Don't expect too much from the global climate-change accord that's expected to emerge from high-stakes international talks in Paris next year.
A new MIT study concludes that even if negotiators reach a deal at the United Nations conference, it probably won't be enough to limit global temperature increases to 2 degrees Celsius above pre-industrial levels. That's the level many scientists say would help stave off some of the most dangerous and disruptive effects of climate change.
Here's the study's bottom line on what to expect from the so-called Conference of the Parties 21 in Paris: "Based on our expectations for the architecture of a COP-21 agreement, and our predictions about the national contributions likely to come forth under it, our analysis concludes that these international efforts will indeed bend the curve of global emissions. However, our results also show that these efforts will not put the globe on a path consistent with commonly stated long-term climate goals," states the paper by economics professor Henry Jacoby and Y-H Henry Chen, who both work with MIT's Joint Program on the Science and Policy of Global Change.
The 2Â°C ceiling has been highly optimistic for a while, as global greenhouse-gas emissions continue to soar.
In a major report last year, the United Nations Intergovernmental Panel on Climate Change modeled the impact of a series of possible emissions trajectories. Runaway emissions growth could further boost temperatures, at the high end of the estimates, by up to 4.8 Â°C by 2100, the IPCC estimated. Only the most aggressive of the emissions-cutting pathways that researchers modeled—in which carbon dioxide stabilizes in the atmosphere at around 450 parts-per-million (it's now at 400 and climbing)—would be "likely" to stay below the 2 degree target, the IPCC predicts.
So the new MIT paper raises a major question looming over emissions pledges that nations will make early next year and for the Paris talks themselves: How should success be measured?
Robert Stavins, a Harvard University expert on international climate negotiations, said the good news is that the MIT study predicts the Paris accord will be successful in changing the global emissions trajectory as nations worldwide pledge to act on pollution targets.
"You can view their results as saying there is a half-full glass of water," said Stavins, director of the Harvard Environmental Economics Program.
While the 2Â°C target is a goal of the negotiations, he said the structure of the agreement that's shaping up is important in other ways that will mark a vast improvement over the existing Kyoto Protocol. This is the 1997 treaty the U.S. never joined, Canada has abandoned, Japan won't meet, and includes no commitments from developing nations at all.
Stavins compared the tricky work of global-emissions agreements to trying to build a 70-story skyscraper. Kyoto began construction but didn't create a foundation that's anywhere close to broad enough, he said.
The emerging architecture of the hoped-for Paris deal will be very different. Instead of a formal treaty with binding emissions agreements from a subset of nations, all countries are expected to determine their own pledges on how much they will curb emissions.
Stavins predicted that initial emissions pledges that nations make will not be very ambitious, and that this will lead some people to be disappointed when they add up the emissions curbs. But Stavins said what's more important than whether these initial pledges are a pathway to the 2Â°C target is creation of a foundation for countries to jointly act on climate, unlike the relatively narrow set of nations covered under Kyoto.
He said the structure that's emerging in recent years is promising. "That policy architecture should be judged in terms of whether it is a foundation for long-term progress," he said. "It makes me vastly more optimistic about a 70-story building being constructed when you start with the right foundation."
In effect, what's expected to emerge is a softer type of pact that's not a formal treaty but does bring all the big polluters on board, including China, which now emits the most greenhouse gases in the world. The MIT paper also pointed out that a formal treaty would not be able to clear the U.S. Senate anyway.
Interim U.N. climate talks in 2011 in Durban, South Africa, produced what is known as the Durban Platform, which is a road map for an accord that takes effect in 2020 and has, at the very least, "legal force." But that's a rather flexible term.
The MIT paper predicts that in Paris next year, "Negotiations will focus on loosely harmonized domestic actions in a system of pledged contributions, with some system of ex-post review. Since a legally binding agreement on emissions targets is unlikely to occur, the Durban Platform specification of an agreed outcome with legal force could only require mandatory participation in a process to review progress in achieving pledged contributions."
Countries including the U.S. are slated to reveal their post-2020 emissions pledges early next year.
Environmentalists will be looking for aggressive cuts from polluting nations. Greenpeace, for instance, has said the "road map" for a deal should include pledges strong enough to meet the 2 Â°C target, or a even a 1.5 Â°C target.
Jamie Henn, a cofounder of the aggressive advocacy group 350.org, noted, "Limiting global warming below 2Â°C, which is already a dangerous amount, is the one concrete target that survived the Copenhagen train wreck." That's a reference to the fractious, late-2009 climate talks in Copenhagen that nearly dealt a mortal blow to the U.N. negotiating process before a hastily crafted interim plan was salvaged.
"Giving up on 2Â°C would be an act of inexcusable cowardice," he said.
But Peter Frumhoff, director of science and policy at the Union of Concerned Scientists, said it's vital for policymakers to prepare for higher temperatures even while seeking strong actions to cut emissions.
"We need to ... prepare for the fact that we are likely to go beyond 2Â°C and think about what that implies for managing the risks of an inarguably dangerously warming climate," he said in an interview. "We need to stop treating a definitive answer around 2Â°C as the sole metric by which we measure our success around climate."
"The 2Â°C target is a terrifically important, ambitious political target informed by science but also informed by our values. We need to both treat Paris and beyond as opportunities to marshal our commitments to reduce emissions as ambitiously as possible and prepare for the hard choices for a world that may warm well beyond it," Frumhoff said.
Jacoby predicted that the Paris agreement will be a "step in the right direction," but that policymakers should be ready to explore what's next if nations' pledges aren't a recipe for staying within the 2Â°C target.
"[Our] expectation is that the pledges will not put the world on the path to meet the existing goals for limiting temperature change. We see emissions increasing through 2030 and, without additional international agreement, continuing to increase in the following decades," he said in comments published alongside the study.
"That raises the question, if it's obvious in the early stages of the negotiation that we're not getting on a path to temperature goals, what will be the nature of the follow-up process? We should be starting to have that discussion as well."