Trees do wonders when it comes to cooling Earth. They suck planet-warming carbon dioxide from the atmosphere, locking it into their trunks, leaves, and roots to the tune of about 2.4 billion metric tons of carbon each year. But a new study has found, somewhat counterintuitively, that more trees might not always mean a cooler planet. In fact, researchers examining 260 years of changes in European forest management found that—despite a 10% increase in wooded land—the continent’s forests have actually caused a slight increase in regional temperatures since 1750.
The new findings show that simply planting trees—at least in temperate areas—isn’t enough to stave off global warming, says Vivek Arora, a research scientist at the Canadian Centre for Climate Modelling and analysis in Victoria, who was not involved in the research. “[This] is not the silver bullet that will save us from climate change.”
But how did more forest coverage lead to warming? Researchers from the Laboratory of Climate Science and Environment in Gif-sur-Yvette, France, came to this conclusion by building a model that uses 260 years of forestry data in Europe, from the distribution of tree species to the methods people have used to harvest wood. From 1750 to 2010, the continent added almost 200,000 square kilometers of forest, and created a 0.12°C rise in temperature.
The researchers pin most of the temperature increase on a shift from broad-leaved tree species like oak to more economically valuable conifer species like spruce and pine, they write this week in Science. “By changing the forest, we also make changes to the amount of radiation, water, and energy that the forest releases,” says lead author and environmental scientist Kim Naudts. The conifers are worse for the climate because they absorb more light with their dark color, trapping heat that would otherwise be reflected back into space. They also release less cooling water into the atmosphere through evaporation. Together, these two factors were to blame for 0.08°C of the region’s warming. Foresters removing trees for wood products contributed another 0.02°C by releasing carbon that would otherwise be stored in forest debris and soil.
Previously, most models focused on big changes between different land types, like farmland and forests. The model created by Naudts and her colleagues drills deeper, and examines how the forests were actually used. For example, the new model includes a historic 3D representation of the forest canopy, allowing researchers to see differences in how various tree species interact with the atmosphere. Naudts’ team also included the removal of trees for wood products or fuel. “The model tries to look at how management, which would thin the forest but not change it entirely, would affect the climate,” Arora says. “That’s new.”
But there is already an understanding among researchers that carbon sinks—areas that sequester carbon—interact with the atmosphere in complex ways, not all of them positive for climate change. “The fact that not all land-use and land-cover change policies may contribute to climate change mitigation is not something new,” says Giorgio Matteucci, a forest ecologist at the Institute of Agriculture and Forestry Systems in the Mediterranean in Ercolano, Italy, who was not involved with the study. For instance, a study published in Nature more than 20 years ago showed that expanding forests in colder areas could actually increase the temperature of snow-covered regions, because snow reflects much more light than dark trees.
Other researchers caution that we shouldn’t read too much into the 0.12°C measurement. “The paper makes sense and these results are consistent with what I would expect,” Arora says. But he adds that this is only one of many possible models. “If a different model were to use the same [parameters], it might find different results”.
It’s also tempting to extend these results to other regions. But Europe’s temperature increase was in large part due to the continent’s specific history of forestry, its location, and the kind of tree species that are present there. The tropics, especially, play by different rules—there, slowing deforestation is almost certain to contribute to cooling, because trees in the tropics release comparatively more water into the atmosphere, seeding clouds that reflect light. The European model does indicate, however, that we should be cautious about the promise of forests to solve our climate woes.