Could a global tree-planting programme really save us from climate change?

Trees are our friends. Image: Getty.

Restoring the world’s forests on an unprecedented scale is “the best climate change solution available”, according to a new study. The researchers claim that covering 900m hectares of land – roughly the size of the continental US – with trees could store up to 205 billion tonnes of carbon, about two thirds of the carbon that humans have already put into the atmosphere.

While the best solution to climate change remains leaving fossil fuels in the ground, we will still need to suck carbon dioxide (CO₂) out of the atmosphere this century if we are to keep global warming below 1.5˚C. So the idea of reforesting much of the world isn’t as far-fetched as it sounds.

Since the dawn of agriculture, humans have cut down three trillion trees – about half the trees on Earth. Already 43 countries have pledged to restore 292m hectares of degraded land to forest worldwide. That’s an area ten times the size of the UK. But what the new study advocates is reforesting something like ten times that amount.

Trees absorb CO₂ from the air and store the carbon as bark and other tissue. Image: author provided.

Rewilding habitats and reforesting may be easier in the future as the world is already becoming a wilder place in many areas. This may seem a strange prediction, given that the global population will grow from 7.7 billion to 10 billion by 2050, but by then nearly 70 per cent of us will live in cities and have abandoned rural areas, making them ripe for restoration. In Europe already, 2.2m hectares of forest regrew per year between 2000-2015, and forest cover in Spain has increased from 8 per cent of the country’s territory in 1900 to 25 per cent today.

Massive reforestation isn’t a pipe dream and it can have real benefits for people. In the late 1990s, environmental deterioration in China became critical, with vast areas resembling the Dust Bowl of the American Midwest in the 1930s. Six bold programmes were introduced, targeting over 100m hectares of land for reforestation.

Grain for Green is the largest and best known of these. It reduced soil erosion and stabilised local rainfall patterns. The ongoing programme has also helped to alleviate poverty by making payments directly to farmers who set aside their land for reforestation.

Better yet, the new study suggests that bringing back 900m hectares of forest wouldn’t impact on our capacity to reserve land for growing food. This is certainly possible, and in line with other estimates. Reforestation may even result in production from farmland increasing, as was found in China when more stable rainfall and fertile soil followed the return of forests.

Where the billion hectares of forest could be planted – excluding desert, farmland and urban areas. Image: Crowther Lab/author provided.

How all of that new forest would look, alongside what’s already there. Image: Crowther Lab/author provided.

No solution without emission cuts

There should be more scepticism about how much CO₂ 900m hectares of new forest could store though. The paper insists on 205 billion tonnes of carbon, but this seems too high when compared to previous studies or climate models. The authors have forgotten the carbon that’s already stored in the vegetation and soil of degraded land that their new forests would replace. The amount of carbon that reforestation could lock up is the difference between the two.

Mature forests can store a lot of carbon, but this capacity is only reached after hundreds of years, not a couple of decades of new forest growth as assumed in this study. The most recent estimate from the IPCC suggests that new forests could store on average an extra 57 billion tonnes of carbon by the end of the century. This is still a huge number and could absorb about one sixth of the carbon that’s already in the atmosphere, but reforestation should be thought of as one solution to climate change among many.

Radically reducing carbon emissions and absorbing the carbon that’s already in the atmosphere will be necessary to avert catastrophic climate change. Image: Mark Maslin/author provided.

Even if warming is stabilised at 1.5˚C, the study indicates that one fifth of the land proposed for reforestation could be rendered too hot for growing new forests by 2050. But this concern ignores the role of carbon dioxide fertilisation – when there are higher levels of carbon dioxide in the atmosphere, photosynthesis is more efficient, meaning plants need less water and can still be productive at higher temperatures. Today, the most immediate threat to tropical forests is deforestation by people and the fires they light which get out of control, not the more subtle impacts of higher temperatures.

Reforesting an area the size of the US will have massive benefits on local environments and will store a huge amount of man-made carbon emissions. It is not, however, a substitute for reducing those carbon emissions.


Even if the world reduces its carbon emissions to zero by 2050, there will still need to be negative global carbon emissions for the rest of the century – drawing CO₂ out of the atmosphere to stabilise global warming at 1.5˚C. Reforestation is essential for creating negative emissions – not reducing the amount of carbon that humans are still emitting.

There is another sting in the tail. Massive reforestation only works if the world’s current forest cover is maintained and increasing. Deforestation of the Amazon rainforest – the world’s largest – has increased since Brazil’s new far-right president, Jair Bolsonaro, came to power. Current estimates suggest areas of rainforest the size of a football pitch are being cleared every single minute.

It won’t be easy, but society needs to protect the forests we’ve got, and protect new forests in perpetuity to permanently keep carbon sequestered in trees and out of the atmosphere.

The Conversation

Mark Maslin, Professor of Earth System Science, UCL and Simon Lewis, Professor of Global Change Science at University of Leeds and, UCL.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

 
 
 
 

CityMetric is now City Monitor! Come see us at our new home

City Monitor is now live in beta at citymonitor.ai.

CityMetric is now City Monitor, a name that reflects both a ramping up of our ambitions as well as our membership in a network of like-minded publications from New Statesman Media Group. Our new site is now live in beta, so please visit us there going forward. Here’s what CityMetric readers should know about this exciting transition.  

Regular CityMetric readers may have already noticed a few changes around here since the spring. CityMetric’s beloved founding editor, Jonn Elledge, has moved on to some new adventures, and a new team has formed to take the site into the future. It’s led by yours truly – I’m Sommer Mathis, the editor-in-chief of City Monitor. Hello!

My background includes having served as the founding editor of CityLab, editor-in-chief of Atlas Obscura, and editor-in-chief of DCist, a local news publication in the District of Columbia. I’ve been reporting on and writing about cities in one way or another for the past 15 years. To me, there is no more important story in the world right now than how cities are changing and adapting to an increasingly challenging global landscape. The majority of the world’s population lives in cities, and if we’re ever going to be able to tackle the most pressing issues currently facing our planet – the climate emergency, rising inequality, the Covid-19 pandemic ­­­– cities are going to have to lead the way.

That’s why City Monitor is now a global publication dedicated to the future of cities everywhere – not just in the UK (nor for that matter just in the US, where I live). Our mission is to help our readers, many of whom are in leadership positions around the globe, navigate how cities are changing and discover what’s next in the world of urban policy. We’ll do that through original reporting, expert opinion and most crucially, a data-driven approach that emphasises evidence and rigorous analysis. We want to arm local decision-makers and those they work in concert with – whether that’s elected officials, bureaucratic leaders, policy advocates, neighbourhood activists, academics and researchers, entrepreneurs, or plain-old engaged citizens – with real insights and potential answers to tough problems. Subjects we cover include transportation, infrastructure, housing, urban design, public safety, the environment, the economy, and much more.

The City Monitor team is made up of some of the most experienced urban policy journalists in the world. Our managing editor is Adam Sneed, also a CityLab alum where he served as a senior associate editor. Before that he was a technology reporter at Politico. Allison Arieff is City Monitor’s senior editor. She was previously editorial director of the urban planning and policy think tank SPUR, as well as a contributing columnist for The New York Times. Staff writer Jake Blumgart most recently covered development, housing and politics for WHYY, the local public radio station in Philadelphia. And our data reporter is Alexandra Kanik, whose previous roles include data reporting for Louisville Public Media in Kentucky and PublicSource in Pittsburgh, Pennsylvania.

Our team will continue to grow in the coming weeks, and we’ll also be collaborating closely with our editorial colleagues across New Statesman Media Group. In fact, we’re launching a whole network of new publications, covering topics such as the clean energy transition, foreign direct investment, technology, banks and more. Many of these sectors will frequently overlap with our cities coverage, and a key part of our plan is make the most of the expertise that all of these newsrooms combined will bring to bear on our journalism.

Please visit citymonitor.ai going forward, where you can also sign up for our free email newsletter.


As for CityMetric, some of its archives have already been moved over to the new website, and the rest will follow not long after. If you’re looking for a favourite piece from CityMetric’s past, for a time you’ll still be able to find it here, but before long the whole archive will move over to City Monitor.

On behalf of the City Monitor team, I’m thrilled to invite you to come along for the ride at our new digs. You can follow City Monitor on LinkedIn and on Twitter. If you’re interested in learning more about the potential for a commercial partnership with City Monitor, please get in touch with our director of partnerships, Joe Maughan.

I want to thank and congratulate Jonn Elledge on a brilliant run. Everything we do from here on out will be building on the legacy of his work, and the community that he built here at CityMetric. Cheers, Jonn!

To our readers, on behalf of the City Monitor team, thank you from all of us for being such loyal CityMetric fans. We couldn’t have done any of this without you.

Sommer Mathis is editor-in-chief of City Monitor.