Mapping Where Planting Trees Can Help with Climate Change Mitigation

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One argument for combating climate change is that perhaps the easiest solution, and potentially least painful financially, would be to plant many trees around the world to absorb the excess carbon and reverse tree loss and high levels of CO2. The Intergovernmental Panel on Climate Change has estimated nearly 1 billion of hectares of trees are needed to reduce climate change to about 1.5°C. The challenge, however, is where to put those trees. A new study indicates where these trees can be planted.

In the new study,[1] rather than focusing on foresting urban areas or agricultural land, the authors assume regions with urban areas and agricultural land should be excluded for large scale tree planting, as those are needed for habitation and food. Looking at different current land use areas, using photo-interpretation, and analysing regions across the globe, where the research uses downloadable imagery from sources such as Google Earth and Bing Maps, where high resolution and lower resolution data are used together, the authors were able to assess regions for suitability for reforestation. The resolution of the derived map using the global dataset is at 30 arc seconds and is useful in providing a range, from 0-100 percent, on the total percentage of trees that could be covered on given land based on current land use and environmental factors such as rainfall.

Researchers mapped out where the Earth currently and could potentially support 4.4 billion hectares of forest.  The gray areas represent surfaces of the Earth that cannot support tree growth such as deserts and densely developed areas.  Source: Bastin et al, 2019.

Researchers mapped out where the Earth currently and could potentially support 4.4 billion hectares of forest. The gray areas represent surfaces of the Earth that cannot support tree growth such as deserts and densely developed areas. Source: Bastin et al, 2019.

The model the apply to the derive land cover potential for forestation uses a random forest machine-learning approach which estimates potential for tree cover in any given region. From this, the authors estimate that we can easily add about 0.9 billion hectares, and up to 1.6 billion hectares, of forests without affecting our habitation and agriculture, which would be about 500 billion more trees. Overall, Russia, the United States, Canada, Australia, Brazil, and China are the most likely countries where trees can be planted at a large scale, which alone could cover 50% of the trees needed. By taking this number of trees, 200 gigatonnes of carbon materials could be added to forest cover. The end result could be a reduction of 25% for global CO2, potentially making enough of a contribution to limit temperature increase to no more than 1.5°C. In effect, there is a potential by simply planting enough trees in areas that can currently have trees without disturbing our economic systems and habitation to blunt major climate change. However, forestation and adding trees would need to be done soon, as in the coming years the land available for planting trees becomes reduced due to increasing climate change and land use change. Overall, the globe could support 4.4 billion hectares of trees.[2]

Map showing tree cover potential for areas where new forest growth can occur (removing existing forest areas, agricultural and urban land cover).  This potential area for new tree growth represents  0.9 billion hectares.  Source: Bastin et al., 2019.

Map showing tree cover potential for areas where new forest growth can occur (removing existing forest areas, agricultural and urban land cover). This potential area for new tree growth represents 0.9 billion hectares. Source: Bastin et al., 2019.

Not all scientists agree with the results, particularly about the fact that other actions could also be effective in limiting climate change, such as reducing overall CO2 emissions. Furthermore, trees are complex, where they can both reduce but also increase heating through their respiration, depending on the local climate and environmental conditions. Nevertheless, the study does demonstrate the potential of different land use potential at a global scale that can accommodate new tree growth, which would have the potential to reduce overall  CO2.[3]

Other studies do caution that biomes are not always suitable for reforesting or planting trees. Take, for example, grasslands in many regions, where grasslands provide important habitats for wildlife. Afforestation, that is planting trees in areas that did not historically have trees, can be devastating and have unforeseen consequences given that a mixture of biomes are important for environmental balance and species across the globe. However, reforestation could be the best option, where areas that should historically have trees could be planted with new trees to carbon sequester.[4]

Furthermore, there may need to be a consideration of planting trees in urban and agricultural areas, including creating more mixed use land. Trees can also help reduce overall pollution. Trees emit different gasses and affect the microclimate and air turbulence in ways that can improve air quality in cities. Thus, to make urban regions healthier with cleaner air, while also possibly reducing  CO2, scientist also suggest that tree planting occurs in urban areas.[5]

The new study on tree cover and climate change has the potential to provide an effective solution for combating global warming. Other scientists warn us that solutions required are more complex than this and a combination of approaches are likely needed, ranging from overall CO2 reduction to planting in areas that should naturally have tree cover to also planting trees in urban regions to reduce pollution as well as combat climate change.

References

[1]    For more on reducing greenhouse emissions using tree planting and the location of where to place these trees, see: Bastin, Jean-Francois, Yelena Finegold, Claude Garcia, Danilo Mollicone, Marcelo Rezende, Devin Routh, Constantin M. Zohner, and Thomas W. Crowther. 2019. “The Global Tree Restoration Potential.” Science365 (6448): 76–79. https://doi.org/10.1126/science.aax0848.

[2]    For more on specific areas around the world that can be covered by additional trees, see:  https://www.crowtherlab.com/maps-2/

[3]    For more on a press piece related to the Science article on global tree coverage, see: https://www.bbc.co.uk/news/science-environment-48870920.

[4]    For more on reforestation and afforestation, see:  Veldman, J. W., G. E. Overbeck, D. Negreiros, G. Mahy, S. Le Stradic, G. W. Fernandes, G. Durigan, E. Buisson, F. E. Putz, and W. J. Bond. 2015. “Tyranny of Trees in Grassy Biomes.” Science347 (6221): 484–85. https://doi.org/10.1126/science.347.6221.484-c.

[5]    For more on environmental benefits from trees, see:  Grote, Rüdiger, Roeland Samson, Rocío Alonso, Jorge Humberto Amorim, Paloma Cariñanos, Galina Churkina, Silvano Fares, et al. 2016. “Functional Traits of Urban Trees: Air Pollution Mitigation Potential.” Frontiers in Ecology and the Environment14 (10): 543–50. https://doi.org/10.1002/fee.1426.

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