The Southern Ocean
The Southern Ocean is changing in ways that are likely to affect regional and global climate, and marine productivity (Rhein et al. 2013). The Southern Ocean has warmed more rapidly and to a greater depth than the global ocean average in recent decades (Böning et al. 2008, Gille 2008, Schmidtko & Johnson 2012, Roemmich et al. 2015), although the warming is less than that observed in the Arctic Ocean (Armour et al. 2016). Most of the Southern Ocean has also freshened, reflecting an increase in precipitation and, possibly, Antarctic ice melt (Böning et al. 2008, Helm et al. 2010). In recent decades, Antarctic bottom water (see Global importance of Antarctica) has warmed, freshened and decreased in volume (Rintoul 2007; Purkey & Johnson 2010, 2012, 2013; van Wijk & Rintoul 2014). Sea ice formation and melting are important drivers of shallow overturning, and changes in sea ice extent may play a role in future changes in circulation near the continent (Bindoff & Hobbs 2016). Increased ocean heat transport has caused Antarctic ice shelves and glaciers to thin and retreat, particularly in West Antarctica, where an irreversible retreat may now be under way (Stan et al. 2012, Joughin et al. 2014, Rignot et al. 2014). The ozone depletion and increase in atmospheric greenhouse gases caused by human activities have resulted in changes in wind patterns, and caused many of the changes observed in the Southern Ocean (Turner et al. 2014).
Global sea levels are rising, primarily because of oceanic uptake of heat, and run-off from melting ice caps and glaciers (Church et al. 2013). The rate of change of sea level has been regionally and globally variable in recent decades because of influences from natural climate variability, particularly the El Niño–Southern Oscillation and the 1991 eruption of the Mount Pinatubo volcano, but is expected to increase because of continuing global warming (Fasullo et al. 2016). From 1993 to 2012, most of the Southern Ocean increased in height, with parts of the Pacific sector showing a modest fall—these changes are consistent with observed changes in ocean heat content (Church et al. 2013).
The Southern Ocean is one of the world’s largest sinks for atmospheric CO2. Approximately 25–30 per cent of the anthropogenic CO2 released to the atmosphere has been taken up by the world’s oceans, some 40 per cent of which has been taken up by cold Southern Ocean waters that lie south of 40°S (Sabine et al. 2004, Doney et al. 2009a, Takahashi et al. 2009).
Although this ocean uptake reduces the accumulation of CO2 in the atmosphere, it also causes ocean acidification (see Human influences on Antarctica). Atmospheric CO2 levels are currently higher than they have been for at least the past 25 million years (Dolman et al. 2008) and reached approximately 397 parts per million in 2014 (Le Quéré et al. 2015; Figure ANT11). From 2005 to 2014, the rate of global CO2 emissions from fossil fuels and industry grew by 2.2 per cent per year, compared with 3.2 per cent per year for 2000–09 and 1 per cent per year for 1990–99 (Le Quéré et al. 2015). Of the approximately 9 Gt of CO2 emitted each year from 2005 to 2014 (Le Quéré et al. 2015):
- around 33 per cent was taken up by land (mainly from forest growth)
- around 29 per cent was taken up by the oceans
- the remainder contributed to the increase in atmospheric CO2.