Changes in IPCC Scenario Assessment Emulators Between SR1.5 and AR6 Unraveled

Area of Science:

• Climate Science
• Earth System Science
• Climate Modeling

Background:

• The Intergovernmental Panel on Climate Change (IPCC) relies on scenario databases for climate assessments.
• Updates between IPCC reports, like SR1.5 and AR6, involve complex and sometimes unclear factors.
• Earth System Model emulators are crucial for estimating global warming from emission scenarios.

Purpose of the Study:

• To isolate and analyze the impact of Earth System Model emulators on updated IPCC climate assessments.
• To compare warming projections from emulators used in SR1.5 and AR6.
• To understand the factors contributing to changes in climate projections between IPCC assessment cycles.

Main Methods:

• Focused analysis on Earth System Model emulators as a specific factor in IPCC assessment updates.
• Compared warming projections derived from SR1.5-era emulators with those from AR6-calibrated emulators.
• Quantified the consistency of warming projections between the two emulator sets.

Main Results:

• Warming projections using AR6-calibrated emulators are consistent within approximately 0.1°C with SR1.5 emulators.
• Two key changes compensated: increased assessed historical warming and reduced projected warming due to improved emulator calibration.
• Improved agreement between emulator response to emissions and the calibrated assessment contributed to projection consistency.

Conclusions:

• Earth System Model emulators show a high degree of consistency between SR1.5 and AR6 assessments.
• The evolution of emulators and historical warming data between IPCC reports has a quantifiable impact on projected warming.
• This analysis clarifies one factor influencing the IPCC's climate change projections and emission targets.