Planting too many trees could spoil the planet

As the world faces the growing threat of climate change, finding ways to reduce greenhouse gases like carbon dioxide (CO2) is more important than ever. Planting trees across large areas, known as forestation, has been suggested as a solution.

While it seems simple, the impact of widespread forestation on our planet is more complicated than we might think.

A recent study from the University of Sheffield has explored this complexity in detail. The researchers used advanced models to examine how adding large amounts of trees to Earth would affect the climate. While forestation is known to absorb CO2 and cool the planet, the study reveals other, less obvious consequences.

Potential impact of large-scale forestation

The experts used two advanced climate models to simulate how the planet’s climate and weather would change if there were a lot more trees in suitable areas around the world.

A key part of their study was creating a scenario where forests covered much more land than they do now. This scenario helped them understand the biggest possible impact that forests could have on climate.

The researchers looked at both the well-known benefits of forests, like storing carbon dioxide, and lesser-known effects, like how forests influence the chemicals in the air and how much sunlight Earth reflects back into space.

They found that these chemical and reflective effects are linked: forests release chemicals that affect clouds and the air, and the amount of sunlight Earth reflects depends on how much forest cover there is. This link is important because it can influence how much Earth warms or cools.

Effects on atmospheric composition

Planting more trees can have both cooling and warming effects on the climate. The good news: When forests grow, they release more biogenic volatile organic compounds (BVOCs). These compounds react with other chemicals in the air to form tiny particles called organic aerosols. 

The aerosols act like tiny mirrors, reflecting sunlight back into space and cooling the Earth. This cooling effect can help counteract the warming caused by other factors, like reduced reflectivity of the Earth’s surface. 

Professor Dominick Spracklen of the University of Leeds, who was not involved in the study, said: “This study highlights the amazingly complex role of forests in our climate system. Through calculating how forests alter atmospheric composition, this study provides one of the most comprehensive assessments of the climate impacts of large-scale forestation.”

Positive radiative forcing 

Planting trees can make the ground absorb more sunlight, and this is particularly the case in areas with a lot of snow. This is because forests are generally darker than snow and reflect less sunlight back into space.

The absorption of sunlight warms the planet. This effect is called positive radiative forcing. The effect is stronger in places farther north where there’s more contrast between the dark forests and reflective snow.

The study suggests that this warming effect can reduce the benefit of forests storing carbon dioxide, which helps cool the planet. This is especially true if we plant a lot of trees in high-latitude regions. Therefore, where we plant trees to fight climate change matters. 

Planting trees and proper planning

Dr. Stephanie Roe, WWF Global Climate and Energy Lead Scientist, and co-author of the study said: “We know that forests are critically important for biodiversity, water, ecosystem services, and the climate. What this research shows is that the effectiveness of reforestation for climate mitigation declines significantly in higher latitudes and unless paired with deep emission reductions which reduces air pollution. 

“It underscores the importance of properly planning reforestation efforts and adequately accounting for biophysical and future climate impacts in different latitudes and regions. Importantly, the study finds that preventing deforestation, when compared to reforestation efforts, is a far more efficient way to mitigate climate change.”

Excessive tree planting can cause problems

Planting more trees seems like a no-brainer for fighting climate change. But there’s a twist: forests can also release other gases that contribute to warming, like methane and ozone.

Methane

This gas comes from rotting plants and wet areas, both common in forests. It’s much more potent than carbon dioxide at trapping heat, so even small amounts matter. Planting trees in wetlands could increase methane emissions, potentially negating some of the carbon storage benefits.

Ozone 

This gas is a pollutant found in smog, not the good kind high up in the atmosphere. Trees emit chemicals that react with other pollutants to create ozone. While ozone protects us from harmful sun rays high up, it’s bad for us and the climate at ground level. More trees could mean more ozone, adding another wrinkle to the climate puzzle.

The warming effect of trees

The study examines the effectiveness of planting trees as a way to fight climate change by removing CO2 from the air. However, it finds that this benefit may be partially offset by other factors, like changes in how sunlight and heat interact with the Earth.

In a scenario where global temperatures rise significantly (4°C), up to a third of the cooling effect from planting trees could be canceled out by changes in the atmosphere.

On the contrary, if we limit warming to 2°C (like the Paris Agreement aims for), the negative effects are smaller, and planting trees is more effective.

So, what does this mean? Planting trees definitely helps, but it’s not a magic bullet. We need to consider all their effects, good and bad, when planning big planting projects. Balancing their carbon-storing power with other factors is key to effectively tackling climate change.

Comparing tree planting strategies

The researchers also compared the different tree-planting strategies (planting new forests, restoring old ones, and thickening existing ones) to see how well they remove carbon dioxide from the air and fight climate change.

Planting new forests on land that was not previously forested (afforestation) seems to be the most effective strategy, followed by restoring old forests (reforestation) and making existing forests denser (enhancement). 

But planting too many trees can have unintended consequences, like making the land darker and absorbing less sunlight (warming it up) or releasing gases that affect the atmosphere in complex ways.

Broader implications 

The study emphasizes that planting trees isn’t a simple solution and needs to be done carefully. Dr James Weber, from the University of Sheffield’s School of Biosciences, said: “The public are bombarded with messages about climate change, and the suggestion that you can plant trees to offset your carbon emissions is widespread. Many businesses now offer to plant a tree with a purchase, and some countries plan to expand, conserve, and restore forests.” 

“Trees can help tackle climate change, but we need to be careful about relying on them. We need to evaluate forestation, and other climate change mitigation strategies, in detail. This will help identify limitations and unintended consequences so these can be minimized where possible.”

The study is published in the journal Science.

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