10:45 AM - 11:00 AM
[ACG46-01] Calibration of a two-layer model emulator for probabilistic assessment of warming levels
Keywords:climate sensitivity, CO2 forcing, ocean heat uptake, parameter ensemble, 1.5°C level
The latest sixth assessment of Intergovernmental Panel on Climate Change (IPCC AR6) presents probabilistic assessment on the long-term evolution of warming levels and the timing of crossing the 1.5°C and 2°C levels under five representative scenarios in the Working Group I (WGI) contribution to AR6. The assessment is based on multiple lines of evidence, most notably in Chapter 4 on future global climate change, emulator calculations with a simple two-layer energy balance model reflecting climate sensitivity assessment, which are referenced along with scenario experiments with multiple full-scale climate models. This study investigates the emulator method based on published data and program codes and explores methodological improvements in warming level assessment through further comparison with the emulator's large-scale parameter ensemble addressed in Chapter 7 on Earth's energy budget.
In Chapter 4, warming levels for each scenario are presented for the best estimate and the 5–95% range. These statistics were calculated separately by the full models and the emulator, with averaging both adopted as the final assessment. The former is observationally constrained estimates of multi-model projections in scenario experiments provided by the Coupled Model Intercomparison Project Phase 6 (CMIP6), while the latter is the results of emulator calculations with calibrated parameters corresponding to the best estimate and the upper and lower bounds of the 5–95% range of climate sensitivity.
Although the 5–95% ranges of the constrained CMIP6 and the climate sensitivity-calibrated emulator are similar to each other, the latter shows noticeably smaller ranges in low-emission scenarios. This difference is presumably because the emulator in Chapter 4 was driven only by the best estimate of effective radiative forcing assessed in Chapter 7. In fact, ensemble calculations considering uncertainty ranges for different forcing factors reduce the difference from the constrained CMIP6. The results suggest the need to pay more attention to uncertainty in forcing when the 1.5–2°C level is the focus in climate assessment of mitigation scenarios.
Calibration of the two-layer model for given climate sensitivity values involves mapping a total of five parameters that control climate feedback and ocean heat uptake onto given values of equilibrium climate sensitivity (ECS) and transient climate response (TCR), along with forcing of CO2 concentration doubling. The Chapter 4 method addresses this underdetermined system by fixing the model layer thicknesses and CO2 doubling forcing, and by combining two parameters for controlling ocean heat uptake. In addition, an approximation is made that treats the ratio of TCR to ECS in a simplified manner. Comparison with the parameter ensemble and mathematical analysis suggest that there is room for improvement in these aspects as well.
In Chapter 4, warming levels for each scenario are presented for the best estimate and the 5–95% range. These statistics were calculated separately by the full models and the emulator, with averaging both adopted as the final assessment. The former is observationally constrained estimates of multi-model projections in scenario experiments provided by the Coupled Model Intercomparison Project Phase 6 (CMIP6), while the latter is the results of emulator calculations with calibrated parameters corresponding to the best estimate and the upper and lower bounds of the 5–95% range of climate sensitivity.
Although the 5–95% ranges of the constrained CMIP6 and the climate sensitivity-calibrated emulator are similar to each other, the latter shows noticeably smaller ranges in low-emission scenarios. This difference is presumably because the emulator in Chapter 4 was driven only by the best estimate of effective radiative forcing assessed in Chapter 7. In fact, ensemble calculations considering uncertainty ranges for different forcing factors reduce the difference from the constrained CMIP6. The results suggest the need to pay more attention to uncertainty in forcing when the 1.5–2°C level is the focus in climate assessment of mitigation scenarios.
Calibration of the two-layer model for given climate sensitivity values involves mapping a total of five parameters that control climate feedback and ocean heat uptake onto given values of equilibrium climate sensitivity (ECS) and transient climate response (TCR), along with forcing of CO2 concentration doubling. The Chapter 4 method addresses this underdetermined system by fixing the model layer thicknesses and CO2 doubling forcing, and by combining two parameters for controlling ocean heat uptake. In addition, an approximation is made that treats the ratio of TCR to ECS in a simplified manner. Comparison with the parameter ensemble and mathematical analysis suggest that there is room for improvement in these aspects as well.