This post originally appeared on Climate Central.

The manmade emissions fueling global warming are accumulating so quickly in the atmosphere that climate change could spiral out of control before humanity can take measures drastic enough to cool the earth’s fever, many climate scientists say.

The most important way the earth’s rising temperature can be tempered is to reduce the use of fossil fuels. But scientists say another critical solution is to physically remove greenhouse gases from the atmosphere — something called “negative emissions” — so that carbon dioxide and rising temperatures could peak, and then begin to decline over time.

Many of the assumptions underlying the landmark Paris Climate Agreement rely on the idea that humans will be actively removing carbon from the atmosphere late this century because reducing emissions won’t be enough to prevent global warming from exceeding levels considered dangerous.

But that assumption relies on technology that hasn’t been proven to work on a global scale. Removing carbon dioxide from the atmosphere on a scale large enough to slow global warming is untested, and the technology is in its infancy. The effect it could have on the earth is largely unknown, and some scientists warn that some of the consequences of using negative emissions technology could be catastrophic.

Because of all those unknowns, it’s critical that humanity doesn’t bet its future on negative emissions, Stanford University Woods Institute for the Environment scientists Katharine Mach and Christopher Field write in a paperpublished Thursday in the journal Science.

The paper argues that both negative emissions technology and a commitment to quickly cutting carbon dioxide emissions as much as possible are critical to solving the climate crisis.

Carbon concentrations in the atmosphere must not exceed 450 ppm if global warming is to be prevented from exceeding a level considered dangerous by most climate scientists —  2°C (3.6°F), the primary goal of the Paris Climate Agreement. The problem, though, is that humanity is quickly running out of time to limit more warming. The atmosphere blew past the 400 ppm mark last September and it’s on a trajectory to pass 450 ppm within 22 years.

Most of the Intergovernmental Panel on Climate Change models underlying the Paris Climate Agreement assume some level of large-scale carbon removal will be occurring in the coming decades, but nobody knows exactly how that will be accomplished.

Ben Sanderson, a climate scientist at the National Center for Atmospheric Research who is unaffiliated with the paper, said the study shows that carbon removal shouldn’t be treated as a cure-all for climate change because the future of humanity can’t rely on untested technology.

“The major risk is that the planned-for CO2 removal might never come to pass — and this is a very real concern,” Sanderson said.

The paper warns of dire consequences if the effects of negative emissions technology aren’t fully accounted for before they’re implemented.

For example, one of the negative emissions technologies carbon-removal proponents often cite as the most promising — bioenergy and carbon capture and storage, or BECCS — could create widespread food insecurity because it could take half of the world’s farmland out of production.

BECCS relies on converting agricultural areas and other land to vast new forests, which absorb atmospheric carbon in tree trunks and roots. The trees would be harvested for biomass energy and burned in power plants. The resulting carbon emissions would be captured and stored permanently — a method some scientists believe could be worse for global warming than burning fossil fuels.

“Converting land on this staggering scale would pit climate change responses against food security and biodiversity protection,” the paper says.

Chief among the many other negative emissions technologies being developed include expanding forests globally to store more carbon naturally, and building hundreds or thousands of facilities that directly remove carbon from the atmosphere and store it permanently. Those facilities, called “direct air capture” plants, have never been built on a large scale and scientists say they would require a large amount of energy to operate and many thousands of them would have to be constructed to make a dent in global warming.

The paper criticizes the idea of peak and decline — the theory that carbon removal could bring about a peak in global temperatures and then begin to cool the planet. That may be risky because the costs and consequences of global cooling following a temperature peak are not well understood. Some of the effects of climate change such as sea level rise and melting polar ice sheets can’t be reversed as the globe cools.