As trees grow, they take in carbon from the air and store it in wood, plant matter, and in the soil, making them what scientists call “carbon sinks.” In this way, forests play an important role in the global carbon cycle by soaking up lots of carbon dioxide that would otherwise live in the atmosphere. Could we plant enough trees to absorb the amount of CO2 that Americans create and, in theory, cancel out our carbon emissions?
It’s tempting to think that a back-of-the-envelope calculation can deliver a ballpark answer. For example, studies have estimated an average American’s carbon footprint at around 16 tons of CO2 annually, one of the highest figures for any country because of the energy-intensive American lifestyle. A single mature tree, meanwhile, may take in about 50 pounds of carbon dioxide per year (1). At this rate, it would take 640 trees per person to account for all American emissions, which adds up to more than 200 billion trees. (A recent study estimated there are about 3 trillion trees on Earth right now(2)). So that’s the answer, right?
The short answer is, no. In reality, the carbon math is much messier. As part of the planet’s natural carbon cycle, carbon sinks such as the forests and oceans absorb an enormous amount of naturally emitted CO2 as well as much of what humans create. Humanity’s emissions have tipped that natural cycle out of balance. And the enormous complexity of this system makes it perhaps impossible to say for sure how many new trees would be required to bring it back into balance.
MIT professor of civil and environmental engineering Charles Harvey explains that although it is a good idea for the world to plant many more trees, the truth is much more complicated than assuming more trees can cancel out our emissions. One key question is: How much treeless land is available for planting? Dense forests once covered the American Midwest, for example. They could grow there again, but much of that treeless land is now used for cities, agriculture, and industry.
Even if the United States could find the space to plant about 200 billion new trees, Harvey notes, it would not reap the climate benefits immediately. The faster trees are growing, the more carbon they can suck up, which means new growth is not as valuable as a carbon sink as are longstanding forests. There’s another problem, too: Trees don’t last forever. When they die and decay, burn in a wildfire, or are chopped down and burned for fuel, trees release all the CO2 they’ve been hiding away. (There are exceptions to this rule: Harvey studies peat forests in places like the peat swamp forests of Borneo, where biomass accumulates on the forest floor rather than fully decaying and releasing its CO2. But most forests cannot theoretically sequester carbon forever.)
It is also not clear that trees could continue to soak up CO2 indefinitely if humans continue our emissions unabated. Trees need not only carbon dioxide but also nutrients from the soil like nitrogen and phosphorus to grow. Research by César Terrer, MIT assistant professor of civil and environmental engineering, has shown that trees will need much more of both nutrients to balance their diets as the CO2 concentration in the atmosphere increases. If the soil does not have enough, that could curtail how much new CO2 a tree is able to store.
For all these reasons, Harvey says, a society could get more bang for its buck by focusing on preserving existing forests rather than prioritizing new growth as a way to offset emissions.
“Planting trees where they aren't is often a good idea, and that will take up CO2,” Harvey says. “But a much more efficient thing to do, to have a larger effect for the same effort, is to stop cutting down trees. It's almost silly to think about [planting a huge number of new trees] while we're just burning and destroying them everywhere, releasing carbon at rates that are much higher than what new growth would take up.”
Thank you to Robert August Dow of Columbus, Georgia, for the question. You can submit your own question to Ask MIT Climate here.
Published June 16, 2022.
1 European Environment Agency: Trees help tackle climate change. Accessed June 16, 2022.
2 Crowther, T., Glick, H., Covey, K. et al. "Mapping tree density at a global scale." Nature, vol. 525, 2015. doi:10.1038/nature14967