‘Plant a Tree, Save the Planet’ Isn’t Enough

(Bloomberg Opinion) -- Limiting climate change means curbing greenhouse-gas emissions, it's well known. But controlling emissions won't be enough by itself to stop global temperatures from warming uncomfortably beyond 1.5 degrees Celsius. Carbon dioxide already in the air needs to be cleared away too. By the end of this century, up to a trillion tons of carbon dioxide will have to be removed, according to the Intergovernmental Panel on Climate Change.

One effective way to do this is to expand forests and woodlands, which soak up greenhouse gases. Indeed, if enough natural tree cover can be added, it can accomplish one-quarter of the needed carbon dioxide sequestration.

A global effort toward reforestation has begun. Dozens of countries have joined the Bonn Challenge, launched in 2011 with a goal of restoring 350 million hectares (more than 1.35 million square miles) of forest by 2030. Participating countries have already pledged nearly half the required area. But a new analysis by British scientists has revealed a big problem: The countries' pledges are not nearly as ambitious as they need to be.

Too many countries seem to assume it will be good enough to merely expand commercial tree plantations or agroforestry operations, which involve planting trees amidst crops such as coffee or corn. Only one-third of the area so far committed to “reforestation” worldwide is slated to return to uncultivated forestland.

Yet natural forests are the only kind that trap ample quantities of carbon dioxide. Plantations absorb little more than empty land does. And every time trees are harvested, more carbon dioxide is released.

Natural forests are six times as effective as agroforestry at storing carbon and 40 times as effective as plantations, scientists at University College London and the University of Edinburgh have calculated. Thus, the current country pledges, if carried out, stand to accomplish only a fraction of the carbon sequestration that could be managed if all the lands in question are left to return to wild forestland.

Especially beneficial are reforestation efforts in the tropics and subtropics, where land is relatively cheap and trees grow fast. Forests near the equator also never shade snow, which helpfully reflects sunlight.

Expanded forests provide additional benefits: As home to two-thirds of all species, natural forests help maintain the planet's biodiversity. By filtering rainwater, they improve water quality. And reforestation creates jobs — in the reforesting work itself, for example, and in outdoor recreation.

Governments worldwide need to become realistic about the amount and kind of tree cover that must be restored and protected to stave off climate change. The urgency of reforestation, like that of emissions reduction, leaves little time for delay.

Editorials are written by the Bloomberg Opinion editorial board.

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