Long-term surface pCO₂ trends from observations and models

We estimate regional long-term surface ocean pCO₂ growth rates using all available underway and bottled biogeochemistry data collected over the past four decades. These observed regional trends are compared with those simulated by five state-of-the-art Earth system models over the historical period. Oceanic pCO₂ growth rates faster than the atmospheric growth rates indicate decreasing atmospheric CO₂ uptake, while ocean pCO₂ growth rates slower than the atmospheric growth rates indicate increasing atmospheric CO₂ uptake. Aside from the western sub-polar North Pacific and the subtropical North Atlantic, our analysis indicates that the current observation-based basin-scale trends may be underestimated, indicating that more observations are needed to determine the trends in these regions. Encouragingly, good agreement between the simulated and observed pCO₂ trends is found when the simulated fields are sub sampled with the observational coverage. In agreement with observations, we see that the simulated pCO₂ trends are primarily associated with the increase in surface dissolved inorganic carbon (DIC) associated with atmospheric carbon uptake, and in part by warming of the sea surface. Under the RCP8.5 future scenario, DIC continues to be the dominant driver of pCO₂ trends, with little change in the relative contribution of SST. However, the changes in the hydrological cycle play an increasingly important role. For the contemporary (1970-2011) period, the simulated regional pCO₂ trends are lower than the atmospheric growth rate over 90% of the ocean. However, by year 2100 more than 40% of the surface ocean area has a higher oceanic pCO₂ trend than the atmosphere, implying a reduction in the atmospheric CO₂ uptake rate. The fastest pCO₂ growth rates are projected for the sub-polar North Atlantic, while the high-latitude Southern Ocean and eastern equatorial Pacific have the weakest growth rates, remaining below the atmospheric pCO₂ growth rate. Our work also highlights the importance and need for a sustained long-term observing strategy to continue monitoring the change in the ocean anthropogenic CO₂ sink and to better understand the potential carbon cycle feedbacks to climate that could arise from it. (authors)