Washington Post: The Magic Number

Getting the world’s governments to reach a global agreement at the United Nations’ climate conference in Paris is going to require careful politics, but the effort may be up against an even tougher challenge: mathematics.

The central goal of the gathering is to forge an agreement that would set the world on a path to ultimately restrict planetary warming to less than two degrees Celsius above preindustrial temperatures. The target was originally proposed by the European Union in the 1990s as a way to avert some of the worst consequences of climate change, such as rising sea levels.

But as ambitious as this effort is, time and numbers are working against it. Some scientists and analysts now suggest the two-degree target may be all but out of reach. The world has waited so long to cut emissions, they say, that sticking to two degrees may require extremely harsh cuts that could damage economies, or the assumption of future technologies that have not been invented yet.

Kevin Anderson, deputy director of the Tyndall Center for Climate Change Research at the University of Manchester in Britain, recently hurled a grenade into the two-degree discussion in an essay in Nature Geoscience by charging that many of his scientific colleagues “are ultimately choosing to censor their own research.”

Anderson particularly objects that many models now rely on “negative emissions” through technologies that remove carbon dioxide from the air or after combustion processes. These technologies, he objects, are at “little more than a conceptual stage of development.”

Oliver Geden, a researcher with the German Institute for International and Security Affairs, leveled similar charges earlier this year and has called for modifying the two-degree target, labeling it “patently unrealistic” and “obviously unattainable.” On the scientific front, meanwhile, a recent analysis by Stanford University earth scientist Rob Jackson and three colleagues found “without immediate and substantial mitigation . . . time has nearly run out for 2°C.”

And the goal is beset by other hurdles, too — such as major uncertainties surroundinghow much carbon is really safe to emit, and around unexpected factors that can arise as the planet warms.

And then there’s the simple, discouraging fact that two degrees may not be a “safe” level of global warming anyway.

The ‘carbon budget’

The key problem is the finite global “carbon budget.” The two-degree goal was formally adopted by parties to the U.N. Framework Convention on Climate Change (UNFCCC) in a 2010 meeting in Cancun, Mexico, but it wasn’t until 2013 that a key innovation clarified he nature of the planetary emissions problem and showed how difficult it really is to solve.

This was the carbon budget.

Noting that the relationship between the amount of carbon dioxide we put in the atmosphere and the eventual global temperature is “near linear,” the United Nations’ Intergovernmental Panel on Climate Change calculated the maximum amount the world could emit for a one-third, 50 percent or two-thirds chance of keeping warming below two degrees.

The resulting headline: As of 2011, the world had about 1,000 gigatons, or billion metric tons, of carbon dioxide left to emit in order to have a two-thirds or greater chance of staying below two degrees. After that, net emissions must go to zero.

From here you simply do the math. Energy-related carbon dioxide emissions alone were 32.3 gigatons last year, according to the International Energy Agency, and that does not include other sources, such as deforestation. Based on such numbers, the remaining carbon budget is already under 900 gigatons of carbon dioxide.

Anderson’s paper calculated that once you take future deforestation and cement-related emissions between now and 2100 into account, the remaining budget is just 650 gigatons of energy-related emissions. That’s about 20 years at current rates — but emissions are still rising. That trend is currently expected to continue out to 2025 or 2030, despite countries’ recent carbon-cutting pledges, in large part because of growing demand for energy in coming decades.

The UNFCCC recently acknowledged that these pledges, on their own, would hold warming to perhaps only 2.7 degrees Celsius — other analyses are still more pessimistic — and, therefore, that much more must be done in Paris and beyond to ensure attainment of the two-degree goal.

And, of course, that’s assuming that we’re using the right number to begin with: There are big error bars around the 1,000-gigaton number. “When we talk about a thousand billion tons left with a probability of 66 percent, we’re saying we think that budget, there is 1 in 3 probabilities that you’ll still go beyond,” said Pep Canadell, executive director of the Global Carbon Project and a scientist at Australia’s Commonwealth Scientific and Industrial Research Organization.

In one recent paper, Canadell, Jackson and two colleagues found that a 90 percent probability shrinks the allowable carbon budget by 500 gigatons — and that we would bust such a budget in about a decade.

So far, 2015 is the hottest year on record.

The permafrost problem

And there are still more issues. For instance, scientists have recently verified a long-standing fear — that as warming increases, the thawing of northern permafrost, frozen soil that contains the remains of dead but un-decomposed plant life, will steadily release carbon into the air. It is not clear how fast or how much, but one recent study found that for moderate warming of 1.2 additional degrees Celsius between the year 2010 and 2100, global permafrost could emit 44.8 to 122.6 gigatons of carbon dioxide, which would tighten the carbon budget further.

More generally, scientists do not fully understand the global carbon cycle, which governs the ultimate fate of all the CO2 we put in the atmosphere. Any revisions in our understanding of how much carbon gets taken up by the ocean or by trees and plants would also require revisions to carbon budgets.

And this does not even address the deepest problem — whether two degrees is actually the right threshold to avoid “dangerous” climate change, a term that comes from the UNFCCC, the international treaty at the center of these negotiations.

In the end, the definition of “safety” is subjective. But recent research suggests that for every degree of warming, the world can ultimately expect to see about 2.3 meters (over seven feet) of gradual, long-term sea-level rise. No wonder many countries — though not the big emitters, such as the United States and China — are holding out for a target of 1.5 degrees Celsius rather than two degrees.

The great ocean conveyor belt

Chilly freshwater runoff from Greenland’s melting ice cap is probably the cause of an abnormally cold region in the North Atlantic that threatens to interrupt the Gulf Stream, which brings critical warmth to Northern Europe.

But not unlike the 90-percent-or-greater chance of staying under two degrees, the carbon budget for 1.5 degrees is much tighter. “I think 1.5 degrees is gone,” Anderson says.

One way to improve these odds is to assume that in the future we will be able to “take back” emissions that we’ve put in the atmosphere — “negative emissions.”

“Negative emissions, in my mind, fall somewhere between a godsend and voodoo,” said Stanford’s Jackson. “The advantage is that it allows us to overshoot targets, then come back down. The disadvantage is, we don’t know that it’s going to work, and we’re not sure how much people are going to be willing to pay for it even if it could work.”

The leading candidate for negative emissions is bioenergy combined with carbon capture and sequestration, or BECCS. The idea is that we would burn trees or biomass to create energy, which would grow again and store carbon, making the process carbon neutral. But if we also sequestered the carbon from burning biomass in the ground, the result would be a net removal of the gas from the atmosphere.

Anderson charges that current two-degree scenarios rely heavily on this concept. “I have no problem with one or two scenarios, or 10 percent of all scenarios having some BECCS in them,” he said. “But once you start to have 80 or 90 or 100 percent of their scenarios having BECCS, that’s a systemic bias.”

Tech assumptions

The U.N. Environment Program recently released a report confirming this concern. It noted that scenarios that are able to keep warming to two degrees, without major emissions cuts before the year 2020, “rely on so-called ‘negative emission technologies.’ ”

Two degrees “only remains a viable goal if we are prepared to accept the implications.”

Although this kind of technology does not exist in any widespread form, researchers are certainly interested in exploring it. “I think the pieces individually work. Carbon capture today works technically, although not economically, yet,” said Dan Kammen, an energy policy expert at the University of California at Berkeley who has published on the subject. “The pieces are all there, and because it does look like we are going to need carbon-negative energy, there’s a big upside in exploring the options.”

There is also the prospect of directly capturing carbon from the air. The firm Carbon Engineering, with funding from Bill Gates, just opened an experimental carbon-capture plant in Squamish, B.C., that can capture “a couple of tons of CO2 per day,” said chief executive Adrian Corless. Scaling that up would, obviously, be a gigantic endeavor. But Corless expects the world will need the technology more in the future and be willing to pay for it — especially in light of “negative emissions” scenarios.

And there’s another glint of good news, Jackson said — rapid growth rates for wind and solar. “We built 50 gigawatts of new wind capacity last year,” he said. “That is more than was on the planet a decade ago.” Still, most scenarios assume only a gradual displacement of carbon-intensive fossil fuels such as coal from the global energy supply.

So as the world heads to Paris, there are rising questions about two degrees and about negative emissions in particular. The prospects for staying within the carbon budget are not gone, because the budget-busting emissions literally have not gone into the air yet. But with every year and, indeed, every day, the quest gets harder.

Two degrees, Anderson said, “only remains a viable goal if we are prepared to accept the implications.”