if we start planting vast areas tomorrow, can trees store enough carbon to buy us time to act on climate change?

The Crowther paper said 0.9 billion hectares could lock up 205 gigatonnes of CO2. Including land-use change, such as forests being cleared for farming, humanity’s annual emissions are about 41 gigatonnes. But House says many researchers were shocked by the paper. “It’s quite harmful because it makes it seem like trees can do more than they can,” she says.

Several experts took to scientific journals to explain why they felt it exaggerated the amount of usable land and how much carbon could be stored. In response, Crowther says a lot of the criticism is well-founded, but it is important to get a global perspective on what it is possible, in order to set meaningful restoration targets.

Based on Roe’s review of the literature, reforestation has the potential to lock up between 1 and 10 gigatonnes of CO2 a year. “In terms of what is feasible, we came to 3 to 4 gigatonnes [a year],” she says.

More research is underway on calculating the carbon storage potential of tree planting.In the meantime, it seems large enough to be attracting big business. Last year, Shell announced that it would spend $300 million over three years on reforestation projects to generate carbon credits for itself and others.

On Crowther’s analysis, Duncan van Bergenat at Shell says: “Even those people who have challenged it, have not challenged the fact that it is really, really big. It’s on the margins between really big and huge.”He says the numbers presented “resonated” with Shell’s own researchers.

Such interest in reforestation from oil companies has set alarm bells ringing in some quarters. “Fossil-fuel industries can say they’re harnessing nature to address their emissions, which is dubious I think, in terms of the scientific case for this significantly having an impact on climate change,” says Hirons.

There is a risk that we plant trillions of trees without firms and countries also deeply cutting their emissions. Shell says that isn’t the case. “We are definitely not doing this instead of other tough things and changes we need to make. This very much comes on top,” says van Bergen.

Even if mass reforestation happens in parallel with decarbonisation of economies, Stolle warns that trees must be planted at the right place and time. As well as picking suitable species for the climate and the soil where they are planted, it will be crucial to plant trees that help rather than hinder biodiversity.

Biodiversity warning

Take the UK, where the government’s climate advisers have called for a tripling of tree planting to hit carbon goals. Jane Memmott at the British Ecological Society says there are huge differences in biodiversity levels between trees you might pick for the UK. “Something like oak and birch is fantastic – there are literally hundreds of species associated with them, whereas something like sycamore has pretty much a single aphid on it,” she says.

Then there are the people who live in and around the places where reforestation might take place, often in developing countries. Restored forests won’t thrive or remain intact long enough to lock up CO2 for centuries if local people aren’t invested in them, says Stolle.

“In developing countries, people very much understand the value of trees,”he says, but only when they play a role in deciding when and where they are planted.

Hirons fears that the urgency of tackling climate change could see the wishes of local communities being ignored. “I think there’s a massive risk of social harm being caused by widespread reforestation. There is an idea that there is lots of underused land, which is a myth.”

While there are international guidelines on how best to do reforestation, set by the Society for Ecological Restoration, there is no requirement to follow them.

Lastly, if the CO2 locked away is to be counted properly, we will need to monitor reforestation for a long time.That is surprisingly tricky. Deforestation is easy to spot – satellites show areas turning from green to brown. But they find it hard to detect new trees, which for the first few years will be tiny saplings hard to discern from space. Higher resolution images may help.

Perhaps the biggest thing missing from today’s focus on reforestation is the great number of trees being lost to deforestation, which is getting worse. The world lost forests the size of the UK every year between 2014 and 2018. Deforestation in the Amazon rainforest has spiralled to the highest level in a decade. Recent bush fires in Australia burned 64,000 square kilometres in Victoria and New South Wales, most of it forests.

“It’s an eternal debate,” says Stolle. “Is [reforestation] a distraction because we really need to stop deforestation? On the other hand, if you look at the IPCC, we need those negative emissions. We can’t wait until we’ve done one before we do the other.”

The New Scientist, 29 February 2020, p.21