A new report from the Potsdam Institute in Germany shows that planting trees and other plants to absorb carbon dioxide from the atmosphere cannot substitute for cutting carbon emissions.

Growing trees and other kinds of "biomass" have been thought of as an effective countermeasure against our rising global carbon emissions. In fact, countries that preserve forests or green spaces can receive carbon credits that they can trade or sell to other countries that are polluters. 

The researchers looked at several scenarios. One was the the "business-as-usual" scenario, in which greenhouse gas emissions continue to rise at current rates, and which scientists fear could lead to a global average temperature rise of 4.5 C by 2100. They found that if we want trees to absorb all that extra carbon, even if we converted all of our agricultural land to biomass cultivation, it cannot be done without experiencing the "most dire consequences for food production or the biosphere."

Even if emissions are reduced to meet the levels agreed to in the UN Paris Agreement, which aim to keep the planet's average temperature within 2 C of pre-industrial levels, tree planting alone is still not enough to reach that goal. While it will be an important player in combating climate change, absorbing carbon with plants will have to be just be one of several mitigation strategies.

Borneo rainforest

A mature tropical rainforest, like this one in Borneo, stores carbon, but doesn't absorb very much. (T. R. Shankar Raman, cc-by-sa-4.0)

Trees and other plants do indeed absorb carbon dioxide from the atmosphere in order to build their tissues, releasing oxygen in the process. Along with phytoplankton in the oceans, this is the system that has built oxygen levels in the atmosphere so that animals like ourselves could thrive on Earth. But not all trees absorb carbon at the same rate. Counterintuitively, the iconic rainforests, with huge mature trees supporting towering canopies of green, do not absorb as much carbon as younger, faster growing forests, because the big trees have slowed their growth and are not turning as much carbon into new wood and organic material.

Part of the concept behind intentionally using trees and plants as carbon sinks is to create enormous plantations of fast-growing species such as poplars, or in more rugged terrain, switchgrass.  As they grow, they draw carbon dioxide out of the atmosphere and turn it into wood, stems and leaves. Once these plants reach maturity, they would be harvested so more young trees take over, providing a constant source of new carbon-absorbing growth.

Switchgrass

Switchgrass is a popular choice as a fast-growing, robust biomass crop. (S E Wilco, cc-by-sa-2.0)

There are just two issues with this scenario. When the scientists calculated the amount of land needed for these plantations, it turned out to equal all of our existing agricultural land and about half of the world's natural forests. That leaves us with ridiculous choices: between growing food and flighting climate change, and between preserving natural forests full of wildlife, and razing them for carbon-absorbing plantations. In addition, these enormous plantations would consume huge amounts of water for irrigation and millions of tons of fertilizer, which have their own emission problems.

The second issue is what to do with the harvested material to keep that carbon from reentering the atmosphere. Some wood could be turned into building materials and furniture, or converted into biofuels. But again, these processes consume energy and produce more emissions, so the overall carbon reduction is less.

The researchers point out that planting trees and preserving green spaces is still an important factor in fighting climate change. It is just not the only solution. As with other large-scale concepts, such as geoengineering the atmosphere, this research shows that it is folly to think we can compensate for carbon emissions with "end of pipe" solutions. The only real answer to beating climate change is to cut the sources of those emissions and move to alternatives.