section_path: Climate/Change
section_name: Change
paragraph_text: Change
  
  As human-caused greenhouse gas emissions cause increased warming, one of the most notable effects of climate change on oceans is the increase in ocean heat content, which accounted for over 90% of the total global heating since 1971. Since 2005, from 67% to 98% of this increase has occurred in the Southern Ocean. In West Antarctica, the temperature in the upper layer of the ocean has warmed 1 °C (1.8 °F) since 1955, and the Antarctic Circumpolar Current (ACC) is also warming faster than the global average. This warming directly affects the flow of warm and cold water masses which make up the overturning circulation, and it also has negative impacts on sea ice cover in Southern Hemisphere, (which is highly reflective and so elevates the albedo of Earth's surface), as well as mass balance of Antarctica's ice shelves and peripheral glaciers. For these reasons, climate models consistently show that the year when global warming will reach 2 °C (3.6 °F) (inevitable in all climate change scenarios where greenhouse gas emissions have not been strongly lowered) depends on the status of the circulation more than any other factor besides the emissions themselves.
  
  Greater warming of this ocean water increases ice loss from Antarctica, and also generates more fresh meltwater, at a rate of 1100-1500 billion tons (GT) per year. This meltwater from the Antarctic ice sheet then mixes back into the Southern Ocean, making its water fresher. This freshening of the Southern Ocean results in increased stratification and stabilization of its layers, and this has the single largest impact on the long-term properties of Southern Ocean circulation. These changes in the Southern Ocean cause the upper cell circulation to speed up, accelerating the flow of major currents, while the lower cell circulation slows down, as it is dependent on the highly saline Antarctic bottom water, which already appears to have been observably weakened by the freshening, in spite of the limited recovery during 2010s. Since the 1970s, the upper cell has strengthened by 3-4 sverdrup (Sv; represents a flow of 1 million cubic meters per second), or 50-60% of its flow, while the lower cell has weakened by a similar amount, but because of its larger volume, these changes represent a 10-20% weakening. However, they weren't fully caused by climate change, as the natural cycle of Interdecadal Pacific Oscillation had also played an important role.
  
  Similar processes are taking place with Atlantic meridional overturning circulation (AMOC), which is also affected by the ocean warming and by meltwater flows from the declining Greenland ice sheet. It is possible that both circulations may not simply continue to weaken in response to increased warming and freshening, but eventually collapse to a much weaker state outright, in a way which would be difficult to reverse and constitute an example of tipping points in the climate system. There is paleoclimate evidence for the overturning circulation being substantially weaker than now during past periods that were both warmer and colder than now. However, Southern Hemisphere is only inhabited by 10% of the world's population, and the Southern Ocean overturning circulation has historically received much less attention than the AMOC. Consequently, while multiple studies have set out to estimate the exact level of global warming which could result in AMOC collapsing, the timeframe over which such collapse may occur, and the regional impacts it would cause, much less equivalent research exists for the Southern Ocean overturning circulation as of the early 2020s. There has been a suggestion that its collapse may occur between 1.7 °C (3.1 °F) and 3 °C (5.4 °F), but this estimate is much less certain than for many other tipping points.