Environment

Earth has room to reforest an area the size of the United States, but it’s no silver bullet for climate change

Earth has room to reforest an ...
A new study has quantified how much land is available for reforestation, and how effective that would be for countering climate change – but there's much more to the story
A new study has quantified how much land is available for reforestation, and how effective that would be for countering climate change – but there's much more to the story
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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
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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
This chart shows the land available for reforestation, excluding deserts, agricultural and urban areas, as well as current forestland
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This chart shows the land available for reforestation, excluding deserts, agricultural and urban areas, as well as current forestland
A new study has quantified how much land is available for reforestation, and how effective that would be for countering climate change – but there's much more to the story
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A new study has quantified how much land is available for reforestation, and how effective that would be for countering climate change – but there's much more to the story

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

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
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

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

15 comments
windykites
I guess one solution is to cut energy consumption. Increased use of LED lighting would save a lot of energy. Carbon capture in power stations should be improved. Stop wasting heat energy with cooling towers!
Nik
As CO2 has no significant effect on climate, the CO2 absorption by trees is of no consequence, however, their cooling effect on the climate is. So reclamation of desert areas by planting trees would be entirely beneficial. In addition the land reclaimed would also be useful for crop growing, within the forested areas, by providing shade, and compost for food plants.
Nobody
This article is much friendlier to the scientific research about the planting of more trees than others I have read recently where the global warming scammers are totally offend by the idea that anything but carbon taxes and more government control is the ONLY answer.
fb36
IMHO, if all the forests humanity destroyed since the beginning of industrial age (fossil fuel usage), were still in place, there would not be Global Warming problem today, for sure! IMHO, restoring forests of Earth is the only realistic & practical solution for Global Warming! & definitely NOT extremely dangerous & costly geoengineering schemes suggested! & neither many new techs which are too far from any chance of practical large scale application! (Trying to reduce all kinds of fossil fuel usage also would help obviously but IMHO it is NOT the main solution!)
HighlanderJuan
Interesting. CO2 is not the reason for global climate change, and neither is Joe Sixpack If there is any abnormal global climate change going on, one does not have to look further than the military use of the atmosphere as a mechanism for creating and using weather as a weapon. Would planing more trees help? Of course. Would stopping the use of chemtrail sprays and HAARP weather controls help? Of course. As far as I know, plant nurseries still use CO2 generators to help their plants grow. That should tell us 1) CO2 is not a climate problem, and 2) that there may not be enough CO2 in the regular atmosphere. So, now that organized government, via the military, is able to control our environment, we should be asking why the government is using weather as a weapon, rather than creating nirvana for earth's animal and plant life.
Brooke
Planting trees is a short term patch, not ever a solution. Trees have a life time and then they die (I live in a forest) and the dead tree gives back all the carbon it captured while it was alive.
Douglas Bennett Rogers
It's a good thing they excluded the desserts! Irrigating the desert is the WORST thing you can do if you are worried about global warming. Increasing the path length water over the desert raises the morning low and the afternoon wet bulb around 10 deg. F. Evaporative cooling is also a major contributor. The desert is actually pretty cold, with morning lows and afternoon wet bulbs being around 50 deg. F in the summer. The thermal energy is radiated away to a cold (dry) sky. A world with few plants (an ice age) has a temperature of around freezing.
Nelson Hyde Chick
Of course they did not include the new farmland needed to accommodate the billions more we are going to ad to humanity this century.
Munoz-Nieves Jose
Of course its not just planting trees but creating an ecosystem. A forest is not just about the plant life but also the animal life. IMO the real diversity in that environment is the animal life. It's an environment so complex that is as complex above ground, and probably even more so below ground. The destruction of a forest is a tragedy that we cannot conceptualize.
Don Duncan
The growasis will grow a tree anywhere a tree grew naturally. It waters and shelters the sapling for as long as it needs.