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Is 2 Degrees the Right Limit for Global Warming? Some Scientists Say No

We've come to think of it as the threshold of catastrophic climate change—but it’s the wrong limit to set, two researchers argue.

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Emissions waft from the smokestacks of a coal-fired power plant during winter.

For more than a decade international climate-policy discussions have revolved around a seemingly simple goal: Limit the rise in average global surface temperature to two degrees Celsius (3.6 degrees Fahrenheit). But a new paper argues that the two-degree target is not only increasingly unrealistic but also misleading.

"More and more, it's a combination of fantasy and irrelevance," says David Victor, a professor of international relations at the University of California, San Diego, and the co-author of a critique of the target published in today's issue of Nature. "Maintaining it forces us to continue to pretend that it's feasible—and focuses people's attention on a number that isn't very well connected to the damage humans are doing to the climate."

The two-degree target first gained prominence in the early 1990s, when a number of international scientific panels suggested the limit as a way to maintain the relatively stable climate conditions that humans (and other species) had adapted to over the previous 12,000 years and to prevent some of the worst impacts of climate-change-driven drought, heat waves, and sea-level rise.

The 2009 Copenhagen Accord—the document that emerged from that year's UN Climate Change Conference—enshrined a two-degree rise in global average temperature as the threshold of "dangerous" human interference in the climate system.

"There was little scientific basis for the 2°C figure that was adopted," Victor and his coauthor, Charles Kennel, director emeritus of the Scripps Institution of Oceanography, write in Nature,  "but it offered a simple focal point and was familiar from earlier discussions ... At the time, the 2°C goal sounded bold and perhaps feasible."

For many nations and advocacy groups, the target has acquired "near totemic status," Andrew Jordan of the Tyndall Centre for Climate Change Research wrote recently. "To question this to challenge the whole rationale for collectively addressing climate change."

Unattainable and Misleading

Victor and other critics, however, say that as the target becomes effectively unachievable, it threatens the relevance of the process it's intended to catalyze. Though some models show that the target can still be met, those make "heroic assumptions"—immediate global cooperation, for instance, or the sudden, wide availability of new technologies.

"Pretending that they are chasing this unattainable goal has also allowed governments to ignore the need for massive adaptation to climate change," Victor and Kennel write.

Victor also points out that average global surface temperatures doesn't fully represent the changing global climate. Although the increase in average surface temperature has stalled over the past 16 years, average temperatures in the deep ocean—where most of the extra heat in the climate system is stored—has continued to rise.

"We think it's an error to boil it all down to a single goal, given how complex the climate system is," he says.

What's the Alternative?

Victor argues that policymakers should instead focus on a suite of "vital signs" that are more tightly linked to carbon emissions, including atmospheric carbon-dioxide concentrations, ocean heat content, and high-latitude temperature changes. Because of their closer correlation with emissions, he says, these indicators could be more easily translated into emissions-reduction goals and actions at national and local levels. He cites the eight United Nations Millennium Development Goals established in 2000—a set of global health and environmental targets, most of which are expected to be met by the end of 2015—as a promising model.

"Surface temperature can be misleading if it's the only metric we use," agrees Jay Gulledge of the Climate Change Science Institute at Oak Ridge National Laboratory. "But I wouldn't take the next step of saying that policymakers should disregard it."

Early last year Gulledge and several colleagues suggested that policymakers modify their all-or-nothing approach to a temperature target, instead adopting a "climate security" strategy borrowed from defense planning: Aim for an ambitious target, but prepare for the consequences should emissions-reduction measures fall short.

Michael Oppenheimer, a former chief scientist at the Environmental Defense Fund who and now a professor at Princeton University, concedes that the two-degree target is imperfect. "Two degrees probably allows us to avoid dangerous impacts in some parts of the world but not in others," he says. "Certain areas have already exceeded the danger threshold."

But Oppenheimer argues that the target can be refined without discarding it. Scientists can predict regional climate-change risks much more reliably than they could 20 years ago, for instance, and those predictions are being incorporated into the international scientific assessments that inform United Nations climate-change negotiations. And while indicators like ocean heat content may respond more quickly or dramatically to the carbon emissions that cause climate change, surface temperature is more closely related to the effects of climate change—and the effects, after all, are what climate policies at any level are intended to ease.

Victor and Oppenheimer do agree that the two-degree target is slipping out of immediate reach. "There is a good chance we will miss it," Oppenheimer says. "But it still tells us where science thinks we should be, and where we need to get back to. Missing it doesn't mean that the Earth will explode or that climate policy will end. It just puts a sharp point on the fact that we goofed."

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