Are anthropogenic changes in the tropical ocean carbon cycle masked by Pacific Decadal Variability?

Abstract:

The tropical Pacific is the ocean’s largest natural source of CO2 to the atmosphere, thus playing a key role in the global carbon cycle. Observational studies show that that since 1980, the outgassing of CO2 over the central equatorial Pacific has remained constant or slightly increased, as ocean partial pressure of CO2 (pCO2) has risen at about the same rate as atmospheric pCO2. On the contrary, Earth System Models (ESMs) project robust decreases in the sea-air pCO2 difference (ΔpCO2) and associated outgassing throughout the 21^st Century. We reconcile these conflictive ideas using an ensemble of 28 simulations performed with the Community Earth System Model Version 1 (CESM1). The large size of the ensemble allows us to explore the interaction of internal and forced changes in Pacific outgassing. The CESM1 ensemble shows that during the 1980-2014 period, internally driven and forced trends can have similar magnitudes, suggesting that they could have constructive or opposing effects, depending on the phase of internal variability. For instance, the changes in ΔpCO2 and outgassing are negligible when Pacific Decadal Variability (PDV) is trending towards its negative phase, just as observed during 1980-2014. Translating this result to nature implies that equatorial outgassing could be already diminishing in response to increasing atmospheric CO2. This signal, however, has not emerged from the background of internal variability, particularly due to the ongoing multi-decadal trends in Pacific climate.