On the whole, the annual average surface ocean pCO2 of global oceans rose 53.6 µatm from 1992 to 2020 (Figure 2a), with a mean increase rate of 1.9 µatm yr-1. According to the globally averaged CO2 records from Ed Dlugokencky and Pieter Tans, NOAA/GML (gml.noaa.gov/ccgg/trends/), an average increase rate of the global mean atmospheric CO2 during the past three decades was 2.0 µatm yr-1. However, the average growth rate of surface ocean pCO2 (2.0 µatm yr-1) was significantly higher than the average growth rate of atmospheric pCO2 (1.6 µatm yr-1) during the 1990s, suggesting a rapidly decreasing ∆pCO2 across the interface. However, the global average atmospheric pCO2 rise more rapidly in the 2000s (1.9 µatm yr-1) and 2010s (2.4 µatm yr-1), while the global mean surface ocean pCO2 increased 1.8 µatm per year in the 2000s and 2.1 µatm per year in the 2010s on average, suggesting a continuously increasing ∆pCO2 across the interface since 2000. However, large difference between the growth rates in basin scale (Figure 2b). The average growth rate of pCO2 in the Atlantic Ocean during the last three decades was 1.6 µatm yr-1, lowest compared to other basins. While the surface ocean pCO2 in the Southern Ocean increased the most rapidly, with an average growth rate of 1.9 µatm yr-1 during the last three decades.
Our result suggested great variability of global ocean carbon sink (Figure 3a). The variability of the global ocean carbon sink in the past 3 decades can be divided into four stages. The first stage was from 1992 to 2001, during which the global ocean carbon sink weakened from -1.61±0.29 Pg C yr-1 to -1.17±0.26 Pg C yr-1 with an average weakening rate of 0.05 Pg C per year. The second stage was from 2001 to 2003, during which the global ocean carbon sink strengthened back rapidly to -1.79±0.30 Pg C yr-1 in only two years. During the third stage, the global sink weakened to -1.58±0.29 Pg C yr-1 in 2008, with an average weakening rate of 0.04 Pg C per year. After that, the global carbon sink continuously strengthened to -2.14±0.33 Pg C yr-1 until 2020, with an average strengthening rate of 0.06 Pg C per year. The total strengthening speed became 0.03 Pg C per year if these four stages are all involved, covering from 1992 to 2020.
On the basin scale, the carbon sink of the Atlantic Ocean and Arctic Ocean was strengthening with a slow growth rate before 2001 (Fig. 3a), while the carbon sink of the Pacific Ocean, Indian Ocean, and the Southern Ocean was weakening. The weakening of the Indian Ocean carbon sink continued until 2007, reaching a weak sink of -0.09±0.06 Pg C yr-1. After 2007, the Indian Ocean continuously strengthened to -0.24±0.07 Pg C yr-1 until 2020. The weakening of the Southern Ocean carbon sink restart at 2003, and turned to a slow strengthening trend in the last decade. Differently, the Pacific Ocean, Atlantic Ocean, and the Arctic Ocean carbon sink started more rapid strengthening since 2001, but the Pacific Ocean show a large short-term variability related to the ENSO events. The weakening of Pacific Ocean and Southern Ocean carbon sink contributed most to the weakening of global sink during 1992-2001. The rapid strengthening of Pacific sink led to the restrengthening of global carbon sink during 2001-2003. Then for the weakening of global sink during 2003-2008, the weakening of Pacific Ocean sink mainly contributes.
As the important sink contributing most in the interannual variability of global sink, the Pacific Ocean carbon sink was mainly driven by the fluctuated equatorial carbon source (Fig.3 b). Compared with the large variability of 0.2-0.4 Pg C yr-1 in a short time, the long-term trend of the equatorial Pacific source seems to be extremely small. The temperate Pacific carbon sink was continuously strengthening in the last three decades, with a close strengthening speed between the north and south temperate Pacific. In the north of 44°N, a carbon source region caused by upwelling make the subpolar Pacific a weak sink in the 1990s, with an average of -0.08±0.05 Pg C yr-1. With the strengthening of the carbon source in the upwelling regions, the subpolar carbon sink continuously weakened and turned to a weak source of +0.01±0.04 Pg C yr-1 until 2020. Due to the strengthening temperate Pacific sink, the total carbon sink of Pacific Ocean shows a long-term strengthening trend of 0.01 Pg C yr-1with a large short-term variability in few years.
Except the equatorial region, the Atlantic Ocean was mainly a carbon sink and especially a strong sink in the north subpolar regions (Fig. 3c). The south temperate Atlantic sink was relative weaker than the north temperate and subpolar regions, and strengthened slightly from -0.02±0.04 Pg C yr-1 in 1992 to -0.07±0.04 Pg C yr-1 in 2020. The north temperate Atlantic sink remains steady around -0.15±0.04 Pg C yr-1 in the 1990s, and start strengthening since 2001, reaching -0.24±0.04 Pg C yr-1 in 2020. The equatorial Atlantic carbon source continuously weakened from +0.14±0.05 Pg C yr-1 1992 to +0.02±0.04 Pg C yr-1 in 2020, and may became a carbon sink in few years if the weakening trend holds. Fluctuation similar to the equatorial Pacific source was not found in the equatorial Atlantic. The strengthening carbon sink in the temperate and subpolar Atlantic and the weakening equatorial Atlantic carbon source led to the continuously strengthening Atlantic Ocean carbon sink from -0.17±0.09 Pg C yr-1 in 1992 to -0.48±0.10 Pg C yr-1 in 2020.
The Indian Ocean carbon sink was -0.14±0.06 Pg C yr-1 in 1992, with a sink of -0.25±0.07 Pg C yr-1 in the south temperate area and a source of +0.08±0.06 Pg C yr-1 in the equatorial area. The north Indian was a relatively weaker carbon source than the equatorial areas, only 0.02±0.06 Pg C yr-1. With the strengthening of equatorial carbon source and the weakening of south temperate sink, the Indian Ocean became a weak sink of -0.09±0.06 Pg C yr-1 in 2007. After that the continuously strengthening sink in the temperate area and weakening source in the equatorial area turned the Indian Ocean back to a strong sink, reaching -0.24±0.07 Pg C yr-1 in 2020. However, the north Indian Ocean carbon source show a small interannual variability, weakening only 0.01 Pg C yr-1 in the last three decades.
Although different interannual variability of CO2 flux exists in each basin, the Pacific Ocean carbon sink contributes the most to the interannual variability of the global ocean carbon sink, and the Southern Ocean sink next (Figure 4). The interannual variability of the Atlantic Ocean and Indian Ocean carbon sink was relatively lower in individual years, but continuously contributes to the strengthening of global ocean carbon sink. However, the interannual variability of the Pacific Ocean and Southern Ocean carbon sink was more dynamic, with alternating strengthening and weakening trends. In previous research, the weakening of the global ocean carbon sink was considered to be driven by the weakening Southern Ocean sink (Landschützer et al., 2016; Gruber et al., 2019). But our result suggested that the weakening sink of the Pacific Ocean also played an important role. The Pacific Ocean carbon sink weakened 0.22 Pg C in the 1990s, similar to the weakening of 0.23 Pg C in the Southern Ocean sink. After that, the strengthening sink of the Pacific Ocean and the Atlantic Ocean contributed most to the restrengthening of the global ocean carbon sink in the 2000s. While in the 2010s, the Pacific Ocean carbon sink strengthened by 0.24 Pg C, near the total strengthening amount in the Atlantic Ocean, the Indian Ocean, and the Southern Ocean carbon sink.
Figure 5 showed a comparison between the anthropogenic carbon sink from the Global Carbon Project (GCP) (Pierre Friedlingstein et al., 2021) and several pCO2-based products (Rodenbeck et al., 2014; Zeng et al., 2014; Landschützer et al., 2016; Denvil-Sommer et al., 2019; Gregor et al., 2019; Chau et al., 2021; Iida et al., 2021; Gregor et al., 2021). While the pCO2-based products were corrected for the pre-industrial source of CO2 from river input to the ocean, by adding a sink of -0.78 Pg C yr-1 (Resplandy et al., 2018). During the 1990s, a similar weakening trend with a short restrengthening in the 1997 was found in the stepwise FFNN product with the GCPs and other pCO2-based products. Since 2008, the stepwise FFNN product gave an extremely close result with the average of GCPs, reaching a same level of near -3.0 Pg C yr-1 in 2020.While the other pCO2-based products suggested a significantly stronger carbon sink in the same period, and the results ranged from -3.2 to -4.0 Pg C yr-1 in 2020. Although most pCO2-based products and the GCPs suggested continuously strengthening global ocean carbon sinks since the 2001, a short-term weakening of carbon sink was found in the stepwise FFNN product used in this paper and also the pCO2-based product from Rodenbeck et al. (2014).
Comparing the carbon sink of each basin from different products (Table 1), the stepwise FFNN product suggested an extremely close result of the Atlantic Ocean carbon sink. The decadal average Atlantic Ocean carbon sink increased from -0.18±0.09 Pg C yr-1 in the 1990s to -0.45±0.10 Pg C yr-1 in the 2010s. A relatively weaker Arctic Ocean carbon sink was found in the stepwise FFNN product than others, with an average of -0.08±0.13 Pg C yr-1 in the 2010s, while the result of other pCO2-based products range from -0.15 Pg C yr-1 to -0.17 Pg C yr-1. The stepwise FFNN product suggested a significantly stronger Pacific carbon sink and a weaker Southern Ocean carbon sink, which contributes most on the difference of global ocean carbon sink between the stepwise FFNN product and other pCO2-based product in the 2000s and 2010s. The other pCO2 products suggested a restrengthen of the Southern Ocean carbon sink in the 2010s (Landschützer et al., 2016; Keppler et al., 2019; Gruber et al., 2019), while the Southern Ocean carbon sink from the Stepwise FFNN product remains a same level as before.