Earth’s carrying capacity for trees and the impact on climate change
Introduction The role of trees in biodiversity & climate change
The solution is also clear: We need to drastically reduce emissions and also draw existing carbon out of the atmosphere. Stop the damage and start to repair. Many tools exist to help address climate change - from transitioning to renewable electricity generation and decarbonizing transportation, to shifting to more sustainable food production and consumption. To address 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 renew Earth’s ecosystems.
However, until recently, it remained unclear if these restoration goals were within reach or ambitious enough. This is partly because researchers lacked even a basic understanding of how much tree cover might be possible under current or future climate conditions and 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 any scientific evidence, it wasn’t possible to quantify the true potential of tree restoration or its impacts on carbon drawdown.
Our study is the first to explicitly link direct tree measurements to environmental characteristics and provide quantitative, spatially explicit global estimates of potential tree cover across the globe.
Our researchThe 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 20km by 20km, 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.
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, but 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 would return to grasslands and wetlands, which in their healthy states often naturally have some 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
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 major role in restoration that some of the world’s leading economies can play.
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. Approximately 10% of the countries have committed to restoring an area of land that considerably exceeds the total area that is available for restoration. Similarly, over 43% of the countries have committed to restore an area less than half the size of the area that has restoration potential. 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 in the area available for global forest restoration by 450 million hectares until 2050. This change in size of the available area is mostly due to the consistent declines of tropical rainforest and areas with high tree cover.
ActionInspiring action requires ecological tools
The world is ready to take Nature-Based Solutions to scale. We’re encouraged by new global efforts like the UN Decade on Ecosystem Restoration (2021-2030) and responsible ‘trillion tree’ initiatives including 1t.org (by the World Economic Forum) Trillion Trees (by WWF, WCS and Birdlife International) and the Trillion Tree Campaign (by Plant for the Planet in support of UNEP) 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.
Principles for Nature-Based Solutions
- Cut emissions
- Restore a diverse mix of native species
- Respect and involve local communities
- Conserve existing ecosystems
The magnitude of the global forest potential categorically places forest restoration and conservation among the most effective solutions to mitigate climate change. Our interactive maps show exactly on which region on Earth to focus on in order to have the biggest impact. Together, we can achieve this!
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? Check out our restoration page to learn more about how the Crowther Lab is supporting Restor.
Want to read the full paper? https://science.sciencemag.org/content/365/6448/76/tab-pdf