It took more than two years to hash out the first step in the IPCC's new approach to scenarios: choosing a handful of emissions trajectories, known as representative carbon pathways (RCPs). The RCPs then became the basis for new runs of the latest climate models. In parallel, the RCPs are also intended to be focal points for research on socio-economic scenarios, to see what kinds of developments might take the world along each pathway.
The IPCC authors chose four emissions trajectories to focus on, and have labelled them based on how much heating they would produce at the end of the century — 8.5, 6, 4.5 and 2.6 watts per square metre (W m−2). They chose four trajectories to avoid the common misconception that the middle scenario is the most likely. The figure shows the annual carbon dioxide emissions (in billions of tons of carbon) out to 2100 for each of the RCPs. Data from RCP database (http://go.nature.com/Rmyxyt).
The range covered by the RCPs is wider than that contemplated in previous IPCC reports, partly reflecting a general shift in outlook to one where possible future emissions trajectories look more extreme than they did a decade ago. But the RCPs also include a scenario that some think is incredibly optimistic. The 2.6 W m−2 scenario has greenhouse-gas emissions dropping to zero by about 2070, and then continuing to fall, so that the world's emissions become negative — actually pulling greenhouse gases out of the air and locking them away — for decades. This pushes the bounds of what is plausible with mitigation, some experts say.
On the high end, the 8.5 W m−2 case, carbon dioxide levels soar above an enormous 1,300 parts per million by the century's end — and are still rising fast. Some resource experts — such as Jean Laherrère, a petroleum geologist retired from French oil company Total — say this is unrealistic, because people won't be able to produce enough oil, coal and gas to produce that much carbon dioxide. Nebojsa Nakicenovic agrees, but for different reasons: “the high end is impossible,” he says, because the impacts would be so severe that it's inconceivable that the world would not take some kind of action.
Mason Inman is a freelance science writer based in California, USA