The inconstancy of the transient climate response parameter under increasing CO2

Area of Science:

• Climate Science
• Earth System Science
• Atmospheric Science

Background:

• The Coupled Model Intercomparison Project Phase 5 (CMIP5) provides a framework for comparing climate model outputs.
• Understanding transient climate response (TCR) is crucial for projecting future climate change.
• The 1pctCO2 scenario simulates a gradual increase in atmospheric carbon dioxide.

Purpose of the Study:

• To investigate why the global mean surface air temperature increase is larger during the second CO(2) doubling compared to the first in CMIP5 models.
• To identify and quantify the contributing factors to this amplified warming.
• To assess the implications for estimating climate sensitivity from observed climate change.

Main Methods:

• Analysis of CMIP5 model outputs under the 1pctCO2 scenario.
• Quantification of heat uptake by the ocean.
• Evaluation of climate feedback parameters and radiative forcing.
• Statistical correlation analysis between temperature at different CO(2) levels.

Main Results:

• The surface climate system's reduced heat loss to the warming ocean is a primary driver of amplified warming.
• Model spread in ocean heat uptake efficiency correlates with thermal coupling between upper and deep ocean layers.
• The climate feedback parameter may decline with rising CO(2) concentration or time, contributing to amplified warming.
• Temperature projections correlate more strongly with quadrupled CO(2) levels than TCR, suggesting TCR might underestimate observed warming.

Conclusions:

• Ocean heat uptake efficiency and changes in climate feedbacks significantly influence the amplified warming observed during the second CO(2) doubling.
• The transient climate response (TCR) may underestimate the actual warming projected for the end of the century.
• Further research is needed to disentangle the precise contributions of radiative forcing and climate feedbacks.