Did we overestimate the benefits of planting trees to fight climate change? A new study says yes

For decades, planting trees has been hailed as the gold standard in the fight against climate change. Corporates, burdened by social responsibility, often paint a picture of their leafy arms reaching skyward to capture and store the ever-growing and problematic carbon dioxide. However, are they really that effective at curbing the ongoing climate crisis?

While planting trees certainly helps, there are a ton of usually uncounted things that prevent it from being the magic bullet we believe it to be. For instance, the darker foliage of certain trees, while adept at capturing carbon, also absorbs sunlight that would otherwise be reflected back into space, creating a warming effect. This is especially true when trees replace snow and ice, like in high-altitude and extreme-latitude regions.

But it doesn't stop there. You might be surprised to know that even trees emit volatile organic compounds (VOCs), the cancer-causing stuff from paints and printers that we’ve become increasingly terrified of. These interact with atmospheric chemicals, extending the life of the potent greenhouse gas methane, or team up with nitrogen oxides to form ozone, another heat-trapper. However, they also have a cooling side, forming aerosols that reflect sunlight into space.

Taking all these things into account, a new study has revealed that the climate benefits of forests might actually be 15-30% lower than earlier estimates!

To understand the full picture, the researchers built a climate model simulating a world completely blanketed by trees, except in built-up areas or farmland. They then ran two scenarios: one with limited climate action besides planting, and another with stronger efforts to reduce warming.

Interestingly, the results differed. In the limited action scenario, the warming forests helped in avoiding a ton of warming, but it was still 23-31% lower than what is typically expected, due to the added influence of all the less-accounted-for effects discussed earlier. But in the scenario with stronger climate action, the reduction was only 14-18%.

The researchers attribute this difference partly to the reduction of air pollution aerosols from cutting fossil fuels. Aerosols are horrible for air pollution, but the large amounts of them in our air also end up inadvertently masking the cooling effect of additional forest-made aerosols in a polluted world. However, in a world where human-made aerosols are sparse, the impact of tree-made ones becomes more profound.

However, the research also has its limitations. The team explains that it doesn't capture all the intricate feedback loops, like how ozone affects the carbon capture potential of vegetation or the impact of carbon-spewing wildfires. Capturing every nuance in a single study is inherently challenging.

The main takeaway is probably that protecting existing forests is far more efficient for climate mitigation than planting new ones. Additionally, the cooling effect of water vapour evaporating from leaves, especially in tropical regions, is not included in the model — but the researchers believe it can be significant. Forests also provide a treasure trove of benefits beyond climate change, like being havens for countless species, regulating water cycles, preventing soil erosion, and supporting communities with food, jobs, and other resources.

So, while planting trees remains an essential part of the climate solution, it's crucial to do so strategically and in conjunction with other efforts like reducing fossil fuel emissions and protecting existing forests.

The findings of this research have been published in Science and can be accessed here.