When it emerged that a miscalculation by the oil company Shell meant that its new scenario for meeting the 1.5C climate target would need a new forest about the size of Brazil, the controversy over the role of trees in the climate emergency again hit international headlines.

The prospect of planting trees to tackle the possibility of irreversible climate change  gives hope. While human activities keep belching out carbon dioxide at a rate irreconcilable with planetary limits, trees not only breathe carbon dioxide in, but they breathe out oxygen. What’s more, planting trees is not only good for the environment, it’s great for our mental and physical health too. Unfortunately, like many apparently easy answers, tree-planting is more complex than it first appears.

The Intergovernmental Panel on Climate Change (IPCC) says in its 2018 Special Report, that if the world wants to limit temperature rise to 1.5C by 2050, an extra 1 billion hectares (2.4bn acres) of trees are needed. But what types of trees should we plant and where, and how do we know what difference it will make? Many different initiatives across the world have attempted to restore woodland to landscapes, but what works best for people and the biosphere? Here we look at several options to understand how trees might best help achieve rapid transition, including some flagship initiatives to combat deforestation and in particular natural forest regeneration, rather than commercial plantations.

Over the last decade, several reforestation and afforestation schemes have sprung up under the programme of United Nations Convention to Combat Desertification (UNCCD), in an effort to mitigate climate upheaval. In addition, there are numerous  national and regional reforestation initiatives. One of the most high profile schemes is China’s forest rehabilitation programme. Another is the African Great Green Wall initiative, which aims to combat climate change, poverty and desertification by creating 780 million hectares of green productive landscape across North Africa, the Sahel (the area south of the Sahara desert) and the Horn of Africa. In order to halt land degradation by 2030, 10 million hectares must be restored each year. In 2019, the FAO launched a similar reforestation plan targeting cities specifically – the Great Green Wall for Cities initiative. To reverse the environmental pressures caused by city-based urban expansion, the plan aims to create up to 500,000 hectares of urban forests by 2030 and restore a further 300,000 hectares of existing natural forests across the Sahel and Central Asia. This project is expected to remove  between 0.5 to 5 Gigatonnes of CO2 per year from the atmosphere.

Many other countries have also stepped up their efforts to plant trees specifically to combat the climate emergency. In 2017, Pakistan met its target of planting a billion trees and  committed to reach 10 billion trees within five years. In India, 66 million trees were planted in a 12-hour record breaking environmental campaign involving 1.5 million volunteers. Other Indian states have followed suit with similar rapid tree-planting campaigns. Some countries relied on more innovative reforestation methods such as Myanmar using drones to plant trees.

Such grand reforestation projects may appear to be particularly attractive solutions to meet the rapid and outstanding decarbonisation targets outlined in the IPCC report. However, their ability to achieve these ambitious goals has proven controversial in some cases.

Several climate and forest scientists favour what they call “natural forest regeneration” – essentially letting the forest grow back naturally – which proves to be the most efficient and low-cost approach in achieving natural carbon sequestration. However, only a third (34%) of the total area dedicated to forest restoration plans covered by the Bonn Challenge – a global reforestation initiative – is earmarked for this approach. A further 21% of land is reserved for agroforestry, which is a method promoting the positive forms of co-production for multiple plants and crops. The remaining 45% of land area is destined for monoculture production of trees, the largest majority of which are located in Brazil, China, Indonesia, Nigeria and the Democratic Republic of Congo. Despite meeting the standard definition of a forest used by the UN Food and Agriculture Organisation (FAO), these plantations fail in several ways when it comes to climate change mitigation and biodiversity protection. For this reason, and while deforestation rates in the Amazon and Boreal forests continue unabated, global tree planting efforts will struggle to achieve much.

But there are many examples of better ways of doing things. In Ghana or Malaysia, for example, people restored their local forests after being granted ownership of the land. In Ghana, the forest restoration projects found to be most successful were those which included a “rights-based approach”. Strengthening community rights contributed to the protection of forests from appropriation and privatisation, and ensured greater accountability, the right to speak out and provide safeguards against illegal practices. In Malaysia, an NGO supported Penan indigenous communities to fight for their legitimate entitlement to land rights in court. The Penan have strong ties with the land and forests, which play a strong role in the maintenance of local biodiversity.

Wider relevance

Reforestation projects date back to the colonial period when Anglo-European powers engaged in vast schemes to reverse desertification, and increase a certain kind of cash-crop productivity in the territories they conquered. 

More recently, under the United Convention to Combat Desertification (UNCCD), tree planting programs have had very mixed results. While many failures occurred because the trees planted were not suitable for local climate conditions, others depleted groundwater reserves, leaving nearby soils desiccated and this further undermined local agricultural production. To preempt  this practice, places like South Africa and the US Southwest enshrined the removal of exotic species into law. However, many organisations and governments continue to support reforestation projects in ecologically problematic areas.

Despite the capacity of fast-growing tree species to quickly absorb carbon, their harvest and the resulting land clearance can release carbon emissions just as quickly, or even faster, through the decomposition of plantation waste and products (paper and wood chip boards). This can also ruin the wider habitat, including underground root and fungal systems that keep soil stable, healthy and able to hold water. By contrast, natural-forest restoration is the most effective way to store carbon from the atmosphere. Natural forests are far more effective than plantations (40 times) and agroforestry (six times) at retaining carbon. Their complex ecological systems are also better for biodiversity.

Protecting land from fire and other human hazards is crucial if trees are to be allowed to regenerate naturally into large forest carbon sinks. To speed up the recovery process, native species can be planted too – a method that was pioneered in Costa Rica using legislation and incentives to encourage and steer the process. This does not mean that every natural forest must be protected forever, but scientists argue that the reforestation agenda must be prioritised  if we are to reach the target of 350 million hectares of new natural forest. The limits of suitable land is an obvious constraint; there are many competing interests vying to use land to grow crops for food, fuel, fodder, fibre and for other ecosystem services. Nevertheless, an increase in agricultural productivity and a change in consumer habits towards vegetarian and vegan diets could help free up a sizeable amount of land.

Four main ways in which nations can encourage successful restoration of natural  forests are proposed: 

  1. Countries should increase the proportion of their land allocated for regeneration  to natural forest.
  2. Priority should be given to humid tropical areas, such as Amazonia, Borneo and the Congo Basin, which support high biomass forest (in comparison with drier regions). International, climate adaptation and conservation funds could be used to support such action.
  3. Countries should focus on naturally regenerating existing carbon stocks such as degraded forests and partly wooded areas, using treeless regions for plantations or agroforestry  – and preferably the latter, removing the plantation model wherever possible.
  4. Restored forest must be protected. This might include giving title rights to indigenous peoples who protect forested land, changing the legal definition of land-use so that it cannot be converted to agriculture, and ensuring that commodities companies cannot clear restored forests.

Context and background

Over recent decades, countless UN commissions, international conventions and scientific reports have looked to tree-planting as a solution to combat the devastating impacts of climate breakdown. Among these are the United Convention to Combat Desertification (UNCCD) (1994), the UNFCCC REDD (2008) and REDD+ (2013) (reducing emissions from deforestation and degradation) programmes as well as the Bonn Challenge – a IUCN initiative to bring 350 million hectares of degraded and deforested landscapes into restoration by 2030. 

However, the scientific community soon  contested the potential of such reforestation plans to meet global decarbonisation targets. One study from Stanford University said that if poorly designed, plantation policies might even lead to adding more carbon to the atmosphere and encouraging biodiversity loss. In other words, in certain circumstances planting trees could actually make things worse. The study highlights that plantations, which include the majority of reforestation plans covered in the Bonn Challenge, have “significantly less potential for carbon sequestration, habitat creation and erosion control than natural forests”. Moreover, if those newly planted trees replace natural forests, grasslands or savannahs, their benefits are even lower, since these ecosystems are particularly valuable  biodiversity.

Another study makes similar claims by outlining the minimal effects on carbon sequestration from a reforestation policy in Chile. The authors report that other global mass tree-planting programs like the Trillion Trees initiative and the Bonn Challenge are also subjected to mixed results as they mainly feature monocultural tree plantations. However, they stress that if subsidy restrictions were to ban the replacement of native forests with tree plantations, these same policies could have very beneficial effects.  

On top of the complexity of natural forest ecology and how to recreate it, the scientific community is still arguing over how to calculate the positive climate-related benefits of trees. One  group of scientists from ETH-Zurich in Switzerland said in 2019 that there is enough space to plant trees that could contribute to a potential 25% reduction in CO2 emissions. However others claimed that the study’s authors had overestimated both the area available for plantation and the trees’ capacity to store carbon. Prof Simon Lewis from University College London argues that, “[t]he estimate that 900 million hectares restoration can store an addition 205 billion tonnes of carbon is too high and not supported by either previous studies or climate models.”

Other critics still pointed to the study’s negligence of the role played by local communities in countering biodiversity loss. The Swiss model did not consider the land ownership or land rights issues, which are key when assessing the exact amount of land available for tree cover. The recent UN IPBES biodiversity study recognised that it is fundamental that we provide indigenous peoples with land rights as part of our global efforts to safeguard biodiversity. Research shows that land managed by indigenous people is also better for forest protection and for biodiversity.

Enabling factors

The concept of tree planting programs to mitigate climate change, in place for over 20 years, was endorsed further as part of the “nature-based solutions” (NbS) advocated under the UN Sustainable Development Goals (SDG). Traditional climate-related engineering solutions paid little attention to social issues, while nature-based solutions have the capacity to address the impacts of climate breakdown while also delivering benefits to people and nature.

Credit: Plant Care by Iezalel Williams (public domain)

As part of the SDGs’ integrated approach to tackling climate change, planting trees is promoted as a low-cost strategy with multiple and mutually reinforcing benefits. For instance, the restoration of natural forests in upper watershed catchments where rivers first rise, can reduce the occurrence of downstream flooding, protecting local communities. In cities, trees not only help reduce the urban heat effect and mitigate against the negative impacts from air pollution, but can also be used for recreational purposes with strong mental health and social benefits. Nature-based solutions are today promoted as a new panacea and form a central pillar of most climate and biodiversity reports and conventions, such as the IPBES Global Assessment, The IPCC Climate Change and Land report, the Global Adaptation Commission report and the 2019 UN Climate Action Summit

Among the many UN initiatives specifically focused on tree planting, the UN REDD programme for carbon offsetting has proven most contentious. The climate justice and indigenous rights’ community decried its supposedly climate-friendly benefits. As part of REDD, communities receive an income based upon performance, which makes them particularly vulnerable to conditions which are beyond their control. Critics also argued that by linking reforestation projects – located miles away, most often in the nations of the Global South – with carbon offsetting programs they provide the biggest carbon emitters with a license to keep polluting. This approach is particularly problematic because it promotes an easy-fix solution to climate change issues that are complex by nature. Furthermore, it fails to address carbon-intensive modes of living in the Global North and the ecological debt accrued by rich nations.

The UN significantly revised the project and its new REDD+ programme aims to address reforestation using a community-led approach. These projects offer a more promising avenue to reverse biodiversity loss and mitigate against climate change by rooting their efforts in inclusive and participatory action at the local level. Other NGOs are working towards “rights-based forest restoration” methods as a way to empower communities on the ground, secure livelihoods and mitigate against climate crisis impacts. Thanks to these changes in perspective, it is now common practice to approach reforestation schemes through an ethical and equitable lens while giving agency and ownership to local communities. In contrast with other untested methods to draw carbon from the atmosphere, forests, if managed carefully, remain the most effective and safe “carbon capture technologies”.

Scope and evidence

  • Established in 1994, the United Nations Convention to Combat Desertification (UNCCD) is a legally binding agreement to address the effects of desertification by linking environment, development and sustainable land management concerns.
  • In 2003, China launched its “Green Wall” project – the world’s largest ever programme to combat the effects of desertification, climate change and poverty. Its completion is scheduled for the year 2074. Many other nations have followed suit by launching grand reforestation schemes with mixed to poor results.
  • In 2008, the UN launched its REDD – reducing emissions from deforestation and forest degradation – programme. Following controversies about its devastating impacts on local communities, the UN revised the programme and launched the REDD+ initiative with a specific focus to local community action and participation.
  • To meet the 2030 UN Sustainable Development Goals (SDG), nature-based solutions are promoted as an integrated approach to meet climate change mitigation targets and promote social development goals.
  • In 2019, a group of scientists from the ETH University of Zürich published a new study revising upwards the estimated space available for tree plantation. The findings of this report were however criticised by several other scientists who argued that they significantly overestimated the reality and failed to account for land rights issues.
  • The latest 2019 UN IBPES (Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services) assessment highlighted the important role played by Indigeneous people around the world in safeguarding biodiversity.