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

Logo https://crowtherlab.pageflow.io/trees-against-climate-change-the-global-restoration-and-carbon-storage-potential?utm_source=Twitter&utm_medium=Toolkit&utm_campaign=TreePotential

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Millions marching the streets and daily articles in every newspaper: never before has the topic of climate change been so omnipresent. The problem: We only have approximately 12 years until we use up the carbon budget that keeps us below 1.5 C outlined in the UN’s latest special report from the Intergovernmental Panel on Climate Change (IPCC). Humans have emitted an additional 300 Gt of carbon (ca. 1,000 Gt of CO2 equivalent) into the atmosphere since the industrial revolution. But how do we make sure we stay within that budget? 

Notable organizations such as Project Drawdown have been compiling a list of strategies. Their solutions span from electricity generation, to transport, food, education and land use, with effective refrigeration management when recycling or disposing of refrigerators topping the list with a potential to save 24 billion tonnes of carbon emissions. This is followed by converting to a plant-rich diet on a global scale that could save up to 18 billion tonnes in future carbon emissions. But while these strategies are vital to reduce the annual average of 10 Gt of carbon emitted into the atmosphere due to human activities, they can only prevent future emissions. To stop climate change, we must draw down the carbon already in the atmosphere. For that we will need an immensely powerful system.
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Ecologically speaking, trees are the most effective means to capture and store carbon. Consequently, a number of international initiatives (e.g. the Bonn Challenge, the Land Degradation Neutrality, REDD+ and the Trillion Tree Campaign) have established ambitious targets to promote forest conservation, afforestation and restoration (AFR) at a global scale. Indeed, the latest special report from the Intergovernmental Panel on Climate Change (IPCC) explores a number of future scenarios that can limit climate change. These propose that an increase of up to 1 billion hectares of forest – a quarter of the current forest area – will be necessary to limit global warming to 1.5° C by 2050.  

However, until recently, it remained unclear if these restoration goals are within reach or ambitious enough. This is partly because we 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, we have had no quantitative information about how much carbon these restored trees could capture. Without any scientific evidence, we have not been able to quantify the true contribution of forest restoration, and we don’t know whether this could capture an extra 10 or 110 gigatons of carbon. 

Despite the critical importance of evaluating the global tree cover potential, this topic has received relatively little scientific attention up until now. 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 distributed 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 ‘random forest’ 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 span the entire range of environmental conditions – from the lowest to the highest possible tree cover – we were then able interpolate these natural tree cover estimates across the globe to generate a predictive understanding of the potential tree cover in the absence of human activity. For this, we calculated the potential “continuous tree cover” equivalent. We refer to “continuous tree cover” as the proportion of the land that is covered by tree crown area vertically projected to the ground. 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 the 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.
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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

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Our maps reveal that under the current climate conditions, Earth’s land could support 4.4 billion hectares of continuous tree coverage. That is 1.6 billion more than the currently existing 2.8 billion hectares. Using the European Space Agency’s global land cover model, we estimate that 0.9 billion out of these 4.4. billion hectares are outside of areas currently used for human development, such as current urban and agriculture land. This means 0.9 billion hectares are the area available for tree restoration could store 205 Gt of carbon.  

Of this 0.9 billion, 229 million hectares exist in Boreal regions, 217 million hectares in Temperate regions, 125 million hectares in Subtropical and 329 million hectares in Tropical biomes. Additionally, we see that more than half of the total tree restoration potential can be found in only 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 must play.


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Our model suggests that the global forest restoration requirement of 1 billion hectares of forest proposed by the IPCC is achievable. Of course, carbon capture associated with global restoration is not instantaneous as it takes several decades for restored areas to reach the same level of maturity of existing forest. But, in contrast to existing trees that are mainly able to handle current emissions, the additional tree cover would be able to tackle the problem of atmospheric carbon. We estimate that the 0.9 billion hectares of restored tree cover would reduce the excess atmospheric carbon pool of 300 gigatons by 2/3 to almost as low as those observed in pre-industrial times.

However, there are two notable limitations to be considered:  

Even though our results show that global forest restoration targets are indeed achievable, 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 that is less than 50% of the area available for restoration. At the moment, it is unclear what proportion of the potential restoration area per country is publicly or privately owned, but this aspect just 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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By breaking down the global carbon cycle into simple, tangible numbers, our approach provides an entirely new perspective on climate change mitigation strategies. Not only is restoring ecosystems a natural solution that does not require technologies that may never be invented or effective, it is also one of the cheapest strategies to date. It does not require governmental policies or scientific discoveries that may never come. It just requires the action of all of us, getting involved to promote the restoration of native ecosystems around the world.

How? We suggest three different, but equally simple and straightforward ways for getting involved as a citizen:
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Of course, any governmental support in the fight against climate change is still pivotal and sets a positive example for other countries. It also is a clear sign of support for the citizen action. Countries such as Australia and New Zealand have already pledged to plant one billion trees each – our maps can guide these restoration projects to be effective and targeted in order to achieve the greatest, fastest and most sustainable impact.
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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!

Want to find out more? Read the full paper here: https://science.sciencemag.org/content/365/6448/76/tab-pdf
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