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.

 
 
 
 

How can cyclists protect themselves against air pollution?

A female cyclist attempts to protect herself from air pollution. Image: Getty.

The popularity of cycling in London continues to rise: according to statistics published by Transport for London (TfL), the number of journeys made by bicycle in London grew by 5 per cent in 2018. The transport agency has attributed the upwards trend in cycling to its investment in cycling infrastructure, not least the seven Cycle Superhighways and 12 Cycle Quietways the city now boasts.

Cycling is widely reported to result in health benefits for participants, and cyclists can expect to achieve improvements in both their physical and mental health as a result of switching from public transport or car to a bike. But with air pollution levels remaining stubbornly high across London, should cyclists be concerned that the health benefits they achieve as a result of cycling are actually being outweighed by the dangers posed by increased exposure to air pollution? 

Unlike during the Great Smog of 1952, air pollution today is often invisible to the naked eye. Nonetheless, London breached the European and UK air quality annual limit on 18  March when, for the 36 time this year, levels of pollution particles recorded at a measuring post exceeded the agreed limit. (EU rules allow 35 breaches a year.) Whilst this is a marked improvement on 2018 when the annual limit was broken on the 5 January, it reminds us of the risk that air pollution continues to pose to Londoners today. 

The rise of respirator masks

Anyone who has cycled or walked along one of London's cycle paths in recent years is likely to have seen someone resembling Darth Vader cycling towards them. Protection masks, which are becoming increasingly popular amongst the cycling community, range from cotton surgical masks to respirators with in-built air filtration systems that cover a significant part of the cyclist’s face. 

But do masks actually work and are they worth the investment? 

Cotton masks categorically do not protect wearers against the inhalation of airborne particles. Whilst they can be somewhat effective in protecting against the spread of illnesses, they will not protect a cyclist from air pollution. 

Respirator cycling masks, which range in price from £25 to over £50, are a more sophisticated option. “N99” respirators are said to remove up to 99 per cent of airborne particles from inhaled air. But the particles that cause air pollution today are extremely small, which makes it particularly challenging for respirators to effectively block them from entering the human body. 

Another complicating factor is the fit of the respirator against the human face. Studies have concluded that under “perfect” conditions respirators do effectively filter pollution out of inhaled air. However, when actually fitted to a human face, respirators are often not able to form an effective seal against skin, which ultimately renders them useless. Features such as facial hair and short noses make is particularly challenging for a seal to form. 

The findings of studies into the effectiveness of respirator cycling masks are somewhat mixed – but point to the ineffectiveness of current designs. 


So what can cyclists do to protect themselves? 

The best intervention a cyclist can make to reduce their exposure to air pollution is to avoid the most polluted streets and roads. TfL’s Quietways are an easy way for cyclists to identify the less busy and less polluted roads (although TfL has announced it will be merging the Quietway and Cycle Superhighway networks into a single Cycleways cycle network during summer 2019). 

Cyclists may also consider reducing their cycling speed to reduce their inhalation of airborne particles. The faster and deeper we breathe in polluted air, the more pollutants are delivered to our lungs. Therefore slowing down and reducing their amount of exertion will go some way to protecting cyclists from air pollution. 

Finally, cyclists should check air quality forecasts and make informed decisions regarding their chosen mode of transport on a particular day. TfL provides daily forecasts on its website. 

So should cyclists stop cycling all together? In a word, no. Although there is currently not an effective way to stop yourself from inhaling air pollution whilst cycling, scientists have concluded that the physical and mental health benefits of cycling continue to outweigh the dangers posed by exposure to air pollution. Cycling remains a healthy method of transport for Londoners. 

If you are a cyclist who is concerned about your exposure to air pollution and you are considering investing in a respirator mask, be aware that research suggests they will not protect you effectively. Instead you may want to consider donating the money you would have spent on a respirator to a charity such as Trees for Cities, whose mission is to transform urban areas by creating Urban Forests.