Abstract
Abstract. Using measurements of the surface-ocean CO2 partial pressure (pCO2) and 14 different pCO2 mapping methods recently collated by the Surface Ocean pCO2 Mapping intercomparison (SOCOM) initiative, variations in regional and global sea–air CO2 fluxes are investigated. Though the available mapping methods use widely different approaches, we find relatively consistent estimates of regional pCO2 seasonality, in line with previous estimates. In terms of interannual variability (IAV), all mapping methods estimate the largest variations to occur in the eastern equatorial Pacific. Despite considerable spread in the detailed variations, mapping methods that fit the data more closely also tend to agree more closely with each other in regional averages. Encouragingly, this includes mapping methods belonging to complementary types – taking variability either directly from the pCO2 data or indirectly from driver data via regression. From a weighted ensemble average, we find an IAV amplitude of the global sea–air CO2 flux of 0.31 PgC yr−1 (standard deviation over 1992–2009), which is larger than simulated by biogeochemical process models. From a decadal perspective, the global ocean CO2 uptake is estimated to have gradually increased since about 2000, with little decadal change prior to that. The weighted mean net global ocean CO2 sink estimated by the SOCOM ensemble is −1.75 PgC yr−1 (1992–2009), consistent within uncertainties with estimates from ocean-interior carbon data or atmospheric oxygen trends.
Highlights
The global ocean acts as a major sink for anthropogenic carbon, and thereby helps to slow down the human-induced warming of the Earth’s climate (Stocker et al, 2013)
We first provide an overview of the estimated seasonal and interannual variations in oceanic biomes (Sect. 4.1), and the ability to estimate them from pressures of CO2 (pCO2) data and available mapping methods
Taking advantage of an ensemble of 14 partially complementary surface-ocean pCO2 mapping methods recently collated by the Surface Ocean pCO2 Mapping intercomparison (SOCOM) initiative, we analysed sea–air CO2 flux variability globally and for a subdivision of the ocean into 17 biomes (Fay and McKinley, 2014)
Summary
The global ocean acts as a major sink for anthropogenic carbon, and thereby helps to slow down the human-induced warming of the Earth’s climate (Stocker et al, 2013). Variations in the oceanic carbon sink, in particular a possible decline under climate change, co-determine the future climate trajectory. In addition to this direct relevance, present-day variations in the sea–air CO2 exchange, when related to possible driving factors, can be employed to provide information on the underlying mechanisms of ocean biogeochemistry. Estimates of the oceanic CO2 uptake rate and its variability were largely based on (1) ocean biogeochemical process models (see e.g. Wanninkhof et al, 2013), (2) inverse estimates based on atmospheric CO2 data (see Peylin et al, 2013), or (3) inverse estimates based on oceaninterior carbon data (Gloor et al, 2003, and subsequent refinements). While ocean-interior inversions offer a strong databased constraint on the long-term flux in larger regions, they do not provide flux variability or finer spatial detail
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