Anthropogenic Attribution of the Increasing Seasonal Amplitude in Surface Ocean pCO2

A positive trend in time has been noted in the seasonal amplitude of surface ocean pCO2 over much of the oceans, which is expected to have detrimental impacts on marine ecosystems. To determine whether or not this has an anthropogenic cause, this study investigates historical climate simulations from the Detection and Attribution Model Intercomparison Project with and without anthropogenic forcing. The simulations with anthropogenic forcing show clear evidence of positive trends, whereas the simulations with constant preindustrial atmospheric CO2 and natural external forcing give only negligible trends. A statistical analysis of five zonal latitudinal bands reveals that the trends detected over 1990–2014 in an ensemble of six observational products are attributable to anthropogenic forcing in mid‐latitudes (40°N–10°N, 10°S–40°S), while no trends are detected and modeled in the tropics and the Southern Ocean. Most models fail to represent the sign of the observed climatological mean seasonal cycle difference in high latitudes. Plain Language Summary Global warming due to fossil fuel use is unequivocal and documented by long records. In contrast, measurements for other important Earth system parameters are only available over recent decades. A short record length makes the attribution of observed trends to human‐induced forcing challenging. Trends may be hard to identify due to chaotic Earth system variability. The seasonal amplitude of the partial pressure of CO2 (pCO2) in the surface ocean is observed to increase, with global‐scale data available since 1990. A continued increase is expected to have negative impacts on fish as the amplified seasonality leads to very high pCO2, hindering the uptake of oxygen during breathing. Here, we analyze pCO2 from climate model historical simulations forced by human‐caused and natural factors versus simulations with natural factors only. The results allow us to firmly attribute the observed trend in the pCO2 seasonal amplitude in mid‐latitudes to human activities. Key Points Trends in seasonal amplitudes of surface ocean pCO2 are analyzed in observations and CMIP6 simulations with/without anthropogenic forcing Applying an Analysis of Covariance, the observed trends in mid‐latitude regions are firmly attributed to anthropogenic forcing The model ensemble shows large data‐model and model‐model discrepancies in the pCO2 seasonal cycle, in particular in high‐latitude regions