Climate scenarios provide a consistent set of possible future conditions to inform analyses of the risks posed by climate change.  

The Fourth National Climate Assessment (NCA4) assesses risks to the United States posed by climate and global change. Scenarios that represent a range of plausible future changes in key risk drivers, such as greenhouse gas emissions levels, weather and climate extremes, sea level, population, and land use, are used to guide NCA4’s evaluation of specific climate-related risks in regions and sectors across the United States.

Consistent with previous NCAs, NCA4 relies on climate-related scenarios generated for the assessments of the Intergovernmental Panel on Climate Change (IPCC). The IPCC’s Fifth Assessment Report, completed in 2014, used model-derived products from the Coupled Model Intercomparison Project Phase 5 (CMIP5) based on four “representative concentration pathways” (RCPs). These RCPs capture a range of plausible future greenhouse gas emissions pathways that drive the climate models.

For its assessment of physical climate science presented in NCA4 Volume I (Climate Science Special Report), NCA4 uses the full range of IPCC RCPs and CMIP5 products. For assessments of impacts, vulnerability, and adaptation responses presented in volume II (Climate Change Impacts, Risks, and Adaptation in the United States), NCA4 will focus on RCP 8.5 as a high-end scenario consistent with continued high emissions levels and RCP 4.5 as a low-end scenario consistent with implementation of significant mitigation measures. 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 RCP 4.5 as core scenarios is generally consistent with the range of emission scenarios used in NCA3.

NCA4 authors have been provided with a suite of high-resolution scenario products for the United States, covering at least the 21^st century, to assist them in the development of their chapters. Data products include changes in temperature and precipitation averages and extremes; changes in local sea level along the entire U.S. coastline; changes in population; and changes in land use driven by population changes. The provided scenario products help ensure consistency and improve the ability to synthesize across chapters. These products were coordinated and developed by the USGCRP Climate Scenarios Task Force, Scenarios and Interpretive Science Coordinating Group, and Interagency Sea Level Rise and Coastal Flood Hazard Scenarios and Tools Task Force. They are available at scenarios.globalchange.gov.