Earth’s carrying capacity for trees and the impact on climate change
Introduction The role of trees in biodiversity & climate changeNature provides the foundation for healthy societies and strong economies.
The problem is clear: each year, humans emit another 11 gigatonnes or so of carbon into the atmosphere (Global Carbon Budget 2019). That’s roughly the weight of 37,000 Empire State Buildings, mostly the result of burning fossil fuels. It leads the United Nations’ 2021 IPCC report to paint a dire picture: at current rates of carbon emission, the world is on track to meet or exceed 1.5 C of warming within the next two decades.
To avoid worst-case scenarios, the solution is clear: We need to quickly and drastically reduce emissions while also drawing existing excess carbon out of the atmosphere. Many tools are needed to help address climate change, from transitioning to renewable electricity generation and decarbonizing transportation to shifting to more sustainable food production and consumption. To limit climate change, we need a robust and holistic approach. This includes tapping into the immense power of nature and using nature-based solutions like conservation and restoration to help draw existing carbon out of the atmosphere and to restore and conserve Earth's ecosystems.
This is where trees come inForests play an important role in the carbon cycle, and climate change
However, when these initiatives were announced, it remained unclear if these restoration goals were within reach - or ambitious enough. That’s partly because researchers lacked even a basic understanding of how much tree cover might be possible under current or future climate conditions, as well as where these trees could exist on Earth. In addition, scientists didn’t have quantitative information about how much carbon these restored trees could capture. Without scientific evidence, it wasn’t possible to quantify the true potential of tree restoration or its impacts on carbon drawdown.
Our study, published in the journal Science in 2019, was the first to explicitly link tree observations to environmental characteristics and provide quantitative, spatially explicit global estimates of potential tree cover across the globe. Since then, other researchers have also been exploring tree restoration’s carbon capture potential.
Understanding what drives the variation in the natural tree cover across the globeMapping Earth’s tree potential
To gain a holistic and quantitative view of which environments could potentially support new trees, we used 78,744 direct observations of 0.5-hectare plots across the globe. These observations were gathered using an augmented visual interpretation approach with a systematic sampling grid design of 20 km by 20 km, providing a clear view on the existing natural tree cover across the globe.
We then used a machine learning approach to examine the dominant environmental characteristics – such as climate, edaphic and topographic variables – to understand what drives the variation in the natural tree cover across the globe.
The first-ever quantitative, spatially explicit map of Earth’s tree carrying capacityThe global tree restoration potential
Our resulting map is the first-ever quantitative, spatially explicit map of Earth’s tree carrying capacity. It defines the tree cover that could potentially exist under any set of environmental conditions on Earth under existing climate conditions. The model accurately predicts the presence of forest in all existing forested land on the planet, and it also reveals the extent of potential tree cover that could exist in regions outside existing forested lands.
For example, a low density of trees could naturally grow in grassland ecosystems, which in their healthy states may naturally support a low amount of trees.
A) Current tree cover and potential tree cover showing the total potential
B) Potential tree cover excluding current cover and potential in deserts, agricultural and urban areas
ResultsThe potential of global tree restoration
That’s an area nearly the size of the United States. If that land could be protected long-term, we estimate the regenerating trees could capture 206 gigatonnes of carbon (GtC) in woody biomass and soil over their lifetime (uncertainty ranges from 133 to 276 GtC).
More than half of the total 0.9 billion hectares of tree restoration potential is found in just six countries:
151 million hectares in Russia
103 million hectares in the USA
78.8 million hectares in Canada
58 million hectares in Australia
49.7 million hectares in Brazil
40.2 million hectares in China
This stresses the critical role in restoration that some of the world’s leading economies can play to address climate change.
Forests - like other natural ecosystems - play an important role in the fight against climate change
Furthermore, while our results show that tree restoration targets are ecologically possible, they also reveal many inconsistencies regarding the restoration goals set by 48 countries in the Bonn Challenge and the actual potential in the respective countries. At the time of our study’s publication, approximately 10% of the countries had committed to restoring an area of land that considerably exceeds the total area that is available for restoration. Similarly, over 43% of the countries had committed to restoring an area less than half the size of the area that has restoration potential.
Since the publication of our study, new initiatives, such as the World Economic Forum’s Trillion Trees campaign (1T.org), are driving additional restoration efforts. Of course, restoration must be pursued in ecologically and socially responsible ways, which includes consideration of many socio-economic factors, such as land tenure rights. The need to understand land ownership further strengthens the need for better country-level forest accounting, which is critical for developing effective management and restoration strategies.
Additionally, our models also reveal the urgency of the situation. By running our potential tree cover model under the slightly optimistic 4.5 Representative Concentration Pathways (RCP) and the pessimistic 8.5 RCP scenario, we see a likely decrease of 450 million hectares by 2050 in the area available for global forest restoration. This change in size of the available area is mostly due to the consistent declines of tropical rainforest and areas with high tree cover.
Read the full study in Science here.
An opportunity for all of us to get involvedNew perspective on natural climate change mitigation strategies
More broadly, ecosystem restoration also has tremendous benefits for biodiversity and human well-being.
The world is ready to take nature-based solutions to scale. We’re encouraged by the UN Decade on Ecosystem Restoration (2021-2030) and responsible ‘trillion tree’ initiatives including 1T.org and Trillion Trees (by WWF, WCS and Birdlife International), all of which we’re honoured to support with our science. This unprecedented backing by governments, corporate leaders, NGOs, civil society and others could be the game-changer that drives a resurgent bottom-up movement of local actions with global significance. And it’s an opportunity for all of us to get involved.
For a global restoration movement to work, restoration must be locally driven and ecologically informed. Following the publication of the tree restoration potential paper, we helped convene a global coalition of over 140 leaders in science, conservation, restoration, development and sustainability around four high-level principles for nature-based solutions.
Principles for Nature-Based Solutions
The Together With Nature principles provide a framework to responsibly tackle the climate crisis, restore biodiversity, and benefit planetary health and human well-being:
- Cut emissions
- Restore a diverse mix of native species
- Respect and involve local communities
- Conserve existing ecosystems
Trees can play a role to play in combating climate changeBut tree restoration is more than just a solution for mitigating climate change
Practiced in an ecologically and socially responsible manner, it offers tremendous benefits to help address biodiversity loss and restore nature’s ecosystem services - from clean air and water to disease suppression to production of natural resources.
Are you a restoration practitioner interested in learning more about what ecological data and insights are available for your projects? Or are you someone who’s interested in exploring existing projects and seeing how you can get involved and support restoration? To learn more, visit Restor, a science-based open data platform offering ecological insights to restoration efforts around the world.
Visit our website: www.crowtherlab.com