Tackling climate change by planting trees has an intuitive appeal. They absorb the greenhouse gas carbon dioxide from the atmosphere without using expensive technology.
The suggestion that you can plant trees to offset your carbon emissions is widespread. Many companies, from those selling shoes to drinks, now 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, expanding tree cover can affect climate in complex ways. Using models of Earth’s atmosphere, land and oceans, we simulated large-scale future afforestation. Our new study shows that this increases the removal of carbon dioxide in the atmosphere, which is beneficial for tackling climate change. But side effects, including changes in other greenhouse gases and land surface reflectivity, can partially counteract this.
Our findings suggest that while afforestation – the restoration and expansion of forests – can play a role in tackling climate change, its potential may be smaller than previously thought.
When afforestation takes place alongside other climate change mitigation strategies, such as reducing greenhouse gas emissions, the negative side effects have a smaller impact. Afforestation will therefore be more effective as part of broader efforts to pursue sustainable development. Trees can help combat climate change, but relying on them alone won’t be enough.
What does the future bring?
Future climate projections suggest that to keep warming below the 2°C target of the Paris Agreement, greenhouse gas emissions must reach net zero by the mid-21st century and become net negative thereafter. Because it will be extremely difficult for some industries, such as aviation and shipping, to fully decarbonize, carbon removal will be necessary.
Afforestation is a commonly proposed strategy for carbon removal. When used sustainably – for example by planting mixtures of native trees rather than monocultures – afforestation can deliver other benefits, including protecting biodiversity, reducing soil erosion and improving flood protection.
We have considered an ‘extensive afforestation’ strategy that expands existing forests over the 21st century, in line with current proposals, adding trees where they are expected to thrive, while avoiding arable land.
In our models, we combined this strategy with two future climate scenarios: a ‘minimal effort’ scenario with average global warming above 4°C, and a ‘Paris compatible’ scenario with extensive climate mitigation efforts . We could then compare the outcome of the extensive afforestation with simulations with the same climate, but where afforestation levels followed more expected trends: in the minimum effort scenario, forest cover decreases as agriculture expands, and in the scenario compatible with Paris, a modest increase in global forest cover is expected.
High in the air
The Earth’s energy balance depends on the energy coming in from the sun and the energy escaping into space. Increasing forest cover changes the Earth’s overall energy balance. In general, changes that reduce outgoing radiation cause warming. The greenhouse effect works this way because outgoing radiation is absorbed by gases in the atmosphere.
The ability of forestry to reduce CO₂ in the atmosphere and thereby increase the amount of radiation escaping to space is well studied. However, the amount of carbon that could feasibly be removed remains a subject of debate.
Afforestation generally reduces the reflectivity of the land surface (albedo), as dark trees replace lighter grassland. A decrease in albedo levels counteracts the beneficial reduction of CO₂ in the atmosphere, because less radiation escapes back into space. This is especially important at higher latitudes, where trees cover land that would otherwise be covered in snow. Our scenario mainly includes forest expansion in temperate and tropical areas.
Forests emit large amounts of volatile organic compounds (VOCs), with these emissions increasing as temperatures rise. VOCs react chemically in the atmosphere and affect the concentrations of methane and ozone, which are also greenhouse gases. We find that increased VOC emissions resulting from increased forest cover and temperatures increase levels of methane and, typically, ozone. This reduces the amount of radiation escaping into space, further counteracting carbon removal.
However, the reaction products of VOCs can contribute to aerosols, which reflect incoming solar radiation and help form clouds. Increasing these aerosols with increasing VOC emissions due to increased forest cover results in more radiation escaping to space.
We find that the net effect of changes in albedo, ozone, methane and aerosol is to reduce the amount of radiation escaping to space, negating some of the benefit of reducing CO₂ in the atmosphere. In a future where climate mitigation is not a priority, up to 30% of the benefit is wiped out, while in a Paris-compatible future this drops to 15%.
Cooler solutions
Tackling climate change requires efforts from all sectors. While afforestation will play a role, our work shows that its benefits may not be as great as previously thought. However, these negative side effects do not have as much impact if we pursue other strategies in addition to afforestation, especially reducing greenhouse gas emissions.
This study did not consider local temperature changes due to afforestation due to evaporative cooling, or the impact of changes in atmospheric composition caused by changes in the frequency and severity of forest fires. Further work in these areas will complement our research.
Nevertheless, our research suggests that afforestation alone is unlikely to reverse global warming. We must rapidly reduce our emissions while increasing the natural world’s ability to store carbon. It is important to test climate mitigation strategies in detail because there are so many complex systems at play.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
James Weber receives funding from UK Research and Innovation (UKRI).
James A. King is on an advisory panel for Ecolog. He receives funding from UK Research and Innovation (UKRI).