Reconciling Observation and Model Trends in North Atlantic Surface CO2.

The North Atlantic Ocean is a region of intense uptake of atmospheric CO2. To assess how this CO2 sink has evolved over recent decades, various approaches have been used to estimate basin‐wide uptake from the irregularly sampled in situ CO2 observations. Until now, the lack of robust uncertainties associated with observation‐based gap‐filling methods required to produce these estimates has limited the capacity to validate climate model simulated surface ocean CO2 concentrations. After robustly quantifying basin‐wide and annually varying interpolation uncertainties using both observational and model data, we show that the North Atlantic surface ocean fugacity of CO2 (fCO2−ocean) increased at a significantly slower rate than that simulated by the latest generation of Earth System Models during the period 1992–2014. We further show, with initialized model simulations, that the inability of these models to capture the observed trend in surface fCO2−ocean is primarily due to biases in the models' ocean biogeochemistry. Our results imply that current projections may underestimate the contribution of the North Atlantic to mitigating increasing future atmospheric CO2 concentrations. Key Points: Robust uncertainties for the recent change in the North Atlantic surface fCO2 are determined by using observational‐based and model productsThe increasing North Atlantic surface fCO2 is overestimated by ESMs during 1992–2014, and not captured by models' internal variabilitySimulation initialised with biogeochemical observations correct for the models' bias in the trend in surface CO2 trends [ABSTRACT FROM AUTHOR]