Why planting some trees could make global warming worse

The observed effect of trees on climate in Europe is challenging widespread beliefs about how to mitigate climate change.

Courtesy of Ernst-Detlef Schulze
Coniferous (dark green) and broadleafed (light green) trees in summer in Alsace, France, reflect sunlight differently, challenging the belief that all trees are good for mitigating climate, French climate researchers found in a study published Thursday in the journal Science.

Not all trees are created equal when it comes to fighting climate change – and that some actually may do more harm than good, climate scientists say.

A new study, published Thursday in the journal Science, shows that an expansion of forests towards dark green conifers in Europe has stoked global warming. The findings challenge the widespread view that planting more trees helps human efforts to slow the Earth’s rising temperatures. Apparently, not all trees have the same mitigating effect.

"Two and a half centuries of forest management in Europe have not cooled the climate," a team of scientists led by France's Laboratoire des Sciences du Climat et de l'Environnement wrote.

While the area of Europe's forests has expanded by 10 percent since 1750, the continent’s summer temperatures have increased 0.12 degree Celsius (0.2 Fahrenheit). The scientists say that’s largely because many nations have planted conifers such as pines and spruces whose dark color traps the sun's heat.

Lighter-colored broad-leafed trees, such as oak or birch, reflect more sunlight. But fast-growing conifers, which are used for everything from building materials to pulp, have long outpaced them.

The authors of the study said such changes have outweighed the role Europe’s forests have played in curbing global warming by reducing their ability to absorb carbon dioxide, the main greenhouse gas from burning fossil fuels.

"It's not all about carbon," lead author Kim Naudts told Reuters, saying forest management policies should take into account factors such as their color and changes to moisture and soils. 

Since 1750, Europe's forests have gained 76,000 square miles, an area bigger than Greece, the study said. Over the same period, conifer forests expanded by 244,000 square miles while broad-leaved forests shrank by 168,000 square miles. 

The researchers calculated that the increase in temperature caused by the trees equates to 6 percent of the global warming attributed to the burning of fossil fuels. They warned that similar effects were likely in regions where the same type of afforestation has occurred, such as China, the United States, and Russia.

“Two and a half centuries of forest management in Europe have not cooled the climate,” the study concludes. “The political imperative to mitigate climate change through afforestation and forest management therefore risks failure, unless it is recognized that not all forestry contributes to climate change mitigation.”

This report includes material from Retuers.

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