To meet the Paris climate deal's goal of deep greenhouse gas cuts, nations appear to be relying on costly, possibly harmful large-scale projects to suck carbon dioxide from the atmosphere, says a new paper with sobering calculations of the risks.

"The Paris agreement shows where we want to go — the brave new world of a balanced carbon budget — but not how to get there," says Phil Williamson, environmental scientist at the University of East Anglia in the United Kingdom and science coordinator for the U.K. government's Natural Environment Research Council.

Williamson warned in a commentary Wednesday in Nature that even seemingly beneficial approaches like tree planting could wreak havoc if they are implemented on the massive scale required to limit the increase in average global temperature to below 2° Celsius.

"There's a lot of optimism based on the assumption it will all be all right, because sometime in the future, we're going to be able to remove the carbon," Williamson said in a phone interview. "Well, that's actually going to be more trouble and more expensive than if you face up to the problem now." He said research is urgently needed on the consequences of these massive carbon removal projects, which he says are essentially geoengineering projects by another name.

Paris negotiators did not specifically discuss carbon removal, but Williamson argues their deal implicitly relies upon large-scale mitigation projects, because nations are not on track to cut fossil fuel burning enough to meet the pact's targets.

For example, he offers stark numbers on the potential impact of bioenergy with carbon capture and storage (BECCS)—or the growing of crops, from grasses to trees, that can be burned at power stations for electricity while the carbon emitted is captured and stored. Williamson calculates that for the cuts envisioned under the Paris deal, crops solely for carbon removal would have to be planted on 430 million to 580 million hectares (1,060 million to 1,440 million acres) of land—about one third of the total arable land on the planet, or half the land area of the United States.

Such dependence on BECCS could cause a loss of terrestrial species at the end of the century perhaps worse than the losses resulting from a temperature increase of about 2.8 °C above pre-industrial levels, Williamson wrote.

He also analyzed the possible impact of techniques to increase carbon sequestered in soil—for example, by plowing biochar, a form of charcoal, into agricultural plots. If millions of acres of soil were darkened by application of biochar—as would be required to achieve deep carbon cuts—the albedo, or reflectivity, of the soil would be reduced, increasing the Earth's heat absorption significantly.

An alternative would be to add pulverized reflective rock-like silicate to the soil surface. But it entails a huge amount of rock. To cut 50 parts per million of CO2 in the atmosphere (a 12 percent decrease from current levels), countries would need to apply one to five kilograms per square meter of silicate rock each year to as much as 45 percent of the Earth's land surface area, mostly in the tropics, Williamson wrote. The volume of rock mined and processed would have to exceed the amount of coal currently produced worldwide, at a cost of more than $60 trillion, with environmental degradation of adjacent water systems.

Ken Caldeira, an atmospheric scientist at the Carnegie Institution for Science who has long argued for increased efforts to develop carbon removal technologies, notes that he and others who have been working on geoengineering also advocate the cutting of carbon emissions as deeply and as rapidly as is practicable. "But [we] are concerned that even our best efforts may not suffice to avoid dangerous climate change," Caldeira said.

Indeed, in a new study published this week in Nature Climate Change, researchers at Oregon State University and Lawrence Livermore National Laboratory found that the longer-term sea level rise impacts due to climate change will continue well past the 21st century. Given the long time scales of the carbon cycle, the authors said reducing emissions slightly or even significantly is not sufficient.

"To spare future generations from the worst impacts of climate change, the target must be zero - or even negative carbon emissions - as soon as possible," said Peter Clark, Oregon State paleoclimatologist.

Caldeira said increased research into the impact of carbon dioxide removal would be a good thing, but it shouldn't take away from investment in promising technologies such as solar power.

Williamson previously led scientific research reviews for the UN Convention on Biological Diversity on one geoengineering proposal: ocean fertilization with iron to increase its ability to store carbon. Initial optimism about that idea faded, he said, after research showed the potential costs and unintended consequences to marine life.

"If you're a government or a policy maker, it's always easier to put these decisions out to the future," he said. "The risk is that then it will be too late."

The story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.