The production rate of the coldest, densest ocean water on earth is in rapid decline. Production of Antarctic bottom water, which rests on the bottom of all of the world's oceans except the Arctic and north Atlantic has been in continuous linear decline since 1980. The investigators at the University of Washington have not concluded whether the rapid decline is caused by natural cycles or a warming climate.
Two oceanographers from NOAA and the University of Washington find that Antarctic Bottom Water has been disappearing at an average rate of about eight million metric tons per second over the past few decades, equivalent to about fifty times the average flow of the Mississippi River or about a quarter of the flow of the Gulf Stream in the Florida Straits.
“Because of its high density, Antarctic Bottom Water fills most of the deep ocean basins around the world, but we found that the amount of this water has been decreasing at a surprisingly fast rate over the last few decades,” said lead author Sarah Purkey, graduate student at the School of Oceanography at the University of Washington in Seattle, Wash. “In every oceanographic survey repeated around the Southern Ocean since about the 1980s, Antarctic Bottom Water has been shrinking at a similar mean rate, giving us confidence that this surprisingly large contraction is robust.”
One possible explanation might be that the melt rate of Antarctic glaciers has increased since 1980. The fresh water added to the southern ocean may lower the density of the water around Antarctica so that deep water production stops in areas where large amounts of melt water have been added. This is my suggestion, not the co-authors'.
A layer of Antarctic Bottom Water colder than 0ºC (colors, with darkest blue areas having the thickest layer, and white none) covers the ocean floor around Antarctica (center, shaded grey). Rates at which this layer is thinning during the study period (red numbers in meters per decade) are shown for for each deep basin (outlined by thin grey lines). These rates are estimated using data from repeated oceanographic expeditions (ship tracks shown by thick black lines). Note that seawater at the ocean surface stays liquid even at temperatures approaching -2ºC because of its high salt content.
The disaster movie story of the weakening of the Gulf Stream and the collapse of the overturning circulation in the north Atlantic Ocean is not occurring. However, the collapse of the Antarctic meridional overturning circulation could have global effects just as disastrous as the slowing of the Gulf Stream. The geologic record shows overturning oscillates between the hemispheres. The flux of warm salty water to the Atlantic and Arctic ocean could be increasing as the Antarctic bottom water production declines. This possible increase of warm salty water moving from the Indian ocean around Africa then up to the north Atlantic could cause rapid warming of the North Atlantic and Arctic Oceans. In fact, that's just what we're observing now. (my speculation, not the report's co-authors')
Changes in the temperature, salinity, dissolved oxygen, and dissolved carbon dioxide of this prominent water mass have important ramifications for Earth’s climate, including contributions to sea level rise and the rate of Earth’s heat uptake.
“People often focus on fluctuations of currents in the North Atlantic Ocean as an indicator of climate change, but the Southern Ocean has undergone some very large changes over the past few decades and also plays a large role in shaping our climate,” said Johnson.