Guest post: Mapping where tree-planting has the greatest climate benefit
Restoring tree cover is now firmly established as a strategy for removing carbon from the atmosphere to help tackle climate change.
But there is an elephant in the room when it comes to estimating just how promising a climate solution it is in different locations. This is “albedo” – the fraction of the solar radiation that is reflected from the Earth’s surface.
In essence, brighter surfaces – such as a large snowy expanse or a grassland – will generally reflect a high proportion of sunlight back into space. Trees, meanwhile, tend to be darker coloured and absorb more sunlight, keeping it on Earth – usually in the form of excess heat.
Because restoring tree cover often involves replacing brighter land covers – such as grasslands – with darker ones – namely, trees – this can lead to some degree of global warming.
In some locations, this warming can partially or even completely outweigh the benefit of increased carbon uptake by the trees. Many know of this problem, but it has been difficult to quantify the impact of albedo in specific locations.
In our new study, published in Nature Communications, we map albedo change from restoring tree cover and show that carbon-only estimates of the global climate benefits of tree-planting may be 20-81% too high.
Our maps reveal that the climate benefits of tree-planting in savannahs in Africa and central Asia would be the most reduced by albedo. But we show that it is possible to find places that provide net-positive climate mitigation benefits in all biomes.
§ Tree cover affects albedo
It is getting harder to ignore albedo when planning projects to restore tree cover for climate mitigation.
For example, a recent study published in Science showed that albedo, among other factors, could substantially reduce the climate mitigation benefit of restoring tree cover.
However, despite its importance, albedo is often only given a brief mention as an important factor in research attempting to quantify the climate benefits of restoring tree cover. Its impact is frequently not accounted for – or only via coarse adjustments.
In some places, restoring tree cover modifies albedo enough to dwarf smaller changes in carbon, leading to an overall (net) increase in global warming. In other locations, the impact of albedo does not outweigh the carbon removal, contributing to an overall global cooling effect.
Understanding and quantifying these variations in albedo and carbon change is crucial to the success of a project that aims to restore tree cover for climate mitigation.
Yet there has been a lack of tools to provide this information. Our study sets out to change that.
§ Mapping albedo change
Our study provides the maps that quantify the absolute and relative changes in albedo anywhere on Earth where we might grow trees.
We first created a series of 24 maps that quantified how albedo would change if an area transitioned from one of four open land cover classes – such as grassland or croplands – to one of six different forest-cover classes, such as deciduous broadleaf or evergreen needleleaf forest. These are useful for individual projects that know their starting and end conditions.
However, to examine general global patterns, we used a data-driven approach to model the albedo change resulting from the “most likely” open-to-forest transition for each part of the world. We then combined that with a map of maximum potential carbon storage to map net climate impact in carbon dioxide equivalents.
In this map (below), red and orange shading indicates regions where restoring tree cover leads to net warming and blue indicates regions where restoring tree cover leads to net cooling.
The map shows that, in many places, increasing tree cover is likely to contribute to global warming. These include the dryland ecosystems of central Asia and the Sahel region of Africa, as well as northern reaches of North America, Europe and Asia.
However, all biomes had at least some climate-positive locations, indicating that the coarse exclusions used in the past have missed opportunities. Moreover, some locations experience little to no albedo change, such as in south-east Asia, central Africa and the Amazon.
This map makes it possible for people to determine the best places to restore tree cover to achieve climate mitigation, as well as evaluate different scenarios of where restoration of tree cover might happen.
For example, we examined three previously published global studies of large-scale increases in tree cover. We find that, after accounting for albedo, the global climate mitigation benefit of restoring tree cover may actually be 20-81% lower than expected from carbon-only estimates.
Notably, the study with the greatest deduction included large areas of tree-planting within the tundra and other locations where we predict very negative climate outcomes. We show that constraining this study’s tree-planting to only the more climate-positive areas – about a third of the total area (311m hectares instead of 889m hectares) – would lead to a 2.5-fold increase in mitigation potential.
This demonstrates the value of strategic project placement to maximise climate benefit, because it is possible to achieve more mitigation with less investment of space.
§ Forest restoration projects
Encouragingly, our study also finds that hundreds of thousands of on-the-ground tree-planting projects tend to be concentrated in places where the potential for carbon removal is high and albedo change is moderate.
One example is the moist tropical ecosystems in Brazil and Indonesia. Most of these on-the-ground projects can be found at Restor, a data-driven and community-based platform that aims to accelerate restoration and makes it possible for the first time to evaluate outcomes of the global restoration movement.
This suggests that ongoing or planned projects are concentrated in places that are good for achieving climate mitigation. However, the majority – around two-thirds – of these on-the-ground projects still face an albedo offset of at least 20%, indicating that most – if not all – projects should consider albedo change in their accounting.
None of this is to criticise projects that fall in places with negative climate outcomes. There are many wider reasons for restoring tree cover in a given landscape, beyond climate mitigation, including cleaner water, wildlife habitat, stabilised soils, sustainable livelihoods and cooler local temperatures.
However, for projects where the emphasis is on achieving climate mitigation, it is important to consider changes in albedo alongside changes in carbon removal, especially now that the tools are available to do so.
In general, climate accounting is not for the faint of heart. There are many factors such as albedo that can alter the total climate mitigation of natural climate solutions. However, we are in a critical time when pragmatic decisions need to be made now about which climate solutions to deploy and where.
Alongside our study, we have produced a dedicated web platform – called “naturebase” – to help policymakers, practitioners, communities and governments identify where, why and how to implement nature-based projects with the highest carbon mitigation.
This tool includes maps, data and case studies to show how different natural climate solutions – including restoration of tree cover – could benefit the climate across the world.
Policymakers and land managers are under growing pressure to make complex choices in line with global agreements. We hope that the science in our study and the tools in the naturebase platform will help enable smarter, more nature-positive decisions.