Scientists Sound Alarm as Antarctica’s Ocean Grows Saltier for the First Time in 130,000 Years

Over the past decade, Antarctic sea ice has rapidly declined, dropping to unprecedented annual lows. This trend has triggered drastic changes in a vast layer of the Southern Ocean that plays a critical role in Earth’s climate system and global ocean dynamics. That’s according to a study published Wednesday in the journal Nature. The findings show that the Antarctic Intermediate Water (AAIW)—a mass of cold, oxygen-rich freshwater that forms around Antarctica at depths of 2,300 to 4,000 feet (700 to 1,200 meters)—is rapidly growing saltier, denser, and deeper. These changes could have major implications for global sea levels, ocean warming, and weather patterns in the decades to come. The last time this happened was at the end of the last interglacial period, roughly 130,000 years ago, according to lead author Bishakhdatta Gayen, a researcher and professor at the University of Melbourne and at the Centre for Atmospheric and Oceanic Sciences in Bengaluru, India. “That transition took thousands of years, not two decades,” Gayen said in a statement. A sputtering freshwater engine The AAIW forms a dynamic barrier between the warm upper ocean and the cold deep interior, regulating deep-ocean warming. When it expands northward into the Indian, Pacific, and Atlantic oceans, this water mass transports heat, carbon, oxygen, and nutrients toward the tropics, serving as a vital marine conveyor belt. Antarctic sea ice melt is the engine that drives that conveyor belt. Every summer in the Southern Hemisphere, melting sea ice dumps fresh water into the Southern Ocean, diluting sea water and feeding the AAIW. The relatively low salinity of this layer is crucial to its functioning, as salinity dictates its density and depth. Up until 2015, healthy influxes of freshwater from ample sea ice maintained the AAIW, but that changed in 2016. Two years after reaching a record high in 2014, Antarctic sea ice sharply declined, and it’s been rapidly disappearing ever since. Less ice means there’s less freshwater feeding the AAIW each summer, which explains why this layer is growing saltier. Gayen and his colleagues obtained direct observational evidence of this change. The researchers analyzed data from the global Argo program—a network of nearly 4,000 robotic floats that measure the temperature and salinity of the upper 1.2 miles (2 kilometers) of the ocean—as well as satellite and atmospheric datasets. Their analysis confirmed that the AAIW is becoming measurably saltier, with its core salinity rising at a rate of approximately 0.002 grams of salt per pound of water (0.005 grams of salt per kilogram of water) with each passing decade. That may not sound like a big change, but over time, it’s enough to significantly alter the properties and dynamics of the AAIW, according to the researchers. Impacts far beyond Antarctica Earth’s oceans absorb and redistribute excess carbon and heat from the atmosphere, playing an essential role in moderating human-driven climate change. However, “if the altered AAIW changes the way surface water is carried down into deeper layers, it could affect how much carbon the ocean locks away—although the magnitude and timing are uncertain,” Gayen said. “This could be the tipping point for the Atlantic Ocean and the AAIW current, leading to larger, longer-lasting changes to the climate in the Southern Hemisphere,” he added. What’s more, altered heat storage and redistribution in the Southern Ocean could accelerate sea ice loss even further, creating a feedback loop that progressively ramps up the salinity of the AAIW. This could also influence atmospheric circulation, storm tracks, rainfall patterns, and regional sea levels—all of which are influenced by how much heat the ocean holds and where it is stored. The new findings add to a growing body of evidence showing that Antarctic sea ice loss is making the AAIW saltier, with global consequences. In 2025, an analysis of satellite data revealed a marked increase in surface salinity across the Southern Ocean since 2015. The year prior, a study showed that record-low sea ice in 2023 ramped up ocean heat loss and storm activity. And earlier this year, a sweeping review concluded that “the rapid decline of Antarctic sea ice is suspected to have profound implications for the global climate system,” influencing “the global distribution of atmospheric heat and moisture, as well as atmospheric variability and several teleconnections.” The rapid alteration of the AAIW adds to the ever-growing list of ways fossil fuel emissions are upending Earth’s systems. Understanding this and other deep-water transformations will be essential to accurately forecasting climate feedbacks and planning for a warming world, according to the researchers.