Negative emissions are the flip side of emissions. The idea is to develop technology that would remove carbon dioxide directly from the atmosphere. This would allow for significantly higher fossil fuel emissions over the next few decades. To compensate, we would start removing more and more carbon dioxide from the atmosphere to eventually reach the I.P.C.C.’s net zero emissions line by 2070 and go even lower afterward.

Climate problem solved — at least according to the climate models.

But there’s a problem with this scenario. The United Nations’ own Environment Program pointed it out in a report last year:

“Theoretically, carbon uptake or net negative emissions could be achieved by extensive reforestation and forest growth, or by schemes that combine bioenergy use with carbon capture and storage. But the feasibility of such large-scale schemes is still uncertain. Even though they seem feasible on a small scale, the question remains as to how much they can be scaled up without having unacceptable social, economic or environmental consequences.”

Climate scientists and economists are betting primarily on a new technology called bioenergy with carbon capture and storage, or Beccs. This involves cultivating fast-growing vegetation, or biomass, to absorb carbon dioxide from the atmosphere. Trees or energy crops would then be burned in power plants, and the emissions would be captured and pumped underground. Beccs could also be attached to industrial processes involving biomass, like pulp factories or ethanol plants.

But the Beccs technology does not exist at scale at present. And even if it did, the immensity of this endeavor would be unlike anything that exists today. To achieve the negative emissions that are an essential component of the I.P.C.C. models, we would have to plant around 500 million hectares of biomass crops — an area one and a half times the size of India. This would also require enormous capacities for transporting and storing the carbon dioxide extracted from the atmosphere.

This vast enterprise is being figured into the calculations of climate researchers, environmental groups and policy makers when they maintain that a 3.6 degree target can still be reached.