In this Dec. 1, 2015 photo, a man visits the Climate Generations Areas, part of the COP21, the United Nations Climate Change Conference, in Le Bourget, north of Paris. (AP/Christophe Ena)

As the climate negotiations in Paris proceed, many might be surprised to learn that the goal of all of this is not for the world to agree on new emissions cuts. Rather, countries have already pledged to make cuts on their own, and the task of the Paris meeting is to forge an agreement that will ensure that they are clear and transparent about these pledges and whether they’re achieving them, and that the pledges become considerably more ambitious over time.

The ultimate goal is to ensure that warming stays “well below” 2 degrees Celsius above pre-industrial levels, according to one option in a recent draft agreement text — and many countries want to hold it even lower than that. The current pledges, the United Nations has said repeatedly, are not strong enough to ensure that goal. So the pledges simply have to get tougher.

However, there’s a problem. In two separate new studies just out in the journals Nature Geoscience and Nature Climate Change, scientists challenge both the 2 degree Celsius target — which was originally proposed back in the 1990s, and has since become nearly omnipresent in the climate discussion — and also one of the key tools that may be needed to get us there: so-called “negative emissions” technologies, which would remove carbon dioxide directly from the air.

The first study, by Reto Knutti of ETH Zurich and several colleagues, argues that “no scientific assessment has clearly justified or defended the 2° C target as a safe level of warming, and indeed, this is not a problem that science alone can address.” The 2° C target is a value judgment, the authors say, but they also list many very severe impacts of climate change that could be expected at 2° C or even below it.

For instance, they note that “the number of very hot days exceeding a percentile-based threshold increases globally about six-fold for a 1 ° C-warmer world, but more than 20-fold for a 2 ° C-warmer world.” They also note that the Greenland ice sheet could disintegrate in a 2° C world. Indeed, if we were “risk-averse,” the authors note, the lack of certainty about just how close we are to “potentially catastrophic outcomes” would lead to much sharper emissions cuts than anybody is currently contemplating.

“In our view, the current 2 ° C UNFCCC target is a compromise between what is deemed possible and desirable, rather than a ‘planetary boundary’ that clearly separates a ‘safe’ from a ‘dangerous’ world,” Knutti and his colleagues conclude. The critique comes at a time when more and more nations are pushing for 1.5° C as a much safer temperature threshold. But setting such a target would mean that the world is even more off pace to cut its emissions.

These targets aren’t just difficult to reach — they may also be technology dependent. The longer we wait to cut emissions sharply, the less likely it is that we’ll be able to stay below 2° C (or 1.5° C) without finding some way of taking carbon dioxide back out of the air again, note Knutti et al, through so-called “negative emissions” technologies.

Indeed, those closely watching the carbon math behind the current pledges have been aware for some time that since these targets aren’t enough on their own to keep temperatures below 2° C, negative emissions may be necessary at some point in the future.

This isn’t always stated up front, but it was explicitly acknowledged by the United Nations’ Environment Programme recently in its assessment of nations’ current  pledges and just how much they fall short. The report noted that because strong climate action has generally been pushed off til after 2020, staying within 2 C now requires “so-called ‘negative emission technologies’ such as bioenergy combined with carbon capture and storage.”

In other words, while negative emissions aren’t explicitly an idea at the center of the Paris climate negotiations, they are very much a subtext and part of the background.

And that’s the problem. The second new study, a large overview of negative emissions technologies penned by no less than 40 separate authors, finds that reliance upon them in the future to pull a lot of carbon out of the air (because we can’t cut our emissions more in the present) is “extremely risky.”

The report looked closely at four major ideas for pulling carbon back out of the air — bioenergy combined with carbon capture and sequestration, direct air capture of carbon dioxide, massive afforestation (planting trees where they did not exist before), and so-called “enhanced weathering” of rocks. The goal was to find out if there are any major tradeoffs or drawbacks to any of these technologies, and it basically found that they all had problems if implemented on a scale significant enough to really remove a lot of carbon from the air.

“The study shows that there’s no free lunch,” said Pete Smith, lead author of the paper and a researcher with the University of Aberdeen in Britain. “With all these negative emissions technologies, they have significant impacts either on water, or energy, or cost, or land.”

Take the leader of the pack — bioenergy combined with carbon capture and sequestration, or BECCS. Here, the idea is that we would burn trees or plants to get energy, and when they grow back, these living things would pull carbon out of the air again — thus, burning them would be carbon neutral. However, if the burning process also sequestered carbon and buried it deep in the ground, then the process becomes carbon negative — carbon dioxide is not only pulled out of the air but then never makes it back there again.

However, the major problem here is that to do this at a scale that would really make a dent, you need a simply massive amount of land. The new Nature Climate Change study calculates, for instance, that in order to have enough bioenergy combined with carbon sequestration to remove 3.3 gigatons of carbon from the atmosphere annually — an amount that is not unrepresentative of some models — you would need to use an amount of land that is “roughly the same as the area of the lower-48 states,” explains Stanford University’s Rob Jackson, one of the study’s authors.

“Recognizing that if we took the whole U.S. it still wouldn’t work because of the arid west,” Jackson adds. “The more we use cheaper, lower productivity land, the more acres we need to produce the biomass.”

All of this is supposed to happen, notably, at the same time that a booming population will need more land to grow food. If it sounds like something doesn’t add up here, that’s because it doesn’t.

It’s not that BECCs is an inherently bad idea — it’s just that the problem of climate change is so massive that anything big enough to reverse it also has to be massive. And if that thing involved using land and growing crops, then you are talking about an enormous footprint on the Earth. That’s simply the nature of the game.

So in sum, the conclusion is ultimately one that scientists have actually known for a long time. As the Carnegie Institution’s climate researcher Ken Caldeira has put it, the world simply needs to “stop using the sky as a waste dump for our carbon pollution.” That’s the best way to solve the global warming problem, not negative emissions.

The faster it happens, the less the problem will be — something diplomats in Paris this week will be extremely mindful of.

Read more:

The one thing that really doesn’t make sense about the climate debate in Paris

Climate-change warnings include wild weather shifts. But giant flying boulders?

The invisible threat to human existence

In Paris, experts cite looming risk from coolants, methane

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