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Trees against climate change: the global restoration and carbon storage potential


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For centuries, nature has provided the foundation for healthy societies and strong economies. Yet, we live at a moment in history when pristine ecosystems are at an all-time low and we face the long-standing global environmental crises of biodiversity loss and climate change. The problem is clear: each year, humans emit another 11 Gt or so of carbon into the atmosphere (Global Carbon Budget 2019). At current rates, the world could use up the carbon budget that keeps us below 1.5 C as soon as 2030 (2018 IPCC report). So, how do we stay within that budget?

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.
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This is where trees come in. Like all plants, trees capture carbon from the air for growth and some is released into the soil where it can stay for hundreds or even thousands of years. International efforts such as the Bonn Challenge and New York Declaration on Forests established ambitious targets to promote forest conservation and restoration at a global scale. Indeed, the 2018 IPCC Special Report on the impacts of global warming explores a number of future scenarios that can limit climate change and proposed that increasing forest cover by up to 1 billion hectares of forest by 2050 – a quarter of the current forest area – could be a valuable part of mitigation portfolios to limit global warming to 1.5° C.  

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.
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According to the Food and Agriculture Organization of the United Nations (FAO) a forest is defined as land covered by at least 10% tree canopy cover and without human activity. Whilst this definition has proven to be useful for global restoration targets, it lacks some of the ecological detail needed to learn more about any particular forest system. For example, a forest with 10% tree canopy cover can have very different ecological characteristics to a denser forest with 70% or 80% tree canopy cover.

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.
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With enough observations that spanned the entire range of environmental conditions – from the lowest to the highest possible tree cover – we were then able to interpolate the natural tree cover across the globe to generate a predictive understanding of the global potential tree cover in the absence of human activity. For this, we calculated the potential “continuous tree cover” equivalent (i.e. the proportion of the land that is covered by tree crown area vertically projected to the ground) which is different from forest area. By accounting for all levels of tree cover, this approach balances the relative contribution of different forest types (e.g. woodlands, open forest, dense forest) across the globe. It also allows us to account for more fine-scale differences between current and potential tree cover, irrespective of the forest definition.

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.
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Before/after view

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

Start before/after view
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Earth currently has 2.8 billion hectares of continuous tree cover. Our maps reveal that under the current climate conditions, Earth could have the biophysical capacity to support an additional 1.6 billion hectares of tree cover. Of that amount, we estimate that there might be 0.9 billion hectares of land available for new tree cover outside of existing forest, urban and agricultural areas (as defined by the European Space Agency’s global land cover model). 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 a staggering 206 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 major role in restoration that some of the world’s leading economies can play.

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The scale of estimated global tree restoration potential supports the idea that the restoration of trees is a powerful natural carbon drawdown tool. Of course, carbon capture associated with global restoration is not instantaneous. It takes many decades for restored areas to reach maturity. All the more reason to start now.

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.
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Slowing climate change is an infinitely complex challenge which requires holistic action. We have to drastically reduce global emissions and draw existing carbon out of the atmosphere. By breaking down the global carbon cycle into tangible numbers, our approach provides an entirely new perspective on natural climate change mitigation strategies. Ecosystem restoration also has tremendous benefits for biodiversity and human well being. Moreover, it’s an opportunity for all of us to get involved.

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 (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.

How? Responsibly.
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Following the publication of the tree restoration potential paper, we helped convene a global coalition of leaders in science, conservation, restoration, development and sustainability around four high-level 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 wellbeing:
  • Cut emissions
  • Restore a diverse mix of native species
  • Respect and involve local communities
  • Conserve existing ecosystems
 Read the full statement here.
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The Global Tree Restoration Potential paper showed the magnitude of what’s possible. Responsible tree restoration is one of the large-scale, lower-cost carbon drawdown tools we have at our fingertips today. Moreover, it offers tremendous co-benefits to help address biodiversity loss and restore nature’s ecosystem services - from clean air and water to disease suppression and 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? Check out our restoration page to learn more about how the Crowther Lab is supporting Restor.

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