A new study warns that tree planting efforts could actually make global warming worse if undertaken in the wrong places. Published just 36 days ago, the study reveals how adding trees can paradoxically lead to heating instead of cooling.
Coincidentally, a record-breaking heat wave hits South and Southeast Asia. Temperatures as high as 44°C force schools to close, strain power grids, disrupt agriculture, and cause heat-linked health issues and deaths.
Climate change mitigation becomes more urgent now more than ever.
Where to plant trees for climate
Not all reforestation efforts are equally beneficial for the climate. In fact, existing restoration opportunity maps dramatically overstate climate benefits. Albedo changes cancel out 20-81% of the maximum potential carbon dioxide reductions.
Albedo is how much sunlight is reflected by the surface of the Earth back into space. Reflecting more sunlight has a cooling effect, while absorbing sunlight generates a warming effect.
Different types of land reflect sunlight differently. For example, snow is very good at reflecting sunlight, while forests are not because they absorb more sunlight.
The team calculated the trade-offs between albedo changes and carbon storage, providing a global map of areas where tree planting would result in net climate benefits.
Their key findings include:
- Tree planting in tropical broadleaf rainforests offers some of the biggest climate wins, with the cooling effect of carbon storage far outweighing albedo impacts in 97% of this biome.
- Drylands and northern latitudes face the highest risk of negative climate impacts from afforestation programs. Over 70% of temperate grasslands and 60% of Mediterranean forests would heat the planet if trees were planted.
- Even in the boreal forests of northern regions like Canada and Russia, over 70% of new tree cover could be offset by changes in albedo as dark trees replace reflective snow cover.
Where exactly should trees be planted to maximize the cooling effects? And how can well-intentioned tree planting efforts avoid the unintended consequences of exacerbating global warming?
The study points to tropical and subtropical moist broadleaf forests, as well as temperate forests, as prime candidates for ramped up reforestation. These regions show high potential for carbon storage with minimal albedo penalties. Grassland and savanna areas, on the other hand, should be largely off-limits for tree planting based on the findings.
In drylands and high latitudes where the calculus is more complex, a nuanced, pixel-by-pixel approach is needed. “An important future direction is higher-resolution analysis in regions with strong albedo changes,” according to the paper.
The study offers a critical wake-up call for the many companies, governments, and organizations engaged in tree planting programs as part of their climate strategies or corporate social responsibility initiatives.
Findings underscore the need for rigorous geo-mapping and climate modeling rather than scattershot planting efforts.
“Our work does not aim to dictate where tree cover should be restored,” the researchers emphasize, “but rather to help better account for net climate impacts when mitigation is a goal.”
Financing and other considerations
Even when tree planting locations are chosen carefully to maximize climate benefits, implementation challenges remain.
To plant trees for climate change mitigation, many organizations consider turning to carbon credits. However, this route is quite costly.
“Carbon credits are very expensive,” said Martín Zúñiga Reátegui of Conservation International. Normally, only big companies like Disney or Procter & Gamble can afford the certifications required, he added.
For carbon credits to be viable, rigorous monitoring, reporting and verification is needed to ensure the tree planting represents real, additional carbon removal.
Ensuring permanence over decades or centuries is also crucial but difficult. “You’re going to receive this money during the first 30 years, but with the condition that you have to save the forest for 100 years,” Martin shared.
In the case that carbon credits are not viable, there are organizations that manage the tree planting themselves. This makes it cheaper, but more time-consuming and labor-intensive. Moreover, the organization may lack the expertise needed to assess best sites and locations for tree planting.
Beyond financing, meaningful engagement with local communities is also essential.
“You find families, you find men, women, children. There is an economy inside and there is an ecosystem of different flora, fauna. It’s a mini-world inside of the forest… You have to reach an agreement with these communities.”
Other considerations for tree planting are the species of trees used, soil conditions, and project goals.
One tree planting project might have the motivation of providing timber and/or non-timber forest products, while another might be shelter from heat, sandstorms, or other weather phenomena. It is possible that for some efforts, climate change mitigation is less of a priority.