A new study shows planting trees to store carbon dioxide they have taken up from the atmosphere is not a viable option to counteract unmitigated emissions from fossil fuel burning.
Credit: United States Department of Agriculture: National Resources Conservation Service.

By Jonas Viering

Growing plants and then storing the carbon dioxide they have taken up from the atmosphere is not a viable option to counteract unmitigated emissions from fossil fuel burning, a new study shows.

Plantations would need to be so large they would eliminate most natural ecosystems or reduce food production if implemented as a late-regret option in the case of substantial failure to reduce emissions, finds the new study in Earth’s Future, a journal of the American Geophysical Union.

However, growing biomass soon in well-selected places with increased irrigation or fertilization could support climate policies of rapid and strong emissions cuts to stabilize Earth’s climate below 2 degrees Celsius, according to the study’s authors.

“If we continue burning coal and oil the way we do today and regret our inaction later, the amounts of greenhouse gas we would need to take out of the atmosphere in order to stabilize the climate would be too huge to manage,” said Lena Boysen, a researcher at the Potsdam Institute for Climate Impact Research (PIK) in Potsdam, Germany, and lead author of the new study.

“Even if we were able to use productive plants such as poplar trees or switchgrass and store 50 percent of the carbon contained in their biomass, in the business-as-usual scenario of continued, unconstrained fossil fuel use, the sheer size of the plantations for staying at or below 2 degrees Celsius of warming would cause devastating environmental consequences,” Boysen said.

In the new study, Boysen and her colleagues investigated the feasibility of biomass plantations and carbon dioxide removal using global dynamic vegetation computer simulations. Based on their results, they calculate that the hypothetical plantations would in fact replace natural ecosystems around the world almost completely.

If carbon dioxide emissions reductions are moderately reduced in line with current national pledges under the Paris Climate Agreement, biomass plantations would still have to be enormous, according to the study. In this scenario, plantations would replace natural ecosystems on fertile land larger than one-third of Earth’s existing forests. Alternatively, more than a quarter of land used for agriculture at present would have to be converted into biomass plantations, putting global food security at risk, according to the authors.

Only ambitious emissions reductions and advancements in land management techniques between mid-century and 2100 could possibly avoid fierce competition for land. But even in this scenario, only high inputs of water, fertilizers and a globally applied high-tech carbon-storage-machinery could likely limit warming to around 2 degrees Celsius by 2100, according to the study. To this end, technologies minimizing carbon emissions from cultivation, harvest, transport and conversion of biomass and, especially, long-term Carbon Capture and Storage (CCS) would need to improve worldwide, the authors said.

So far, biomass plantations as a means for carbon dioxide removal have often been considered as a comparatively safe, affordable and effective approach. “Our work shows that carbon removal via the biosphere cannot be used as a late-regret option to tackle climate change,” said Tim Lenton, a researcher at the University of Exeter, United Kingdom and co-author of the new study. “Instead we have to act now, using all possible measures instead of waiting for first-best solutions. Reducing fossil fuel use is a precondition for stabilizing the climate, but we also need to make use of a range of options from reforestation on degraded land to low-till agriculture and from efficient irrigation systems to limiting food waste.”

“In the climate drama currently unfolding on that big stage we call Earth, carbon dioxide removal is not the hero who finally saves the day after everything else has failed,” said Hans Joachim Schellnhuber, Director of PIK and co-author of the new study. “It is rather a supporting actor that has to come into play right from the beginning, while the major part is up to the mitigation protagonist. So this is a positive message: We know what to do – rapidly ending fossil fuel use complemented by a great variety of CO2 removal techniques. We know when to do it – now. And if we do it, we find it is still possible to avoid the bulk of climate risks by limiting temperature rise to below 2 degrees Celsius.”

— Jonas Viering is in the public relations department at PIK. This post originally appeared as a press release on the PIK website.