High radiative forcing climate scenario relevance analyzed with a ten-million-member ensemble

Area of Science:

• Climate Science
• Climate Modeling
• Earth System Science

Background:

• Future climate projections rely on emissions scenarios.
• Traditional scenarios often use qualitative storylines.
• Probabilistic approaches offer a quantitative alternative for assessing future forcing.

Purpose of the Study:

• To develop a probabilistic ensemble of radiative forcing trajectories.
• To assess the relevance of future forcing thresholds.
• To quantify the likelihood of different climate futures.

Main Methods:

• Coupled a probabilistic database of greenhouse gas emission scenarios with a reduced complexity climate model.
• Employed probabilistic calibration for the climate model.
• Generated a multi-million-member ensemble of radiative forcing trajectories.

Main Results:

• Projected median radiative forcing of 5.1 W/m² by 2100 (5th-95th percentiles: 3.3-7.1 W/m²).
• Low probability (0.5%) of exceeding 8.5 W/m² by 2100.
• Probability of exceeding 8.5 W/m² increases to ~7% by 2150.

Conclusions:

• Probabilistic projections provide valuable insights into future climate forcing.
• High forcing scenarios (e.g., 8.5 W/m²) remain relevant for damage function calibration and assessing low-probability/high-impact events.
• Continued utility of extreme scenarios for understanding long-term climate impacts and 22nd-century climate characterization.