An international study detected that the deepest layer of the Southern Ocean, known as Antarctic Bottom Water (AABW), has shown a reduction in volume since 2002, with a notable acceleration in loss starting from 2015. This phenomenon was observed throughout the area surrounding Antarctica.
The survey, released by the Australian Antarctic Program, indicates that the decrease in this reservoir of cold, dense water is linked to reduced sea ice formation and the increase in freshwater entering the ocean due to continental warming.
The analysis was conducted by scientists from the Australian Antarctic Program Partnership, in collaboration with the University of Tasmania. They integrated satellite data, measurements taken by vessels, and information captured by ocean buoys to map the transformations that occurred between 2002 and 2023.
Antarctic Bottom Water plays a crucial role in the functioning of the oceans, as when it forms, it sinks to the deepest parts and disperses throughout the Atlantic, Indian, and Pacific oceans. This movement drives part of the global circulation known as the oceanic 'conveyor belt.'
This mass of water originates mainly during winter, when marine freezing near Antarctica concentrates salt in the remaining water. This increase in salinity makes the liquid heavier, facilitating its sinking to the ocean floor.
The study indicates that this mechanism is weakening. The influx of freshwater, resulting from the melting of ice shelves, decreases the salt concentration at the ocean surface, making the water less dense and hindering its descent into the depths.
The research concluded that AABW lost about 3% of its accumulated circumpolar volume by 2023, using the volume recorded in 2002 as a reference point. Furthermore, the rate of reduction after 2015 was four times higher than the trend seen in the previous period.
Researchers correlated this increase in loss with the accelerated decline of Antarctic sea ice, recorded since 2016. According to the study, there was a strong correlation between changes in ice extent and variations in the volume of Antarctic Bottom Water.
The survey utilized SatGEM-2, a model that combines sea surface height data obtained by satellites with temperature and salinity information collected at various depths. This tool allowed for the estimation of changes in areas with scarce direct observation.
James Wyatt, the study's lead author and a researcher at the Australian Antarctic Program Partnership at the University of Tasmania, stated that reconstructing the evolution of AABW was made possible by combining multiple sources of ocean monitoring.
Wyatt declared: 'Our database gathers satellite observations and continuous monitoring of the ocean interior conducted by ships and floats over the last century. We need to maintain these observations to continue calibrating and ensuring the reliability of SatGEM-2.'
The researcher also clarified that the developed method allows tracking changes almost in real time across the entire water column of the Southern Ocean, even in areas with little measurement coverage.
Nathan Bindoff, co-author and professor at the Institute of Marine and Antarctic Studies and a fellow of the Australian Research Council, commented that the set of observed changes signals a slowdown in deep ocean circulation.
Bindoff stated: 'The simultaneous warming, loss of salinity, reduction of oxygen, and decrease in volume point to a weakening of deep ocean circulation.'
The reduction of Antarctic Bottom Water raises concern among scientists, given that this system is responsible for transporting oxygen to great depths, storing carbon for long periods, and assisting in controlling heat distribution on the planet. Changes in this process can affect the climate and sea level for centuries.
The authors emphasize that monitoring must continue in areas covered by sea ice, where satellites cannot directly measure ocean surface height. Expanding these observations can deepen the understanding of the climate system's evolution.
Research conducted by the University of California, Davis, has shown that iceberg melt in the northeastern part of the Pacific Ocean may be a contributing factor to the weakening of the AMOC, one of the planet's largest ocean currents.
This finding changes how scientists interpret past climate phenomena, demonstrating that modifications in a distant area can impact the balance of the oceans. The research, published in the journal Nature Communications, focused on the Atlantic Meridional Overturning Circulation (AMOC), a mechanism that acts as a vast heat conveyor belt.
This current is responsible for transporting warm, salty water from the tropics to the North Atlantic, accounting for approximately 70% of oceanic heat transport across the Equator. Consequently, any alteration in its functioning can generate global climate consequences.
Historically, scientists believed that large releases of ice in the North Atlantic were the main cause of AMOC weakening during events known as Heinrich events, which are periods of severe climatic variations recorded in the last glacial era. However, the analysis of the sequence of occurrences suggested a different scenario.
According to Chijun Sun, an assistant professor in the Department of Earth and Planetary Sciences at the University of California, Davis, these events in the Atlantic occurred after the current's strength decreased. He clarified that more recently, it was discovered that such episodes of iceberg melt in the North Atlantic followed the weakening of the AMOC and the cooling of Greenland.
To understand the origin of these transformations, researchers integrated paleoclimate data with models created on supercomputers. The model managed to replicate Heinrich Stadial 1, which occurred about 19 thousand years ago. During this period, sea level was approximately 119 meters below the current level, a massive layer of ice covered North America, and the carbon dioxide concentration was around 180 parts per million.
The simulations showed that the fresh water released by Pacific icebergs reached the North Atlantic, modifying the essential conditions for the AMOC to function. The results indicated that Pacific icebergs could have influenced changes in the AMOC, the freshwater altered the characteristics of the North Atlantic waters, and the entire process was linked to phases of global climate change, revealing connections between distinct oceanic regions.
In addition to elucidating past events, the study offers indications about the future climate. Chijun Sun mentioned that there is a scientific consensus that the AMOC is expected to weaken by the end of the 21st century, with some experts considering the possibility of a system collapse.
The researcher emphasized that the current is not only sensitive to events in the North Atlantic but can react to any discharge of freshwater. Previous research by the same team had already associated the weakening of the AMOC with a significant reduction in rainfall in areas such as Central America, the Amazon, and West Africa. This new work reinforces the idea that the behavior of the oceans depends on an intricate network of global connections, and that changes in ice can cause effects far beyond where they occur.
