For almost two decades, the push to beat back climate change has rallied around an easy-to-grasp goal: Slash carbon-dioxide emissions enough to prevent the average global temperature from surpassing the pre-industrial level by more than two degrees Celsius, a threshold scientists warn could trigger dangerous effects, including major sea-level rise.

As a marketing catchphrase, “two degrees” has succeeded stupendously. There’s a 2 Degrees movie. There’s a Two Degrees book. When officials anywhere around the world discuss climate policy, holding below the two-degrees limit is almost always the stated goal.

But as an impetus for real action, two degrees has failed. Earlier this year, the Intergovernmental Panel on Climate Change (IPCC), a United Nations-affiliated network of climate scientists that constitutes conventional wisdom on global warming, projected in its latest voluminous climate-science report that the world is likely to blow past the two-degrees marker by 2100. That may seem a long time away, but because carbon dioxide piles up in the atmosphere over time, as water blasting from a faucet accumulates in a sink, avoiding the two-degrees tipping point would require slashing emissions starting now, the IPCC said. To be sure, the IPCC noted, it’s conceivable the world still could stay below that level—but only if governments immediately imposed stringent and internationally uniform carbon limits, and if a host of new low-carbon energy technologies proved able to scale up. Those are massive “ifs,” and though the IPCC wasn’t so impolite as to say so, there’s little to suggest that perfect trajectory will play out.

More gloom came late last month, when the Global Carbon Project, an international scientific group, reported that carbon emissions not only are increasing, but are increasing at a quickening pace, one that’s likely to push global temperatures beyond the two-degrees barrier within just three decades. Thirty years is a single generation—a blink of an eye in climatic terms, and a blink that, once triggered, is hard to stop.

The conclusion that global temperatures are on track to burst through the two-degrees ceiling isn’t much in dispute. What’s controversial is what the world should do about it. In recent days, influential voices have advocated scrapping the two-degrees threshold as a goal, saying that to continue to structure climate policy around an all-but-dashed hope of averting that limit is both politically dishonest and environmentally counterproductive. This call to nix the two-degrees metric has spurred a backlash from the climate-science establishment, and, more importantly, it raises big financial questions for companies and consumers worldwide. If the two-degrees goal changes, then so might the many climate policies framed around it—policies that translate into costs for polluters and profitable markets for clean-energy providers. At stake in this fight over a couple of degrees is potentially billions of dollars.


Here’s why: Various studies, notably this year’s IPCC report, estimate that rolling out policies and technologies to keep annual carbon emissions low enough to avoid crossing the accepted goal would shave median annual economic growth by 0.06 percentage points through 2100. Climate activists call that a small price to pay for the environmental benefit, and perhaps they’re right. But the apparently small hit to global economic growth wouldn’t hit everyone evenly. Some countries—and some companies, and some people—would pay more than others. The disagreement over how to distribute that cost is arguably the biggest reason that diplomats have failed to reach a global agreement on combating climate change—and thus why the United States has failed to adopt a national carbon cap. People, being people, want to avoid paying up.

Against this backdrop, advocates of scrapping the two-degrees goal argue for new climate-change metrics that they say will resonate more broadly. Writing in the October 1 issue of Nature, David Victor and Charles Kennel, both of the University of California, San Diego, argue for pegging climate policy to a new “array of planetary vital signs,” such as changes in the heat content of the oceans. These metrics, they contend, are more accurate measures not just of how the Earth as a whole is warming, but also of where on the planet bad things are likely to happen as a result. “What is ultimately needed is a volatility index that measures the evolving risk from extreme events—so that global vital signs can be coupled to local information on what people care most about,” Victor and Kennel write. “Getting serious about climate change requires wrangling about the cost of emissions goals, sharing the burdens and drawing up international funding mechanisms,” they add, so it makes sense to shift from a simple but esoteric measure of global-temperature change to a range of indicators to which larger numbers of people are likelier to relate—indicators the authors argue are thus likelier to spur policies that have a real climate-curbing impact.

The suggestion to, as Victor’s and Kennel’s article is titled, “Ditch the 2° C warming goal,” has unleashed fury from many climate advocates, particularly those who have worked for years to brand the importance of the two-degrees threshold into the global body politic. They’re particularly frustrated at the prospect of having to reframe the basic carbon-cutting goal precisely at a moment when they sense rising public concern about climate change, and in the lead-up to a big international climate conference next year in Paris, at which they’re crossing their fingers that global diplomats will agree on a newly stringent carbon crackdown. “If one wanted to sabotage the chances for a meaningful agreement in Paris next year, towards which the negotiations have been ongoing for several years, there’d hardly be a better way than restarting a debate about the finally-agreed foundation once again, namely the global long-term goal of limiting warming to at most 2 degrees C,” Stefan Rahmstorf, an expert at Germany’s Potsdam Institute for Climate Impact Research, wrote last week in an online response to the Nature piece. “This would be a sure recipe to delay the process by years. That is time which we do not have if we want to prevent dangerous climate change.”

Ultimately, however, what’s likely to be more politically important than the question of what jettisoning the two-degrees limit would do to the planet is the question of what it would do to pocketbooks. Who would win and lose is impossible to peg at this point, but there’s no doubt that coming up with a new set of climate metrics would redistribute the economic hit. It could end up costing different industries—or different states, or different countries—more. Global negotiators might, for instance, decide to go after the emitters of certain greenhouse gases that are more potent than carbon dioxide—gases produced particularly by companies that make chemicals. Or they might decide to stick it harder to airlines, on the theory that carbon dioxide emitted at altitude creates special climate problems.  Or they might put a harder squeeze on producers of a particular energy source, such as natural gas—a move whose economic effect would be felt most acutely in the regions that produce it. The point is that climate change is mind-bogglingly complex. The devil is in the details. And so this seemingly wonky but economically high-stakes debate over how to accurately measure global warming is likely to heat up much more. For now, many climate advocates are lashing out in a bet that the devil they know is preferable to the devil they don’t.