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Abstract

There are inherent difficulties in quantifying carbon cycle-climate feedbacks over the 21st century because the system is in a transient state. The conventional approach of deriving gain factors only strictly applies at equilibrium, and they differ with scenario and with respect to different climate variables (e.g., CO2, radiative forcing, and temperature) which have different time lags. Here we show that the positive feedback whereby ocean heating reduces the solubility of CO2 can be quantified in a scenario-independent way, directly from ocean heat content changes, by expressing it as an ‘equivalent carbon emission’. On annual to centennial timescales, the feedback has the same impact on atmospheric CO2 as an equivalent emission flux of fossil fuel carbon. From ocean heat-content data we quantify the ocean heating-CO2 solubility positive feedback, which increased in average strength from an equivalent emission of ∼0.08 PgC yr−1 over 1961-2003 to ∼0.19 PgC yr−1 during 1993-2003.