Climate Modeling, Scenarios, and Time Periods 

The Fifth Assessment Regional Reports were generated through the lenses of the following climate and scientific models:  

CMIP6 (Coupled Model Intercomparison Project Phase 6)

CMIP6 (Coupled Model Intercomparison Project Phase 6) CMIP6 (Coupled Model Intercomparison Project Phase 6) is the most recent version of a global collaboration that standardizes and combines the most advanced climate models to project how Earth’s systems will react to our changing atmosphere. CMIP6, shows the most likely climate trends and the less likely, but still possible, extremes. 

Greenhouse Gas Emissions Scenarios

Current international climate models integrate two kinds of scenarios: Shared Socioeconomic Pathways (SSPs) and Representative Concentration Pathways (RCPs). The SSPs model how global society might evolve over the next century, including changes in population, policy, technology, and economic growth, while tracking the amount of heat trapped in the atmosphere. The RCPs correspond to emissions and capture a quantifiable range of emissions and warming—how much do we expect the world will continue to emit, and the associated warming, throughout the century?  Combined, these describe how global social conditions (e.g., population, technological, and economic growth) affect emission trajectories. This shift offers a more nuanced understanding of how our collective decisions today will shape the climate of tomorrow. The Fifth Assessment research primarily focuses on two RCP/SSP combinations:  

• SSP 2–4.5 (i.e., utilizing the second SSP narrative and RCP 4.5) describes an intermediate scenario indicating improvements toward meeting mitigation commitments. This scenario shows carbon dioxide equivalent (CO2e) emissions initially hover at current levels, then start to fall by mid-century. 
• SSP 3–7.0 articulates a high emissions scenario and less optimistic pathway, assuming current emission levels continue throughout the year 2100. This scenario shows CO2e emissions doubling by 2100. 
• The Fifth Assessment also references a more aggressive emissions scenario (SSP5–8.5), which describes a very high and unlikely emissions pathway that shows CO2e emissions tripling by 2075. The Assessment includes this scenario as a reference tool for a worst-case emissions outcome for informational and planning purposes rather than a plausible climate change future.

Global Warming Levels

Global Warming Levels (GWLs) ask “What happens to California once the world reaches a certain level of warming?” The focus is on adapting to a change in the expected climate, such as a 2°C (35.6°F) increase. While these models identify a range of years when these thresholds might be reached, the emphasis is on the intensity of the impact rather than the calendar date. This is a different and complementary approach to the previously used date-based approach. 

Downscaling with Localized Constructed Analogues (LOCA) and Weather Research and Forecasting (WRF)

To make global climate models (GCMs) useful for local planning, scientists use a technique called downscaling. While global models provide a big picture view of the planet, the Fifth Assessment uses the LOCA2 statistical and WRF dynamical downscaling techniques and a hybridized version of these techniques, LOCA2-Hybrid, to focus on California’s particular climate future.  

Time Periods

The Fifth Assessment uses a 30-year timeframe for analyses, as shown in the table below, alongside historical projections to draw comparisons between current and future time periods.