What if there were 1 trillion more trees? - Jean-François Bastin

Standing at almost 84 meters tall, this is the largest known living tree on the planet. Nicknamed General Sherman, this giant sequoia has sequestered roughly 1,400 tons of atmospheric carbon over its estimated 2,500 years on earth. Very few trees can compete with this carbon impact, but today, humanity produces more than 1,400 tons of carbon every minute. To combat climate change, we need to steeply reduce fossil fuel emissions and draw down excess CO2 to restore our atmosphere’s balance of greenhouse gases.

But what can trees do to help in this fight? And how do they sequester carbon in the first place? Like all plants, trees consume atmospheric carbon through a chemical reaction called photosynthesis. This process uses energy from sunlight to convert water and carbon dioxide into oxygen and energy-storing carbohydrates. Plants then consume these carbohydrates in a reverse process called respiration, converting them to energy and releasing carbon back into the atmosphere.

In trees, however, a large portion of that carbon isn’t released and instead is stored as newly formed wood tissue. During their lifetimes, trees act as carbon vaults, and they continue to draw down carbon for as long as they grow. However, when a tree dies and decays, some of its carbon will be released back into the air. A significant amount of CO2 is stored in the soil, where it can remain for thousands of years. But eventually, that carbon also seeps back into the atmosphere.

So if trees are going to help fight a long-term problem like climate change, they need to survive to sequester their carbon for the longest period possible while also reproducing quickly. Is there one type of tree we could plant that meets these criteria? Some fast growing, long-lived, super sequestering species we could scatter worldwide? Not that we know of. But even if such a tree existed, it wouldn’t be a good long-term solution.

Forests are complex networks of living organisms, and there’s no one species that can thrive in every ecosystem. The most sustainable trees to plant are always native ones; species that already play a role in their local environment. Preliminary research shows that ecosystems with a naturally occurring diversity of trees have less competition for resources and better resist climate change. This means we can’t just plant trees to draw down carbon; we need to restore depleted ecosystems.

There are numerous regions that have been clear cut or developed that are ripe for restoring. In 2019, a study led by Zurich’s Crowtherlab analyzed satellite imagery of the world’s existing tree cover. By combining it with climate and soil data and excluding areas necessary for human use, they determined Earth could support nearly one billion hectares of additional forest. That’s roughly 1.2 trillion trees.

This staggering number surprised the scientific community, prompting additional research. Scientists now cite a more conservative but still remarkable figure. By their revised estimates, these restored ecosystems could capture anywhere from 100 to 200 billion tons of carbon, accounting for over one-sixth of humanity’s carbon emissions. More than half of the potential forest canopy for new restoration efforts can be found in just six countries. And the study can also provide insight into existing restoration projects, like The Bonn Challenge, which aims to restore 350 million hectares of forest by 2030.

But this is where it gets complicated. Ecosystems are incredibly complex, and it’s unclear whether they’re best restored by human intervention. It’s possible the right thing to do for certain areas is to simply leave them alone. Additionally, some researchers worry that restoring forests on this scale may have unintended consequences, like producing natural bio-chemicals at a pace that could actually accelerate climate change. And even if we succeed in restoring these areas, future generations would need to protect them from the natural and economic forces that previously depleted them.

Taken together, these challenges have damaged confidence in restoration projects worldwide. And the complexity of rebuilding ecosystems demonstrates how important it is to protect our existing forests. But hopefully, restoring some of these depleted regions will give us the data and conviction necessary to combat climate change on a larger scale. If we get it right, maybe these modern trees will have time to grow into carbon carrying titans.