Scientists tracked Antarctica’s bottom water across the Southern Ocean for 21 years, and found it has been thinning faster since 2015

Antarctic Bottom Water volume has sharply declined since two thousand and two. This significant decrease accelerated notably after two thousand and fifteen. Image Credit: Wikimedia Commons

Why Antarctic Bottom Water matters

A reduction in sea ice may be linked to weaker bottom-water formation

Changes in this deep ocean current may affect global heat and carbon absorption. These shifts could eventually influence climates in regions like the United Kingdom. Image Credit: Wikimedia Commons

Deep-ocean changes

Implications for the UK and Europe

Scientists advocate ongoing observations

A recent study monitoring Antarctic Bottom Water for more than 20 years found that the huge volume of cold, dense water flowing from Antarctica into the ocean depths has been declining sharply in recent years. Researchers found that the volume of Antarctic Bottom Water (AABW), a major part of Earth’s ocean circulation system, steadily decreased from 2002 to 2023, while the rate of decline accelerated sharply after 2015.According to a study published in Geophysical Research Letters , the scientists used an innovative data set called SatGEM-2 to analyse continuous changes in Antarctic Bottom Water across the Southern Ocean. According to researchers James Wyatt, Nathaniel L. Bindoff, and colleagues, the rate of decline has increased fourfold since 2015 compared with the years before then. By 2023, the volume of Antarctic Bottom Water had decreased by 2.94 percent from the 2002 baseline.Antarctic Bottom Water forms around Antarctica as seawater cools, becomes denser, and sinks to the ocean floor. It then flows northward along the ocean floor into deeper parts of the global ocean. It acts as a conveyor of material like oxygen, heat, and carbon into the deepest part of the ocean. According to the National Oceanic and Atmospheric Administration (NOAA), the ocean conveyor belt is the movement of heat, nutrients, and other important materials in the deep and surface currents of the oceans.Change in the Antarctic Bottom Water may be associated with changes in the ocean’s heat and carbon absorption.The authors said the weakening of Antarctic Bottom Water after 2015 was accompanied by major changes in Antarctic sea ice. Sea-ice formation is closely linked to the formation of dense Antarctic water. When seawater freezes, salt is excluded from the ice crystals, enriching the surrounding water and making it denser.The research demonstrated a clear relationship between changes in Antarctic sea ice and changes in the volume of Antarctic Bottom Water. In recent years, Antarctic sea ice has reached very low levels, including the lowest minimum extent in the satellite record. According to the National Snow and Ice Data Center (NSIDC), Antarctic sea ice has experienced an unusually large decline in recent years, with record-low extents observed during the satellite era.New studies add to previous research on changes in Antarctic Bottom Water. In particular, there have been claims of the water becoming warmer, more saline, and less dense in numerous locations. Researchers found evidence of long-term changes in Antarctic Bottom Water composition, including warming and freshening linked to environmental changes in the Southern Ocean. When this deep-ocean water becomes less dense, it may reduce its ability to transport oxygen and retain heat. The Southern Ocean absorbs a significant amount of excess heat and carbon dioxide generated by human activity.While the changes have been observed around Antarctica, scientists believe that their impacts could ripple out into the wider global oceans. Among the most dramatic reductions in Antarctic Bottom Water were seen in the Atlantic portion of the Southern Ocean, an area that is part of larger ocean circulation processes that affect European climates.The UK’s climate is strongly influenced by the Atlantic Ocean, which helps moderate temperatures by moving warm air northward from the tropics. According to the UK Met Office , ocean circulation plays an important role in regulating climate by transporting heat around the planet, with the Atlantic Meridional Overturning Circulation helping keep regions such as the UK warmer than they would otherwise be. Nevertheless, scientists are clear to point out that such research does not imply any immediate changes in UK weather.According to the authors, their results allow them to observe changes in Antarctic Bottom Water on a larger scale than previous studies, which usually focused on regional areas or single expeditions. Although some areas of the Southern Ocean saw increases in bottom-water volume, the overall trend is a decline, with the fastest changes observed recently.Scientists say further observations will help determine whether the slowdown in Antarctic Bottom Water formation is temporary or reflects a long-term climate response to rising temperatures. Observations of Antarctica’s response to global warming can reveal important signs of climate change in the deep ocean.