New study says restoring trees can cut carbon levels by 25% and is the 'most effective climate change solution'

A landmark study—by researchers from Crowther Lab at ETH Zurich—is the first to quantify how many trees the Earth can support, where they could exist and how much carbon they could store


                            New study says restoring trees can cut carbon levels by 25% and is the 'most effective climate change solution'

An area of the size of the United States could be used for forest restoration, which would reduce human-made carbon emissions by nearly 25% to levels not seen for almost 100 years.

Planting trees to cut carbon levels in the atmosphere is a no-brainer. But scientists have now mapped precisely how much land is available across the world for tree restoration, which can make a significant difference to global warming.

The landmark study—by researchers from Crowther Lab at ETH Zurich—is the first to quantify how many trees the Earth can support, where they could exist and how much carbon they could store.

The study finds that there is potential to increase the world’s forest land by a third without affecting existing cities or agriculture, regrowing trees over an area the size of the US or larger than Brazil.

This, according to the team, makes forest restoration the biggest and most effective climate change solution to date. 

The researchers mapped the global potential tree coverage to show that under the current climate conditions, Earth’s land could support 4.4 billion hectares of continuous tree cover—which is 1.6 billion more than the currently existing 2.8 billion hectares.

Of these 1.6 billion hectares, 0.9 billion hectares are not being used by humans and they exist outside of urban and agricultural regions, implying that presently an area of the size of the US is available for tree restoration.

Once mature, these new forests could store 205 billion tonnes of carbon.

In other words, it could capture two-thirds of the 300 billion tonnes of carbon that has been released into the atmosphere as a result of human activity since the Industrial Revolution, say the researchers.

The figure shows the land available for forest restoration (excluding deserts, agricultural and urban areas; current forestland is not shown). (ETH Zurich / Crowther Lab)

The researchers, however, warn that climate change will change this potential tree coverage.

The findings state that the climate is already changing and every year it reduces the area of land that can support new forests. Even if global warming is limited to 1.5°C, the area available for forest restoration could be reduced by a fifth by 2050, says the paper.

The researchers explain that the need for action is urgent, and countries must act now as it takes decades for new forests to mature and achieve their potential as a source of natural carbon storage. They say it is also critical to simultaneously protect the forests that exist today, pursue other climate solutions, and continue to phase out fossil fuels from economies to avoid dangerous climate change.

“We estimate that if we cannot deviate from the current trajectory, the global potential canopy cover may shrink by approximately 223 million hectares by 2050, with the vast majority of losses occurring in the tropics. Our results highlight the opportunity of climate change mitigation through global tree restoration but also the urgent need for action,” says the findings.

The study also shows which parts of the world are most suited to forest restoration. They found the greatest potential in just six countries: Russia (151 million hectares); the US (103 million hectares); Canada (78.4 million hectares); Australia (58 million hectares); Brazil (49.7 million hectares); and China (40.2 million hectares).

Figure shows the total land available that can support trees across the globe (total of currently forested areas and forest cover potential available for restoration). (ETH Zurich / Crowther Lab)

“We have increased the atmospheric budget by about 300 gigatons of carbon. We know that trees capture that carbon. But we have never had any idea what kind of impact it could have on a global scale. That is why we have never been able to place it as one of the top climate change solutions. The (Crowther) lab actively works with multiple restoration projects globally, trying to restore ecosystems. But if the right ecological knowledge is not available, such projects will end up planting the wrong trees in the wrong soils. Making the right decisions require that ecological understanding,” say the researchers in their findings. 

The team used a methodology called “photo interpretation", which essentially refers to people zooming in on Google Earth using a standardized methodology to characterize tree canopy cover in thousands of locations across the world. They analyzed tree cover in protected forest areas largely unaffected by human activity across the Earth's ecosystems, from arctic tundra to equatorial rainforest, by studying 78,774 high-resolution satellite photographs. The group used this to determine the natural level of tree cover in each ecosystem. This was combined with the mapping software of Google Earth Engine, which was used to generate a predictive model to map potential tree cover worldwide under current conditions, says the paper. “Results show the area for extra tree restoration potential is far greater than we could have imagined. About 0.9 billion hectares are available for forest growth if we restore them effectively right now. If we convert that into carbons, it is 200 gigatons of carbon,” say the findings. 

The team says while the global map is essential for making more effective global scale restoration targets, it is equally critical for guiding local scale restoration projects by revealing what the potential for tree capacity and carbon storage is in all regions around the world. 

Because trees capture and remove carbon dioxide from the atmosphere, widespread reforestation has been considered one of the most effective weapons against climate change. According to the latest report by the Intergovernmental Panel on Climate Change (IPCC), an additional 1 billion hectares of forest will be required to limit global warming to 1.5 degrees Celsius by 2050. However, it remained unclear until now whether these restoration goals are achievable because researchers did not know how much tree cover might be possible under current or future climate conditions. The present study answers these questions and confirms that this scenario projection is "undoubtedly achievable under the current climate".

The study was published on July 5 in the journal Science.

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