Long-term surface pCO₂ trends from observations and models

Abstract

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 subpolar 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 subsampled 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 subpolar 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.

• surface pCO₂
• ocean CO₂ sinks
• Earth system models
• CMIP5 projections
• ocean biogeochemistry

To access the supplementary material to this article, please see Supplementary files under Article Tools online.

To access the supplementary material to this article, please see Supplementary files under Article Tools online.

6. Acknowledgements

We thank three anonymous reviewers for their critical feedback, which helped to improve the manuscript significantly. The research leading to these results was supported by the Centre for Climate Dynamics at the Bjerknes Centre and the EU FP7 project CARBOCHANGE ‘Changes in carbon uptake and emissions by oceans in a changing climate’, which received funding from the European Commission's Seventh Framework Programme under grant agreement no. 264879. SOCAT, GLODAP, CARINA, and PACIFICA are international efforts supported by the International Ocean Carbon Coordination Project (IOCCP). The many researchers and funding agencies responsible for the collection of data and quality control are thanked for their contributions to the above data products. We acknowledge the World Climate Research Programme's Working Group on Coupled Modelling, which is responsible for CMIP5. For CMIP the U.S. Department of Energy's Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. JFT acknowledges the Research Council of Norway funded project EarthClim (no. 207711/E10) and the NOTUR projects nn2980k and nn2345k as well as the NorStore projects ns2980k and ns2345k for supercomputer time and storage resources. AL acknowledges support from the Australian Climate Change Science Program. The collection of data from CLIVAR cruises was compiled and generously shared with us by Robert Key at Princeton University. The CESM1-BGC output was provided by Keith Lindsay.

Notes

To access the supplementary material to this article, please see Supplementary files under Article Tools online.