164 NATURE CLIMATE CHANGE | VOL 4 | MARCH 2014 | www.nature.com/natureclimatechange opinion & comment COMMENTARY: The climate policy narrative for a dangerously warming world Todd Sanford, Peter C. Frumhof, Amy Luers and Jay Gulledge It is time to acknowledge that global average temperatures are likely to rise above the 2 °C policy target and consider how that deeply troubling prospect should afect priorities for communicating and managing the risks of a dangerously warming climate. W hen world leaders signed the Copenhagen Accord in 2009, they agreed to limit the increase in global average surface temperature to less than 2 °C above the pre-industrial level, a target then widely viewed as consistent with avoiding dangerous climate change and feasible to achieve through ambitious reductions in heat-trapping emissions. he climate policy agenda has since been dominated by the narrative that swit and deep reductions in emissions are urgently needed to stay below 2 °C (refs 1–4). his global temperature target has brought a valuable focus to international climate negotiations, motivating commitment to emissions reductions from several nations 5 . But 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, because 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. Inadvertently, the Intergovernmental Panel on Climate Change (IPCC) — the scientiic body charged with informing governments about climate change — 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. Science and the climate policy narrative Since Copenhagen, the foundation on which the 2 °C target was built has steadily eroded. Both human populations and natural systems are now understood to face serious risks of substantial climate change damages with less than 2 °C warming 6 , leading many of the most vulnerable developing nations to argue, with just cause, that the target should be lowered to 1.5 °C. Indeed, the relatively modest warming experienced so far (0.85 °C increase since 1880) 7 is already driving arguably dangerous impacts, including more deaths from extreme heat 8 , widespread forest die-of from climate-driven heat stress and drought 9 , and more extreme coastal looding from higher storm surges resulting from sea-level rise 10 . Such impacts underlie recent demands by developing countries for so- called loss and damage payments, a prevalent topic of negotiations during the COP19 climate meeting in November 2013. Global carbon emissions have also continued to rise, unabated, on average by 3% per year since 2000, including the years since the Copenhagen Accord was signed 2 . At present, emissions are tracking just above the highest Representative Concentration Pathway (RCP 8.5) used by the IPCC to assess projected climate change; a pathway in which emissions would hurtle past the 2 °C carbon budget before mid-century (Fig. 1). Keeping global temperatures from rising above 2 °C could be achieved by rapidly transitioning to a trajectory similar to RCP 2.6, the lowest concentration pathway used by the IPCC. Following RCP 2.6 would require global carbon emissions to decline by 50% below 1990 levels by mid-century and, according to several models, may well require sustained global net negative emissions a few decades later (Fig. 1). his might, in principle, be achieved by coupling biomass energy production with carbon capture and storage on a massive scale or by other yet-to-be- developed technologies 2 . Such heroic assumptions lead a growing number of analysts to conclude that prospects for limiting warming to 2 °C are becoming vanishingly small 2,11 . One recent study excluded climate model outputs using RCP 2.6 from interscenario comparisons of projected changes on the assumption that RCP 2.6 is currently unfeasible 12 . A projection is not destiny, of course, but some are surely more likely than others. Yet, in its most recent assessment, the IPCC makes no judgement on the relative likelihood of the magnitude of future warming associated with each RCP in presenting climate model projections, implicitly treating all scenarios as equivalently plausible. Some scenarios are also projected to lead to very divergent futures in terms of impacts 1 . his leads to efective responses to manage climate risk heavily depending, in some cases, on the scenario actually realized. Policymakers thus have no clear scientiic guidance for confronting and managing the growing risk of high-magnitude warming. Building on its strong legacy of rigorous and detailed treatments of likelihood and uncertainty of observed trends, attribution of change and model output (including future projections) 13 , and on recommendations irst made 14 and subsequently elaborated on 15 more than a decade ago, the IPCC should provide policymakers with guidance on the relative likelihood of diferent magnitudes of future warming. One path forward would be to build on the approach of soliciting expert judgement found on other subjects, such as transient climate response to alternative radiative forcing trajectories 16 , eliciting input that considers both climate sensitivities and the biophysical, socioeconomic, technological and policy drivers of future emissions and concentrations. Towards a new climate policy narrative An ambitious goal for stabilizing global temperatures must remain a central focus of climate policy within a comprehensive risk- management framework. But calling for swit and deep reductions in emissions, although essential, is not suicient. Confronting and managing the risks of high-magnitude warming will require a science-based policy narrative that honestly communicates these risks, accounts for potential policy failures © 2014 Macmillan Publishers Limited. All rights reserved