Why ‘sustainable forest management’ does not make wood a good climate alternative to fossil fuels

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It is true that cutting down trees today creates additional capacity for trees to grow in the future. But this refilling unfortunately takes a long time and means more carbon in the atmosphere for decades to centuries, writes Tim Searchinger. [Oregon Department of Forestry / Flickr]

Net forest growth is now holding down the rate of climate change, making forests an invaluable “carbon sink”. Reducing this sink by cutting down more trees adds carbon to the air and makes climate change worse just like burning any other carbon-based fuel, write Tim Searchinger and Wolfgang Lucht.

Tim Searchinger is a research scholar at Princeton University. Wolfgang Lucht is a professor at Humboldt University and the Potsdam Institute for Climate Impact Research.

The authors are among the signatories of a letter by over 600 scientists to the European Parliament, who urged MEPs to amend a “flaw” in the renewable energy directive that would allow deliberately cutting down trees to burn them for energy. 

On 14 December, a number of world-renowned scientists warned that the present version of the Renewable Energy Directive moving through the European Parliament gravely threatens the world’s forests and climate by encouraging countries to cut down trees just to burn them as “renewable energy”, instead of burning only residues and wastes.

More than 500 additional scientists, many with similar world reputations, are today sending a letter to the European Parliament with the same message.

Yet also in December, a small group of bioenergy academics disagreed. They essentially argued that it is defensible to cut down trees to burn them so long as other trees in the forest or in the country are growing enough to keep the forests in balance.

This is an intuitive and seductive response. However, the authors of the first piece, among them a former Chief Scientist of the United Kingdom, two winners of the US National Medal of Science, and prize-winning experts on global forests did not write casually.

They understand well that trees grow and that a country’s forests contain many trees. But they find that, unfortunately, this argument for burning trees is both mathematically wrong and profoundly dangerous for climate change and biodiversity.

Simple analogies explain the math problem. As a whole, EU companies are profitable. But this does not mean that an EU company losing money is also profitable. If I sit around and eat junk food all day, I am not healthy because most of my neighbours eat fibre and run five kilometres every morning.

Similarly, those cutting down trees in one location to burn them do not help the world just because trees grow elsewhere. Without the cutting, there would be more trees and more carbon in the forest overall.

It is true that cutting down trees today creates additional capacity for trees to grow on the same land in the future – in the way that emptying a bucket creates the capacity to refill it. But because burning wood is inefficient and trees regrow slowly, this refilling unfortunately takes decades to centuries even if the wood displaces fossil fuels.

Under the present versions of the Renewable Energy Directive, most Europeans alive would pay more only to increase global warming throughout their lifetimes, and future Europeans would still face the potentially irreversible consequences of thawing permafrost and melting glaciers.

In addition, the forest bucket is heavily depleted as a new paper in Nature just showed. The real challenge to rebuilding forests stocks is to reduce the effects of logging.

Yet why would it be so bad to cut down trees so long as the carbon stocks in a country’s forests or the world’s forests remain stable overall? The answer from a climate perspective is that doing so reduces or eliminates the removal of carbon from the air that would otherwise occur and is now occurring through the net growth of the world’s forests.

Known as the “forest carbon sink,” that growth is now holding down the rate of climate change. In fact, a major cause is climate change itself. Higher levels of carbon dioxide and warmer temperatures lead to more tree growth, and scientists already count this beneficial feedback in estimating future climate change.

Reducing this sink by cutting down more trees adds carbon to the air and makes climate change worse just like burning any other carbon-based fuel. Increased forest growth is required to keep Earth from warming faster and therefore not a disposable extra.

The mere fact that the carbon in forests is stored above and in the ground, rather than below ground like fossil fuels, provides no help. Roughly a third of the carbon humans have added to the air has come from vegetation and soils. And the remaining plants and soils hold enough carbon to cook the planet thoroughly if released, even without further combustion of fossil fuels.

Requiring balance between a country’s forest harvests and growth would also allow the clearcutting of large areas of tropical rainforests. Because climate change is spurring faster growth of tropical trees too, many tropical forest countries can plausibly claim to have growing forests overall even as they clear millions of hectares of forests for agriculture.

If the international rule is that cutting and burning forests in one place helps the climate so long as forests are thickening in others, then rainforest countries could cut down their trees too.

European and US forests are also re-growing because of the decline of traditional bioenergy over the last century. As fossil fuels replaced the harvest and use of wood in factories and homes, our forests recovered.

As people turned to cars and tractors, forests also regrew on tens of millions of hectares of land devoted as late as the 1930s to oats and pasture for horses. (See a history of the recovery of southern US forests and maps showing the European forest recovery as bioenergy declined.)

The fact that forests recovered after we reduced our harvest of wood for energy is hardly a reason to start burning wood again – particularly now that our energy demands are dramatically larger.

Intensive forest management does make it possible to produce more useable wood, faster on the same land (although at high ecological costs), but calculations show that even using such managed forests for fuel results in more carbon in the air for decades. (See calculations and links reproduced here.)

Expansion of managed forests has also led to high carbon loss from draining wetlands. Regardless, because highly managed forests cannot meet even the world’s existing wood demand, more wood demand for bioenergy will ultimately have to come from natural forests somewhere in the world if only to replace wood from managed forests diverted to bioenergy.

All the world’s annual, commercial wood harvest could at most provide around 3% of the world’s energy. Although the details of this one directive may seem like a small issue, the wrong vote would effectively launch a major new assault on the world’s forests. To avoid that, legislators should instead restrict the use of forests for renewable energy to the traditional sources of residues and waste.