Increasingly, companies are releasing new net-zero goals to neutralize their greenhouse gas emissions in the future. And instead of substantially reducing their carbon emissions, a lot of those pledges rely on using a technique called forest carbon capture.

But a new study reinforces what scientists have been saying for a while: counting on trees to do the dirty work of removing the CO2 we have put on the atmosphere won’t be enough to stop climate change. 

A paper published today in Science posits that, if we stop all human management on forests (for example, wood harvesting) under current climatic conditions and with the CO2 concentration that already exists, their aboveground biomass could increase by up to 44.1 gigatons of carbon. 

If that seems like a lot, it’s because it is. However, it would mean a 15–16 percent increase over current levels of carbon storage, which equals about four years of current CO2 emissions by human activities. 

Without strong reductions in emissions, the paper concludes that this strategy has a low potential to reduce the amount of carbon in the atmosphere. The researchers also highlight that the forest carbon sink (its ability to absorb carbon) should be preserved to offset residual emissions from sectors where they are unavoidable, rather than to compensate for present emission levels.

Forests act as carbon sinks because, as trees grow, they use CO2 in the photosynthesis process and turn it into plant matter, or biomass. They also store carbon on the ground by enhancing the soil’s organic matter. That is why forests play a key role in preventing the planet from overheating.

In places where logging or other issues have damaged the forest, restoring the area could enhance the amount of carbon absorbed there. This kind of “repairing” is what is being sold as one of the solutions to suck CO2 from the atmosphere — while giving a free pass for companies to keep their “business as usual” emission levels. 

But there is a limit to what forests can actually do.

“We focused more critically on naturally occurring disturbances that happen in forests without any human intervention, such as fires, storm related damage, insects etc. This led to an estimated additional carbon storage potential that is more than twice as small as the previous estimates,” says Caspar Roebroek, the lead author of the study and a researcher at ETH Zurich and the European Commission Joint Research Centre. 

They were also able to estimate that an area of 1.6 million kilometers squared (almost 618 million square miles) would need to be reforested to compensate for a single year of our current carbon emission levels. This is roughly the size of Alaska and larger than Spain, Germany, and France combined. 

Besides, it takes time for forests to grow to maturity, from decades to centuries. Meanwhile, the climate crisis is already here, and humanity needs to act fast to limit global warming to under 1.5 degrees Celsius in order to meet the goals of the Paris agreement.

Mathematician Thelma Krug, one of the vice presidents of the United Nations’ Intergovernmental Panel on Climate Change (IPCC), the world’s most important forum on climate, thinks that the study delivers an important message. “It shows that forests, alone, won’t save the planet.”

She explains that all the models the IPCC works with now show that carbon dioxide removal will be necessary to reach net-zero emissions in particular sectors. However, the models show that many different strategies will be needed to do it fast enough — the most significant being ambitious emission cuts. 

Krug says that companies that are investing in forest carbon capture may not realize two important things: that this approach takes time and that forests are also vulnerable to climate change, so their own carbon sink capacity may be hurt by Earth’s higher temperatures. 

“Climate change is happening very fast and things tend to get worse really quickly. At the same time, companies are not taking action where it’s most needed, which is the cut of fossil emissions.”

Although it demonstrates their limits to carbon absorption, the new study in no way diminishes the environmental and climate value of protecting forests. 

“From a climate change perspective, the more carbon we can keep on land the better,” says Roebroek, as deforestation and forest fires are major sources of greenhouse gas emissions.  

“At current emission levels, forests can only contribute to a limited extent to climate change mitigation, but if we collectively reduce substantially the emissions of CO2 from fossil sources, the relative contribution of forests might become very important to close the gap to carbon neutrality,” he points out.

In addition to that, forests have other roles that go way beyond the carbon cycle. Among other services, they modulate temperature changes, help to prevent the spread of diseases, and are home to millions of species and thousands of traditional communities around the world.