As a consequence of the early introduction of standardized methods and the relatively wide interest in the distribution of dissolved oxygen, the historical record of marine oxygen observations is generally richer than that of other biogeochemical parameters, although still sparse compared to measurements of temperature and salinity (Appendix 3.A). Dissolved oxygen changes in the ocean thermocline has generally decreased since 1960, but with strong regional variations (Keeling et al., 2010; Keeling and Manning, 2014). Oxygen concentrations at 300 dbar decreased between 50°S and 50°N at a mean rate of 0.63 µmol kg –1 per decade between 1960 and 2010 (Stramma et al., 2012). For the period 1970 to 1990, the mean annual global oxygen loss between 100 m and 1000 m was calculated to be 0.55 ± 0.13 × 10 14 mol yr –1 (Helm et al., 2011).
The long-term deoxygenation of the open ocean thermocline is consistent with the expectation that warmer waters can hold less dissolved oxygen (solubility effect), and that warming-induced stratification leads to a decrease in the transport of dissolved oxygen from surface to subsurface waters (stratification effect) (Matear and Hirst, 2003; Deutsch et al., 2005; Frölicher et al., 2009). Observations of oxygen change suggested that about 15% of the oxygen decline between 1970 and 1990 could be explained by warming and the remainder by reduced ventilation due to increased stratification (Helm et al., 2011; see Table 6.14).
Oxygen concentrations in the tropical ocean thermocline decreased in each of the ocean basins over the last 50 years (Ono et al., 2001; Stramma et al., 2008; Keeling et al., 2010; Helm et al., 2011), resulting in an expansion of the dissolved oxygen minimum zones. A comparison of data between 1960 and 1974 with those from 1990 to 2008 showed that oxygen concentrations decreased in most tropical regions at an average rate of 2 to 3 µmol kg –1 per decade (Figure 3.20; Stramma et al., 2010). Data from one of the longest time-series sites in the subpolar North Pacific (Station Papa, 50°N, 145°W) reveal a persistent declining oxygen trend in the thermocline over the last 50 years (Whitney et al., 2007), superimposed on oscillations with time scales of a few years to two decades. Stendardo and Gruber (2012) found dissolved oxygen decreases in upper water masses of the North Atlantic and increases in intermediate water masses. The changes were caused by changes in solubility as well as changes in ventilation and circulation over time. In contrast to the widely distributed oxygen declines, oxygen increased in the thermoclines of the Indian and South Pacific Oceans from the 1990s to the 2000s (McDonagh et al., 2005; Álvarez et al., 2011), apparently due to strengthened circulation driven by stronger winds (Cai, 2006; Roemmich et al., 2007). In the southern Indian Ocean below the thermocline, east of 75°E, oxygen decreased between 1960 and 2010 most prominently on the isopycnals σ q = 26.9 to 27.0 (Kobayashi et al., 2012). While some studies suggest a widespread decline of oxygen in the Southern Ocean (e.g., Helm et al., 2011), other studies show regions of alternating sign (e.g., Stramma et al., 2010), reflecting differences in data and period considered.
Coastal regions have also experienced long-term dissolved oxygen changes. Bograd et al. (2008) reported a substantial reduction of the thermocline oxygen content in the southern part of the California Current from 1984 to 2002, resulting in a shoaling of the hypoxic boundary (marked by oxygen concentrations of about 60 µmol kg –1 ). Off the British Columbia coast, oxygen concentrations in the near bottom waters decreased an average of 1.1 µmol kg –1 yr –1 over a 30-year period (Chan et al., 2008). These changes along the west coast of North America appear to have been largely caused by the open ocean dissolved oxygen decrease and local processes associated with decreased vertical dissolved oxygen transport following near-surface warming and increased stratification. Gilbert et al. (2010) found evidence that for the time period 1976–2000 oxygen concentrations between 0 and 300 m depth were declining about 10 times faster in the coastal ocean than in the open ocean, and an increase in the number of hypoxic zones was observed since the 1960s (Diaz and Rosenberg, 2008).