A tract of Amazon rain forest, which has been cleared by loggers and farmers for agriculture, is seen near Santarem, Brazil. (Nacho Doce/Reuters)

Scientists are expressing increasing skepticism that we’re going to be able to get out of the climate change mess by relying on a variety of large-scale land-use and technical solutions that have been not only proposed but often relied upon in scientific calculations.

Two papers published last week debunk the idea of planting large volumes of trees to pull carbon dioxide out of the air — saying there just isn’t enough land available to pull it off — and also various other strategies for “carbon dioxide removal,” some of which also include massive tree plantings combined with burning their biomass and storing it below the ground.

“Biomass plantations are always seen as a green kind of climate engineering because, you know, everybody likes trees,” said Lena Boysen, a climate researcher at the Max Planck Institute for Meteorology in Germany, who led one of the new studies while a researcher at the Potsdam Institute for Climate Impact Research. “But we just want to show that that’s not the complete story. They cannot do that much.”

Forests have long been recognized as one of the world’s most important natural carbon sinks, capable of storing large amounts of carbon that would otherwise end up in the atmosphere. Simply preserving the world’s forest resources — and replanting areas that have already been deforested — is viewed as an important step in protecting the climate.

But for years, scientists have discussed the idea of going further by using large plantations full of fast-growing, carbon-storing trees to pull extra carbon emissions out of the atmosphere, a strategy sometimes called “afforestation.” But the amount of land and other resources this strategy would require to actually help us meet our global climate goals — namely, keeping global temperatures within at least two degrees of their pre-industrial levels — is completely impractical, according to Boysen’s new study in the journal Earth’s Future, and would require the destruction of huge amounts of natural ecosystems or productive agricultural land.

Considering different scenarios for planetary emissions, Boysen and her colleagues find the land space that would be required for the amount of trees necessary to keep temperatures within a 2-degree threshold under our current climate trajectory could have “dire consequences for food production or the biosphere.” And even under more optimistic scenarios, where future carbon emissions are lower and fewer trees would be necessary, they conclude that “high inputs of managed water and fertilizers would be needed in order to avoid fierce competition for land — with potentially negative side-effects for climate and society.”

It isn’t just planting forests on their own: Many scientists have suggested that a more advanced strategy may be necessary to keep to a two-degree pathway in the future — particularly now, as analyses increasingly suggest that even the emissions reduction pledges established by countries participating in the Paris climate agreement are still not enough to keep us on track.

The solution that’s been proposed in numerous reports and climate models, including those released over the years by the Intergovernmental Panel on Climate Change (IPCC), is a technology known as bioenergy and carbon capture and storage, or BECCS. This strategy involves establishing large plantations of fast-growing trees, capable of storing large quantities of carbon, which can then be harvested and used for fuel. Biomass burning facilities would need to be outfitted with a special carbon-capturing technology, which would capture the carbon dioxide produced and store it safely away, potentially in geological formations deep underground.

It’s an ambitious proposal, and one that many scientists have pointed out is nowhere near the point of becoming feasible, even from a technological perspective. Carbon capture and storage technology is just getting on its feet from a commercial standpoint, and there are only a few facilities around the world — several of them in the United States — that use it. Without this technology, the planting and harvesting of biomass becomes far less climate friendly, essentially releasing all the stored carbon right back into the atmosphere.

These biomass energy systems could be valuable, “but only if the biomass is entirely sustainable,” said Daniel Kammen, an energy professor at the University of California at Berkeley and director of the Renewable and Appropriate Energy Laboratory, in an emailed comment to The Washington Post. Kammen was not involved with the new study but has conducted much of his own research on biomass energy and carbon capture and storage systems.

The new study, he says, agrees well with what previous work has suggested — that when the system is not completely sustainable, “the value of a biomass crop for meeting climate targets is non-existent,” he said. Large plantations of fast-growing trees, as explored in the recent paper, are clearly not sustainable, he pointed out.

And other experts agree as well. In a separate paper, published Thursday in the journal Science, Stanford researchers Christopher Field and Katharine Mach argue that carbon dioxide removal strategies, including BECCS, are still too risky to rely on for limiting climate change. In regard to BECCS, they also point out that the massive amount of resources required to sustain the system makes it unfeasible.

“Converting land on this staggering scale would pit climate change responses against food security and biodiversity protection,” they write. “Massively expanding managed land for CDR [carbon dioxide removal] could crash through the planetary boundary for sustainable land use.”

Worryingly, they add, carbon dioxide removal is increasingly assumed by climate models and planning tools as a future mitigation tactic. While they encourage continued research and development of the technology, the authors also urge the necessity of “avoiding cavalier assumptions of future technological breakthroughs.”

That’s not to say that there isn’t any place for this type of technology in the future, Boysen added.

“There are areas indeed where we can plant biomass plantations where they would be beneficial for the region,” she pointed out. But on a global scale, she said, research suggests that it’s just not an effective mitigation tool.

“There are other options,” she said, such as more sustainable agriculture techniques, that can help make a dent in global carbon emissions. “But the most important option is, of course, not to release the CO2 into the atmosphere.”