Does planting trees actually cool the planet? - Carolyn Beans

In the fight to curb climate change, there are few solutions more discussed than planting lots and lots of trees. It sounds simple enough— trees absorb CO2 from the atmosphere to grow, so planting more of them should help eliminate greenhouse gases. The trouble is, tree planting efforts don't always work as planned. For example, between 1974 and 2012, the Chilean government helped fund the planting of over a million hectares of new trees.

Yet a recent analysis suggests this multi-million dollar effort resulted in no major carbon storage gains. What went wrong? Chile focused on afforestation: the planting of trees in places long devoid of— or never home to— any forest, as opposed to reforestation, the practice of restoring recently degraded forests. Many governments and international organizations champion afforestation in their efforts to meet lofty tree planting goals that require massive amounts of land.

The Bonn Challenge, launched in 2011 by the German government and the International Union for Conservation of Nature, aims to restore 350 million hectares of degraded land across the globe by 2030, in part through afforestation. Many companies have also seized on tree planting to offset emissions and, in some cases, negative PR from their contributions to the climate emergency. A 2021 Oxfam analysis found that the area needed to match the tree planting goals set by four of the largest oil and gas producers would require land twice the size of the UK.

Here's where things get complicated. In order to be a long-term carbon sink, trees need to grow to maturity and stay put. Most of Chile's afforestation funding went to the commercial forestry industry, which drastically expanded plantations of mostly non-native trees— in some cases even by plowing into native forests. According to one study, on average, natural forests are 40 times better at carbon storage than plantations. A mature tree can absorb up to 22 kilograms of carbon dioxide each year.

But plantation trees are harvested. Further, pine and eucalyptus— two commonly grown plantation trees— are highly flammable, so gains in carbon storage can quickly go up in smoke. And not all land should be forested. In 2019, researchers estimated that, of the 100 million hectares of land targeted for a tree planting initiative in Africa, most is savanna. Dropping trees into Africa’s savannas threatens wildlife that thrive in sunlight and open spaces.

And wildfires, not to mention a passing elephant, can quickly stomp out years of tree growth. Meanwhile, a savanna’s natural vegetation tucks most of its carbon safely away below ground, where it’s protected from fire and hungry herbivores. Planting trees can also have unintended consequences in places that naturally reflect sunlight like drylands or snowy terrain. Trees in these regions could absorb more of the Sun’s rays, contributing to a warmer planet.

It’s not that we shouldn’t plant more trees. But for the best chance of success, programs should consider which species to plant, which lands to forest, and how to protect the land long-term. Today, Chile is prioritizing planting native trees rather than timber plantations. Some researchers argue that a more efficient way to re-green the planet is to protect forests and let nature do the work.

On recently deforested land, seeds wait in soil and new sprouts grow from old stumps. As time passes, birds and winds deliver seeds from forests nearby. Others support programs that practice assisted natural regeneration— helping nature along by removing grasses that compete with saplings, preventing grazing, and even planting trees in small patches.

So when is it bad to plant trees? When programs put the wrong trees in the wrong places. It’s bad when it’s mistakenly used as a catch-all solution, rather than addressing more complex issues like carbon emissions and active deforestation. It’s also bad to plant trees when it allows companies and governments to practice greenwashing, throwing money at initiatives that have no real chance of achieving the carbon offsets they promise.

The best investments in a greener future are to cut carbon emissions, while protecting these forests from being destroyed in the first place.