It took a long time for nations to set a speed limit on the road to a warming world.

But for the past four years, even though negotiators have never arrived at a plan for avoiding dangerous climate change, they have agreed on a goal: limiting the increase in the Earth's global average surface temperature to 2°C (3.6°F) above the preindustrial level.

Now, two Union of Concerned Scientists (UCS) climate scientists and two colleagues argue that policymakers need to acknowledge that the world is already on track for warming beyond 2°C. (See related "Quiz: What You Don't Know About Climate Change Science.")

"A policy narrative that continues to frame this target as the sole metric of success or failure to constrain climate change risk is now itself becoming dangerous," wrote Todd Sanford and Peter Frumhoff of UCS in the commentary published Wednesday in Nature Climate Change. "[It] ill-prepares society to confront and manage the risks of a world that is increasingly likely to experience warming well in excess of 2°C this century," said the piece, co-authored by Amy Luers of the San Francisco-based Skoll Global Threats Fund, and Jay Gulledge, of the U.S. Department of Energy's Oak Ridge National Laboratory. (See a blog on the commentary by Gulledge.)

Rethinking the Target

The authors are by no means the first to suggest a rethinking of the 2°C goal. Todd Stern, the lead U.S. climate negotiator in President Barack Obama's administration, provoked anger in 2012 when he said a more "flexible, evolving" approach might be more effective in spurring a political accord. Coming at the issue from an entirely different angle, retired NASA climate scientist James Hansen and a group of colleagues wrote in December the 2°C target was not stringent enough, and "so dangerous" as to be "foolhardy." At that level, the world risked initiating feedbacks in the climate system, such as the melting of ice sheet area, that could trigger irreversible warming out of humanity's control.

Hansen and colleagues suggested a 1°C target was far less dangerous. The Earth has warmed 0.85°C from 1880 (preindustrial times) to 2012, according to the latest consensus science reported in September by the Intergovernmental Panel on Climate Change (IPCC), the scientific body established by the United Nations to inform governments of climate risks.

The UCS scientists and colleagues took the IPCC to task for issuing reports that present different future scenarios, while making no judgment on the relative likelihood of the varying projections, "implicitly treating all scenarios as equivalently plausible."

"Inadvertently, the [IPCC] reinforces the present narrative by failing to provide policymakers with guidance on how to weigh the relative likelihood of the scenarios of future concentrations of heat-trapping gases and other drivers of warming on which its climate change projections are based," the authors said. (See related, "UN Climate Report Relevance Debated Amid Rollout.")

Gavin Schmidt, a climate scientist with the NASA Goddard Institute for Space Studies, agreed that the world needs to assess the risks of high-magnitude warming, but he said criticism of the IPCC is misplaced.

"We do need to assess the risks of high scenarios, not just assume that [2°C] guiderail is achievable, but the call for IPCC to give probabilistic information about different [scenarios] is just a non-starter," Schmidt said in an email. "What is the probability of an international carbon tax? A breakthrough in nuclear power generation? Solar? Of missing feedbacks in models becoming important? These are undefinable, and yet essential for what they are calling for.

"We need instead to work around these limitations, not pretend they can be vanished away by an IPCC statistician's pen," Schmidt said.

Andrew Jordan of the Tyndall Centre for Climate Change Research in Norwich, England, co-author of a similar call for a reassessment of the 2°C target, said the IPCC actually "suggested" in its September report that the 2°C target would be breached. The IPCC report showed that the world's "carbon budget," the amount of greenhouse gas that can be emitted without exceeding 2°C, could be used up entirely by 2040, Jordan noted.

"Given where emissions are in 2014, now is, as the authors of this new paper argue, precisely the time to re-think the current policy narrative," Jordan said in an email. "At present, policy makers are stuck in a binary debate about whether or not the target of [2°C] should or will be met."

Jordan noted there are risks in focusing a new debate on the 2°C target. "If the target is downplayed and/or higher temperature limits are debated, breaching the [2°C] ceiling could easily become a self-fulfilling prophecy," he said. "But failing to debate it could mean that by 2020, international policy is premised on an unrealistic target, further undermining its credibility and under-preparing the world for the challenge of adapting to higher temperatures."

Prepare for the Worst

Climate scientists first proposed the 2°C target in the mid-1990s as a way of giving substance to the commitment nations had made to address climate change at the 1992 Earth Summit at Rio de Janeiro. It was not until 2009 in Copenhagen, after the 15th annual session of negotiations around that goal failed to reach a binding treaty, that nations instead signed on to an accord pledging to work toward the 2°C limit.

Since then, the UCS scientists and colleagues wrote in their new commentary, "the foundation on which the 2°C target was built has steadily eroded." Not only have global carbon emissions continued to rise 3 percent a year, but the science has made more clear that human populations and natural systems face serious risk of substantial climate damage at warming less than 2°C, they said.

The scientists didn't spell out a different target in the new commentary, but instead, a different approach. They endorsed an idea borrowed from national security and defense planning, a framework for "climate security," that has been proposed by scientists at the London-based nonprofit EG3. This "ABC" approach says policymakers should aim for an "ambitious" target for reducing carbon emissions, while "building" for, or adapting for, greater warming than targeted, and engaging in "contingency" planning for future climate emergency.

Such an approach, the scientists said, would better communicate to the public the magnitude of climate risks the planet faces. It might increase public willingness to make necessary trade-offs, for example, to accept local impact on wildlife and ecosystems from the siting of renewable energy projects, such as wind turbines and large-scale solar plants, or to consider the risks of geoengineering for forced climate cooling. (See related "Video: The Tortoise and the Solar Plant" and "Mojave Mirrors: World's Largest Solar Plant Ready to Shine" and "Pictures: Seven Emergency Climate Fixes.")

The authors said it might also motivate "difficult but much-needed dialogue and planning" for the drastic measures needed to address "truly disruptive impacts." Two examples: relocation of development from floodplains around London after 2060 and the creation of water-efficient corn varieties for Africa, would require planning and investment now, they said. (See related "Quiz: What You Don't Know About Food, Water, and Energy.")