‘Restoring tree cover may lead to global warming but only in some places’

Photo: Bridget Besaw / The Nature Conservancy

Planting trees has been considered a panacea for all environmental challenges, from pollution and biodiversity loss to climate change. But in the last few years, scientists have warned about the dangers of mindless and gimmicky afforestation practices. 

Trees indeed provide vital ecological services, but if reforestation is done improperly, it can do more harm than good. Planting the wrong species of tree can kill native flora, harm wildlife populations and deplete groundwater unnecessarily. 

Certain reforestation practices by corporates have come under the scanner for inflated decarbonisation claims and human rights violations. 

The compensatory afforestation plan for the about 900,000 trees to be felled in the tropical rainforest on the Great Nicobar island to make way for the Centre’s grand development project has also attracted criticism from environmentalists. The government had initially said that trees will be planted in the Aravallis, some 2,000 kilometres away from the project site, and later changed the location of the afforestation site to Madhya Pradesh, which is almost just as far away. Also, both the areas are very different ecologically than the Great Nicobar island, which is home to more than 300 species of flora and fauna, many of which are endemic and already endangered.

Now, a new study has highlighted that adding green cover on plains or grasslands may not necessarily reduce global warming. By lowering the albedo (the share of sunlight reflected back), it may actually cause warming, the report published in the journal Nature Communications showed. “Although previous work has emphasised the importance of accounting for albedo change, albedo is either omitted from — or only coarsely modeled in — most assessments of the mitigation potential of restoring tree cover,” the authors of the report noted. 

Natalia Hasler, research scientist at Clark University, alongside Susan Cook-Patton, senior forest restoration scientist at The Nature Conservancy and other colleagues from the United States and Switzerland, mapped the albedo changes in converting land across the globe to forests, providing a guide for future exercises in greening. These maps can help predict the net climate benefit of reforestation. 

In a conversation with Down To Earth, Cook-Patton talks about the most scientific ways of doing reforestation as well as what their map says about India. Edited excerpts:

Preetha Banerjee (PB): In your latest report, you and your colleagues have mapped the spots where reforestation will be the most climate-positive. Help us understand the science behind the findings. How did you arrive at these calculations and what do the findings mean for natural climate mitigation activities?

 

Susan Cook-Patton (SCP): The climate benefit of a reforestation project depends on more than just the carbon capture. Restoring tree cover will also alter “albedo”, which is a measure of how much sunlight is reflected from the Earth’s surface back into space. Because trees are often darker than the land covers they replace, such as grasslands, this can lead to some degree of planetary warming. Sometimes, this albedo-driven warming will outweigh the cooling that comes from increased carbon capture. When this happens, reforestation leads to global warming. But this only happens in some places.

Scientists have known for a long time that changes in albedo will impact the climate mitigation benefits of reforestation, but before our work there were no tools to properly account for albedo change. We first used well-established albedo maps to determine how albedo would change if land transitioned from four different open land covers to six different forest cover classes. These 24 maps are available for reforestation projects that know what the land looks like when they start and what type of forest they are establishing. But to look at general patterns, we used a data-driven approach to predict the most likely open land to forest transition for every spot on Earth, and assembled an albedo change map that reflected those land cover transitions. This gives us a single “global albedo change” map that we can then compare with a carbon map to find the places where carbon benefits far outweigh albedo change and thus represent good places to restore tree cover for climate mitigation (blue land areas in the maps). These tend to be more humid tropical forests where trees capture a lot of carbon. It also shows us where albedo outweighs carbon benefits (orange areas in the map).

PB: In the last couple of years, the efficacy of tree planting for climate mitigation became a topic for public scrutiny. While the benefits of reforestation are recognised globally, what are the disadvantages? What is your prescription for a truly climate-positive reforestation initiative?

SCP: It is important to remember that there are many good reasons to plant trees — or even let them naturally regrow when the conditions are right. Trees can help to provide clean air and clean water. They can provide essential habitat for biodiversity. They can provide local cooling benefits through the shade they offer and the water vapor released by their leaves. But reforestation does not work everywhere. For example, trees in native grasslands can harm biodiversity. Trees in arid landscapes can exacerbate water scarcity. It really comes down to putting the right trees in the right places to achieve the most benefits for people and nature. Projects that occur in places that we identify as climate negative will not be providing climate change mitigation, but can still provide a whole host of benefits. We do not intend to criticise reforestation efforts writ large, but instead help target reforestation efforts towards the places that will have the greatest climate benefit per hectare of investment when climate change mitigation is the main goal.

As to reforestation versus other climate solution, the fact of the matter is that we will need to deploy all feasible climate solutions to tackle climate change. Reducing fossil fuels is the most important action, but even if we were to stop all emissions today we would still need to remove additional carbon from the atmosphere. Reforestation remains one of the most cost-effective and scalable solutions we have for carbon removal.

PB: In your new report, what observations did you make for India? What are your suggestions for the country in terms of reforestation?  

SCP: The maps we created are all publicly available and can be accessed via our paper. What we found in India is that spatial patterns are quite complex and show how important it is to have detailed maps of the places that are climate beneficial versus climate negative. And that there are both very climate positive locations and climate negative locations within India. Our hope is that decision makers will use these maps to find the most climate positive locations for reforestation when climate mitigation is the principle goal, and that carbon project developers will now include albedo change in their climate accounting ledger to get a more accurate estimate of the overall mitigation potential of their projects.

This map shows the expected net climate impact from restoring tree cover, after accounting for both albedo change and carbon storage. Orange colors indicate net climate-negative locations, whereas the blues indicate net climate-positive. (Adapted from Hasler et al, 2024 Nature Communications.)

PB: If you had to assess India’s reforestation efforts of the past decade, how would the report card look? 

SCP: I would not attempt to grade India’s reforestation efforts, but instead ask what the intended outcomes were from the reforestation efforts. If the primary goal is climate mitigation, our maps can help show whether past efforts have generally occurred in climate positive locations and provide a method to calculate a more precise climate outcome. At the global level, we were pleased to find that the vast majority of reforestation projects were already occurring in climate positive locations. These are projects implemented for all kinds of reasons — not just climate change mitigation — so it is an encouraging sign that through good luck and / or good planning, reforestation projects are already falling in places that are good for climate change mitigation.

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