Science Snippets: Planting Trees in Some Locations Warms Earth

Draft script:

A paper in Phys.Org published 31 March 2024 is titled Planting trees in wrong places heats the planet: Study. The study referenced in the title appeared in Nature Communications on 26 March 2024. It was produced by 11 scholars.

One of the study’s co-authors was quoted in the Phys.Org article: “There are some places where putting trees back leads to net climate negative outcomes.” In fact, the peer-reviewed paper indicates that reforestation projects that failed to include albedo in the analysis overestimated the benefit to the climate by 20 to 80%.

Hot, humid environments such as the Amazon Basin and central Africa store abundant carbon with little change to albedo. These are ideal places for reforestation. On the other hand, temperate grasslands and savannas sequester and store relatively little carbon and can negatively alter albedo. As a result, promises of enhanced carbon capture tend to be overblown in these areas.

The co-author quoted above indicated in the Phys.Org article that even projects in the best locations were probably delivering 20% less cooling than estimated when albedo changes are included in the analysis. Although there are good reasons beyond carbon sequestration to plant trees, it is important to focus reforestation efforts in proper locations. Given the limited money available for reforestation projects throughout the world, the Phys.Org article and the peer-reviewed paper indicate the importance of prioritizing projects.

The peer-reviewed, open-access paper in Nature Communications is titled Accounting for albedo change to identify climate-positive tree cover restoration. It was published 6 March 2024. I’ll read the entire Abstract: “Restoring tree cover changes albedo, which is the fraction of sunlight reflected from the Earth’s surface. In most locations, these changes in albedo offset or even negate the carbon removal benefits with the latter leading to global warming. Previous efforts to quantify the global climate mitigation benefit of restoring tree cover have not accounted robustly for albedo given a lack of spatially explicit data. Here we produce maps that show that carbon-only estimates may be up to 81% too high. While dryland and boreal settings have especially severe albedo offsets, it is possible to find places that provide net-positive climate mitigation benefits in all biomes. We further find that on-the-ground projects are concentrated in these more climate-positive locations, but that the majority still face at least a 20% albedo offset. Thus, strategically deploying restoration of tree cover for maximum climate benefit requires accounting for albedo change and we provide the tools to do so.”

That second sentence is particularly germane: “In most locations, these changes in albedo offset or even negate the carbon removal benefits with the latter leading to global warming.” In other words, most reforestation projects intended to store carbon have the opposite effect. Rather than storing carbon, most reforestation projects are sources of atmospheric carbon.

The Introduction of the peer-reviewed paper cites several peer-reviewed papers in producing this information: “Restoring tree cover to places that would naturally support trees is a prominent strategy for removing carbon from the atmosphere and tackling the climate crisis. However, the net climate impact of restoring tree cover depends on more than carbon sequestration; it also alters albedo, which is the fraction of solar radiation reflected from the land surface back to the atmosphere. Because tree cover often absorbs more solar radiation than other land covers, this can lead to local and global warming. In some locations, global warming from albedo change can partially or even completely countervail the cooling benefit of increased carbon storage in trees.

The Results section of the peer-reviewed paper includes especially telling information, which is contrary to conventional wisdom. This paragraph references two figures and two tables contained within the paper: “Contrary to previous work that suggests the greatest albedo concerns are in the boreal, we find that drylands have a greater proportion of net climate-negative areas. In particular, 72% of the temperate grasslands, savannas, and shrubland biome would be climate-negative, and 83% of the biome would experience a substantial albedo offset. Across the Mediterranean forests, woodlands, and scrub biome, 60% of the area would be climate-negative … and across the tropical and subtropical grasslands, savannas, and shrubland biome 38% would be net climate-negative. In comparison, 34% of the total area in the boreal forest biome would be net climate-negative, but 72% would experience a substantial albedo offset. Thus, despite the lower proportion of net climate-negative areas in the boreal relative to these dryland settings, changes in albedo remain a concern across most of the boreal. At the other end of the spectrum, only 3% of the total area in the tropical and subtropical moist broadleaf forests biome is predicted to be net climate-negative.”

The Discussion section of the peer-reviewed paper cites several other papers in reaching this conclusion: “[A]lthough we focus on climate change mitigation in this paper, there are many additional benefits from restoring tree cover, such as creation of habitat, improved livelihoods, enhanced hydrological benefits. Moreover, removal of atmospheric CO₂ confers additional environmental benefit by helping to mitigate ocean acidification.”

The final paragraph of the Discussion provides an optimistic conclusion: “Restoring tree cover is not a panacea for climate change. It is also critical to reduce fossil fuel emissions and protect intact ecosystems. However, restoring tree cover remains a promising natural climate solution for removing carbon dioxide from the atmosphere if it is located in climate-positive locations. Our work shows the need to account for albedo change when restoring tree cover for climate change mitigation and provides the tools to do so in a robust and spatially explicit way.”

It’s long been obvious that “restoring tree cover is not a panacea for climate change.” However, concluding that “[it] is also critical to reduce fossil fuel emissions” fails to acknowledge the aerosol masking effect. The reference to protecting intact ecosystems is obvious and therefore unnecessary.

I agree that the change in albedo is important when identifying relevant projects. It is also important to acknowledge aerosol masking when addressing climate change. Very few peer-reviewed articles include this information.