by Stan Hirst
A fine alliterative title, but it presents a problem in visioning. Just what does a trillion trees actually look like? What size area would, or could they cover?
The story starts with Kenyan Wangari Maathai (1940-2011) who, while serving in the National Council of Women in 1976, introduced the idea of community-based tree planting in Kenya. She continued to develop this idea into the Green Belt Movement, a grassroots organization focused on poverty reduction and environmental conservation through tree planting. For her efforts she was awarded the Nobel Peace Prize in 2004.
In 2006 an international Billion Tree Campaign was launched by the United Nations Environment Programme (UNEP) as a response to the multiple challenges of climate change, sustainability decline and biodiversity loss. The campaign achieved its target of planting a billion trees by 2007.
By this time the era of international “planetary emergency” was in full swing, with global warming widely recognized as the defining issue of the era. The Billion Tree Campaign had come to symbolize the readiness of people everywhere to work to protect Earth’s climate and the collective home of humanity. Planted seedlings were seen as the first steps to restoring springs, preventing soil erosion and creating fertilizer to boost harvests. They were promoted as key tools in halting the spread of desert sand dunes, and providing food resources in both rural and urban areas. Other cited objectives of tree-planting were the supply of forage for livestock and for crop-pollinating insects, the production of wood and natural oils for building and for fuel, the creation of medicines to heal and essential oils to ease.
All planted trees were seen as a level of protection against the threats posed by climate change. The range of participants had grown to include groups as diverse as children, giant corporations, women’s groups, scientific and technocratic organizations, farmers and national governments. Trees had attained an enhanced level of significance for communities around the world.
Felix Finkbeiner, German environmentalist and the founder of the international tree-planting and environmental advocacy organization Plant-for-the-Planet, pushed the limit to the trillion tree mark in 2011. Addressing the United Nations to open the International Year of Forests 2011 he famously stated that “it was time to combine the forces of old and young, rich and poor, to forge ahead and plant a trillion trees”. In December 2011, after more than 12 billion trees had been planted, UNEP formally handed management of the program to the youth-led not-for-profit Plant-for-the-Planet Foundation based in Tutzing, Germany. Momentum has since continued, with 40,000 young ambassadors spreading the message in over 100 countries
Questions as to whether tree-planting on such a vast scale would actually bring about the desired ecological and social benefits have been posed many times over the decades. The more specific question as to whether planting one billion trees across various habitats on earth would do anything significant towards ameliorating global climate change by reducing global CO2 levels was eventually posed by Plant-for-the-Planet to the Swiss Federal Institute of Technology (EPFL) in Zurich, Switzerland.
For Prof. Thomas Crowther and his co-workers at EPFL the question led somewhat obviously to the next one, i.e. how many trees are there now on earth? The best estimate then available, made less than a decade earlier, was 400 billion (give or take the odd million). This was admittedly a very rough estimate based on counts made of tree cover on satellite images taken from many locations around the globe. However, Crowther and his associates had much more sophisticated equipment at their disposal. Using a combination of satellite measurements and computer models they measured actual tree cover and density world-wide. They then calibrated their measurements by comparisons with some 400,000 ground-based measurements. The final best estimate for the the entire globe totalled roughly 3 trillion trees (~7.5 times the earlier estimate). Nearly 1.4 trillion of Earth’s trees were found to exist in tropical and subtropical forests where the rate of forest loss is the highest.
Crowther’s and his colleagues’ technology allowed them to go a step further. Knowing that Earth has supported far more trees in the historic past, they used their computer models to estimate that the pre-human Earth must have had something like 5 to 6 trillion trees, i.e. double that which now exists. Alternatively stated, that represents a net loss of some 10 billion trees per year. The study found that 15 billion trees have been cut down each year while only about 5 billion were replanted. That indicates an ongoing net loss of 10 billion trees a year which in turn points to a future some 300 years from now when all or most of Earth’s trees will be gone.
Critics of the study have pointed out that if there are some 3 trillion trees on the planet then the planting of a billion more wouldn’t do much to counteract climate change. Plant-for-the-Planet duly raised their tree-planting target to 18 billion and has transformed the Billion Tree Campaign into the Trillion Tree Campaign.
Crowther’s research has revealed that a 1 °C increase in global mean climate temperature could lead to the release, by vegetation and soil organisms, of an additional 55 billion tonnes of soil carbon (equivalent to 200 billion tonnes of CO2) into the atmosphere by 2050. This addition had not previously been accounted for in climate change computations. His research further suggested that warming generally stimulates decomposition more than it does photosynthesis. Increased activity of microbes and soil animals such as worms would be a source of additional carbon emissions which could accelerate global warming by as much as 17%.
In July 2019 the Crowther Lab published a major paper on global tree restoration potential. Studying nearly 80,000 high resolution satellite photographs they analysed tree cover in protected areas largely unaffected by human activity across Earth’s ecosystems, from arctic tundra to equatorial rainforest. They used machine learning to identify correlations with key soil and climactic variables in Google’s Earth Engine to predict the natural level of tree cover which could potentially exist in each ecosystem. They concluded that a global area of 0.9 billion hectares (outside of agricultural and urban areas) was available for tree restoration.
The data are pointing to an uncomfortable reality. There is not enough room on Earth to offset all of our carbon emissions with trees. The best society can hope to achieve through tree-planting is to draw down some of the excess atmospheric carbon. Cutting fossil fuel emissions, not tree-planting, has to be the top priority for climate action. Despite these conclusions from the international ecological arena, the World Economic Forum in 2020 launched a further initiative to plant one trillion trees to tackle climate change.
The response from global ecologists and natural resource managers has been to attempt to clarify the appropriate role of tree plantings as a climate solution. They have stated that focusing too heavily on planting trees can distract from other important climate solutions. Counting heavily on planted forests as offsets for greenhouse gas emissions presents some risks. If a planted forest is mismanaged and cut and/or burned, researchers say, much of that stored carbon could go up in smoke.
Tropical forests have been one of Earth’s best defenses against rising atmospheric CO2 levels. Despite ongoing deforestation, tropical forests hold more carbon than burning coal, oil and natural gas have emitted over the past 30 years. But scientists point out that the ability of tropical forests to act as carbon sinks will diminish and ultimately reverse as global warming persists and trees, stressed by heat and drought, die and release their carbon. If warming reaches 2°C above preindustrial levels, the study finds huge swaths of the world’s tropical forests will begin to lose more carbon than they accumulate. Many forests in South America have already reached that point.
The prevailing professional views on the use of forest practices to mitigate climate change all point in the same general direction.
- Avoiding deforestation is a cheaper and more effective climate solution than planting new forests.
- Reducing greenhouse gas emissions from fossil fuel combustion and deforestation are far and away the best climate solutions.
- Tree planting is a potentially powerful tool in addressing climate change, but it is not a panacea.
- Benefits and limitations of tree planting as a climate solution vary considerably depending on geographic region, local ecology, social context, planting approach, and a multitude of other factors.
Forests cannot sequester all global carbon releases. Global fossil fuel emissions from 2009-2018 totaled about 9.5 Gt C per year Over the same period the amount of carbon estimated to have been sequestered by forests and grasslands was about 3.2 Gt C per year – only one-third as much.
Forests have many biophysical effects on the environment, including lowering albedo, emitting volatile compounds and methane, altering the roughness of the landscape, and increasing evapotranspiration. Tree planting in temperate and tropical ecosystems will likely lead to cooling since the effects of carbon sequestration outweigh the effects of lower surface albedo. In boreal forests, however, planting trees can increase warming because forests reflect less sunlight than the snow-covered landscapes they replace.
Trees also alter the roughness of the landscape, which affects the amount of energy and water transferred to the atmosphere. Tropical forests have high rates of transpiration that contribute to cloud formation, considerably reducing both surface temperatures and the amount of sunlight reaching the Earth’s surface.
Reforestation and afforestation usually favour the use of single-species plantings over restoring multi- species native forests (much more expensive), which can lead to the global spread of monocultures and invasive tree species. Public pledges of financial support for tree-planting tends to focus more on the extent rather than the quality of forest to be protected, afforested or reforested. The approach encourages the establishment of monoculture plantations of fast growing species, including exotics. As a result, some tree planting efforts can replace native ecosystems and reduce biodiversity.
Reforestation and afforestation can also negatively impact local communities if they remove land available for food production, reduce water supplies, increase social inequity, or displace people from the land.
In summary: tree planting as a climate solution has the potential to distract from other important approaches, especially the reduction of fossil fuels and avoiding deforestation. Solving climate change is not about how to reengineer the biosphere with more forests to take up industrial carbon pollution. The problem is how to end industrial carbon emissions, which are the primary cause of climate change.