Climate Scenarios Development for the Fourth National Climate Assessment

The Fourth National Climate Assessment (NCA4) will use a set of climate-related scenarios as the basis of physical climate and impacts analyses, as done for previous Assessments. These scenarios have been developed under the guidance of the U.S. Global Change Research Program Climate Scenarios Task Force and largely consist of model-derived products based on the four “representative concentration pathways” (RCPs) from the IPCC’s Fifth Assessment Report (AR5).

NCA4 will use the full range of RCPs for physical climate science analyses. Assessments of impacts, vulnerability, and adaptation responses will be primarily based on RCP 8.5 as a high-end scenario and RCP 4.5 as a low-end scenario. Other scenarios (e.g., RCP 2.6) may be used in addition where instructive, such as in analyses of mitigation issues. The use of RCPs 8.5 and 4.5 as core scenarios is generally consistent with the range of emission scenarios used in the Third National Climate Assessment (NCA3). In addition, using a low-end and a high-end scenario will facilitate communications of assessment findings.

The NCA4 scenarios consist of simulations from the Coupled Model Intercomparison Project, Phase 5 (CMIP5) and its derivatives, including Localized Constructed Analogs (LOCA) downscaled simulations. The LOCA dataset consists of statistically downscaled CMIP5 projections for North America at a daily temporal resolution and a 1/16th degree spatial resolution. More than 30 temperature and precipitation extremes metrics have been processed using LOCA data, including threshold, percentile, and degree days calculations.

Previous assessments developed scenarios by creating ensemble averages across all available model simulations. For NCA4, however, both CMIP5 and LOCA scenarios utilize a model weighting scheme based on both model independence and reproducibility of current climate conditions.

Methods for incorporating risk-based framing are also being developed for the scenarios in order to provide information relevant to assess risk as a function of changes in climate, specifically with regard to extremes, worst-case scenarios, and thresholds.