by Tackling climate change by planting trees has an intuitive appeal. They absorb the greenhouse gas carbon dioxide from the atmosphere without using expensive technology.
It is widely recommended that you can plant trees to offset your carbon emissions. Many businesses, from those that sell shoes to drinks, offer to plant a tree with every purchase, and more than 60 countries have signed up to the Bonn Challenge, which aims to restore degraded and deforested landscapes.
However, increasing tree cover may affect the climate in complex ways. Using models of the Earth’s atmosphere, land and oceans, we simulated future large-scale afforestation. Our new study shows that this increases the removal of carbon dioxide in the atmosphere, which is beneficial in combating climate change. But side effects, including changes in other greenhouse gases and the reflectivity of the land surface, could partially counteract this.
Our findings suggest that while afforestation – the restoration and expansion of forests – can play a role in combating climate change, its potential may be less than previously thought.
When afforestation occurs alongside other climate change mitigation strategies, such as reducing greenhouse gas emissions, the negative side effects have a smaller impact. Therefore, forestry will be more effective as part of wider efforts to pursue sustainable development. Trees can help fight climate change, but relying on them alone is not enough.
What does the future hold?
Future climate projections suggest that greenhouse gas emissions must reach net zero by the mid to late 21st century to keep warming below the Paris Agreement’s 2°C target, and that they will net negative afterwards. As certain industries, such as aviation and shipping, will be extremely difficult to completely decarbonise, carbon removal will be necessary.
Afforestation is a widely proposed strategy for carbon sequestration. If deployed sustainably – by planting mixtures of native trees rather than monocultures, for example – forestry can provide other benefits including protecting biodiversity, reducing soil erosion, and improving flood protection.
We considered an “extensive afforestation” strategy that expands existing forests during the 21st century in line with current recommendations, planting trees where they are expected to thrive and avoiding croplands.
In our models, we paired this strategy with two future climate scenarios – a “minimum effort” scenario with average global warming above 4°C, and a “Paris-compatible” scenario with extensive climate mitigation efforts. We could compare the extensive afforestation result with simulations with the same climate but where the afforestation levels followed more expected trends: in the minimum effort scenario a decrease in forest cover is seen as agriculture increases, and in the case compatible with Paris there are small increases in global forest cover.
Up in the air
The Earth’s energy balance depends on the energy that comes in from the Sun and the energy that escapes back into space. Increasing the forest cover changes the overall energy balance of the Earth. In general, warming is caused by changes that reduce the outgoing radiation. The greenhouse effect works this way, because outgoing radiation is trapped by gases in the atmosphere.
The ability of forestry to lower atmospheric CO₂, thereby increasing radiation escaping into space, has been well studied. However, the amount of carbon that can be easily removed is a matter of debate.
Land surface reflectance (albedo) generally decreases as lighter grasses are replaced by darker trees. Reductions in albedo levels counteract the beneficial reduction of atmospheric CO₂, as less radiation escapes back into space. This is especially important at higher latitudes, where trees cover otherwise snow-covered ground. Our case involves forest expansion mainly in temperate and tropical regions.
Forests emit large quantities of volatile organic compounds (VOCs), and these emissions increase with rising temperatures. VOCs react chemically in the atmosphere, affecting the concentration of methane and ozone, which are also greenhouse gases. We find that the enhanced VOC emissions from greater forest cover and temperatures increase methane levels and, typically, ozone. This reduces the amount of radiation that escapes to space, counteracting the removal of more carbon.
However, the reaction products of VOCs can contribute to aerosols, which reflect incoming solar radiation and help form clouds. Due to increases in these aerosols and rising VOC emissions from greater forest cover, more radiation escapes into space.
We find that the net effect of changes in albedo, ozone, methane and aerosols is to reduce the amount of radiation escaping to space, which cancels out some of the benefit of reducing atmospheric CO₂. In a future where climate mitigation is not a priority, up to 30% of the benefit is cancelled, while in a future suitable for Paris, this drops to 15%.
Cooler solutions
Tackling climate change requires efforts from all sectors. Although afforestation will play a role, our work shows that the benefits may not be as great as previously thought. However, these negative side effects are less impactful if we pursue other strategies, especially reducing our greenhouse gas emissions, along with forestry.
This study did not consider local temperature changes from afforestation as a result of evaporative cooling, or the impact of changes in atmospheric composition due to changes in the frequency and severity of wildfires. Further work in these areas will complement our research.
However, our study suggests that forestry alone is unlikely to solve our warming planet. We need to rapidly reduce our emissions while improving the natural earth’s ability to store carbon. It is important to test climate mitigation strategies in detail because there are so many complex systems in place.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
James Weber receives funding from UK Research and Innovation (UKRI).
James A. King sits on an advisory panel for Ecology. It receives funding from UK Research and Innovation (UKRI).
