Trees are an unquestionably important part of our environment, and their ability to convert carbon dioxide into oxygen means they could play a key role in offsetting the worst effects of climate change. But just how effective would they be? A new study from ETH Zurich has quantified that the Earth has room to reforest an area the size of the US, and calculates what benefits that might bring. But of course, the story is more complicated than just planting a trillion new trees.

Not all of the carbon dioxide released into the atmosphere stays up in the air – huge amounts of the gas are absorbed by natural "carbon sinks," such as the oceans and forests of the world. These could help reduce the greenhouse effects of atmospheric CO2, but sadly their functions can't keep up with our current output.

Planting more trees seems like a straightforward strategy. The researchers on the new study, hailing from the Crowther Lab at ETH Zurich, calculated just how much space we have for new tree coverage, and how much more carbon that could potentially store if planted.

This chart shows the land available for reforestation, excluding deserts, agricultural and urban areas, as well as current forestland

Under the current climate conditions, the team calculated that Earth's land area could support 4.4 billion hectares of continuous tree cover. At the moment we have 2.8 billion hectares of trees. But we can't just fill the remaining 1.6 billion hectares with trees, of course – humans are using a substantial amount of that land.

"One aspect was of particular importance to us as we did the calculations: we excluded cities or agricultural areas from the total restoration potential as these areas are needed for human life," says Jean-François Bastin, lead author of the study.

With those spaces removed from the equation, the researchers arrived at a total of 0.9 billion hectares – or an area about the size of the United States – that's ripe for reforestation. Once these new forests have matured, the team calculated that they would be able to store about 205 billion tonnes of carbon. That's a fair chunk of the estimated 380 billion tonnes that humans have produced since 1901.

"We all knew that restoring forests could play a part in tackling climate change, but we didn't really know how big the impact would be," says Thomas Crowther, co-author of the paper. "Our study shows clearly that forest restoration is the best climate change solution available today. But we must act quickly, as new forests will take decades to mature and achieve their full potential as a source of natural carbon storage."

This chart shows the total land available that can support trees, including what's currently covered with trees and what could be reforested in future

The team also investigated where these forests are best suited, and found that six countries are home to the biggest available spaces for reforestation. Unsurprisingly Russia is number one, at 151 million hectares, followed by the US on 103 million, Canada on 78.4 million, Australia at 58 million, Brazil at 49.7 million and finally China on 40.2 million hectares.

But if the study sounds too good to be true, that might just be the case.

Seeing the forest for the trees

Trees have a complicated relationship with the climate, and other scientists point out that the new research misses some key factors, instead painting a simplistic and overly-optimistic view of the tree planting plan.

"The estimate that 900 million hectares restoration can store an addition 205 billion tones of carbon is too high and not supported by either previous studies or climate model," says Simon Lewis, a Professor of Global Change Science at UCL. "The authors forgot to subtract the carbon already on the land and in the soil that was there before any restoration happens. Plus the biome specific carbon storage estimates are too high as they are the end points of hundreds or years of succession, not a couple of decades of forest growth."

There are also questions about how effective forests are as carbon sinks in the first place. Another recent study found that a warming world is reducing the long-term carbon storage potential of trees. More CO2 in the air makes trees grow faster, which is a positive, but they also tend to die younger, releasing stored carbon back into the atmosphere sooner.

Trees also aren't completely innocent when it comes to their own emissions. Besides life-giving oxygen, the plants have been found to emit volatile organic compounds and even methane, which work to warm the planet.

And finally, different types of trees, growing in different environments, have different effects on the climate. A key contributor to whether the planet is warming or cooling is Earth's albedo – essentially, how reflective the surface is. More reflective surfaces, like snow, bounce more sunlight back out into space, but covering that with trees keeps more of the warmth close to the ground. That means trees planted in snowy regions – such as the 151 million hectares the new study suggests for Russia – might be less effective trees planted in places like the Amazon.

Of course, nobody is lobbying for less trees, and it's clear that reforestation will play an important role in managing our changing climate. But with the climate being such an intricate system, it's not a straightforward equation.

"The median estimate from the IPCC 1.5° C report scenarios to meet the 1.5° C target is 57 billion tones of carbon sequestered by new forests this century, which is certainly possible, if these new forests are adequately protected into the long-term," says Lewis.

The research was published in the journal Science.

Source: ETH Zurich

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