Trees against climate change: the global restoration and carbon storage potential
Introduction The role of trees in climate change
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.
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.
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 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. .
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.
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 restoration to offset climate change
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.
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.
ActionInspiring action requires ecological tools
How? We suggest three different, but equally simple and straightforward ways for getting involved as a citizen:
Restore! Start planting trees – in your garden or in your local community, it’s easy to do and fun! Find all maps that guide effective restoration on www.crowtherlab.com
Support! Many organizations and NGOs around the world are actively working on the restoration of vegetation and soils. To donate to one of these organizations, visit our website to find an ever-growing list of projects around the world.
Invest wisely! The financial market has immense power to shape our society. As consumers we can make conscious decisions in supporting companies that aim for a sustainable planet when buying products or investing money – buyer power can go a long way in the fight against climate change.
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