gases (GHGs) and aerosols but not including forcing from albedo changes or from mineral dust. The four RCPs are known by their levels of forcing in 2100: 8.5, 6, 4.5, and 3 (in w/m2). Each was developed by a different research group using a different model, and thus the socioeconomic scenarios do not constitute a set, for example, with population or GDP of the highest RCP being the highest for those variables when compared with the other RCPs. All will start from the same historical baseline in 2000 (RCP 6 was still being harmonized at the time of the workshop).2

The RCPs are designed for climate modelers and therefore include the full suite of relevant gases, aerosols, and land use and land cover. They are downscaled to 0.5 degree, and scenarios are extended in a stylized way to 2300 to allow for climate model research on equilibrium behavior of the climate system. They are consistent with data back to 1850 for gases and land cover to 1700. They have data for over a dozen sectors. Edmonds emphasized that the RCPs were selected to bound a wide range of possible future forcing characteristics over time, not to bound socioeconomic uncertainty.

Emissions trajectories for the RCPs are openly available, but the underlying socioeconomic data are not yet available. Edmonds said that some in the community want to make more data available than went into the RCPs, so as not to overemphasize the RCPs. The research teams are trying to create additional socioeconomic scenarios, called replication ensembles, which would yield the same end states that their models produced. Noting that the drivers are not all downscaled in the RCPs, Edmonds emphasized that many different socioeconomic scenarios are consistent with any of these levels of forcing. Even the 2.6 results can be reached from any socioeconomic scenario—if the requisite policies are adopted.

During the parallel phase of the process, while the climate modeling groups are conducting climate model experiments, the IAM community is producing new socioeconomic scenarios, with alternative backgrounds, different technology availability regimes, and alternative shapes of the emissions pathways leading to the same end points—including even 2.6, which can be produced in various ways. An open question is what range of socioeconomic scenarios should be explored in the IAMs. It may be that the 8.5 scenario requires a more tightly specified socioeconomic future than the 2.6 scenario does.

Edmonds concluded by noting that the new scenarios process could provide an embarrassment of riches—hundreds of scenarios with different combinations of socioeconomic and climate changes. The RCPs

2

They are documented in Moss et al. (2010), and data can be found at http://www.iiasa.ac.at/web-apps/tnt/RcpDb/ [November 2010].



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