Now a team from the US and Germany has used cross-impact balance (CIB) analysis to look at the internal consistency of storylines in the IPCC Special Report on Emissions Scenarios (SRES). The four principal storylines employed vary widely in internal consistency, the team found, and storylines involving highly carbon-intensive development are under represented.

"The strong internal consistency of the high emission scenarios and the robustness of this finding to changes in our analytical assumptions suggest there may be tendencies in the global-energy system toward higher emissions," Vanessa Schweizer of the US National Center for Atmospheric Research (NCAR) told environmentalresearchweb. "Scenarios with lower emission profiles did not share this quality. This may mean that scenario analysts do decision-makers a disservice when they claim that low emission scenarios are equally plausible to high emission scenarios without climate policy."

As Schweizer relates in a video abstract in ERL, the team believes this may actually lead to an understatement of the global mitigation challenge.

"In our analysis, we did find some low emission scenarios with strong internal consistency, but there were many more with higher emissions," said Schweizer. "This suggests that in the absence of policy intervention, there may be more ways to retain a high-emission energy system than to achieve a low-emission energy system."

Schweizer and Kriegler examined six socioeconomic drivers of emissions – population, economy, energy, technology, economic policy and environmental policy. Using the cross-impact balance method, they recorded semi-quantitative judgements on the consistency of interactions of these drivers, based on today's knowledge, in a matrix. For example, a scenario describing high levels of wealth, low educational attainment and low fertility rates would seem questionable.

The team found that the internal consistency of scenarios in the IPCC Special Report showed considerable variation. Also, scenarios incorporating future reliance on coal and high economic growth showed high internal consistency but were not considered. The 14 sensitivity tests, in which different influences on the scenario were considered, revealed that four of the IPCC scenarios and 17 non-IPCC scenarios were robust. Of these, only two scenarios involved energy structures with low carbon intensity.

"Through this sensitivity testing, we found a subset of scenarios that repeatedly retained their internal consistency, and they happened to describe futures of high energy-related emissions, where global economic growth is high or very high and the energy system is predominantly powered by coal," said Schweizer.

Previously researchers have examined the consistency of qualitative aspects of scenarios by using quantitative simulation. However, since the qualitative and quantitative aspects of scenarios represent different details, Schweizer believes this could constrain the range of plausible futures considered.

"We have shown that the conventional wisdom for the roles of qualitative and quantitative aspects of scenarios – specifically, that quantitative modelling ensures the internal consistency of the qualitative description – does not necessarily hold," she said.

But qualitative details in a scenario that are not represented by a quantitative model still need to be tested for internal consistency. "This can only be done with appropriate methodological tools such as cross-impact balance analysis," said Schweizer. "Thus, rather than subsume the qualitative aspects of scenarios to their quantitative aspects, investigation of qualitative and quantitative aspects should be iterative, as scenarios contain some details that are better understood quantitatively and others qualitatively."

Schweizer reckons that the cross-impact balance method for qualitative scenarios could resolve the criticism that such scenarios are often the product of a relatively unstructured and creative process. "Qualitative scenarios are certainly useful for communicating the working assumptions of complex models such as integrated assessment models," she said. "My co-author, Elmar Kriegler, and I demonstrated that qualitative scenarios can be generated in a systematic way. This is an alternative to the traditional creative approach."

Now the team, along with Brian O'Neill at the National Center for Atmospheric Research, is using cross-impact balance analysis to inform the next generation of qualitative socio-economic scenarios for climate-change research. "As part of this work, we have developed survey instruments to collect judgments from interdisciplinary panels of experts who research energy systems or social science," said Schweizer. "As was the case with the SRES scenarios, the new scenarios will describe globally aggregated socioeconomic trends that could occur this century. However, in our survey work we found that many experts have concluded that socio-economic scenarios that are this aggregated are of limited usefulness. For this reason, we also have plans to systematically develop socioeconomic scenarios that are regionally disaggregated."

The scientists reported their work in Environmental Research Letters (ERL).