One of the saltiest stretches of the global ocean is undergoing a striking transformation. The Southern Indian Ocean off the southwest coast of Australia has become significantly fresher over the past six decades, a change researchers link directly to global warming.
According to a study published on February 3 in Nature Climate Change, the expanse of highly saline water in the region has shrunk by nearly 30 per cent since the 1960s. The research, led by scientists at University of Colorado Boulder, describes the shift as the most rapid increase in freshwater observed anywhere in the Southern Hemisphere.
The scale of the change is dramatic. Researchers estimate the freshening is equivalent to adding about 60 per cent of Lake Tahoe’s volume in freshwater to the region every year enough, they say, to supply the entire United States population with drinking water for centuries.
The waters off southwest Australia form part of what scientists call the Indo-Pacific freshwater pool, a vast region stretching from the eastern Indian Ocean into the western Pacific. This system plays a pivotal role in global ocean circulation through the thermohaline circulation—often described as a planetary “conveyor belt” that redistributes heat, salt and freshwater around the globe.
Traditionally, the ocean in this part of the Southern Indian Ocean has been highly saline because evaporation far exceeds rainfall. But the new study finds that the freshening is not due to local precipitation changes. Instead, global warming is altering surface wind patterns over the Indian and tropical Pacific Oceans. These wind shifts are redirecting currents, transporting more freshwater from the Indo-Pacific pool into the Southern Indian Ocean.
The consequences could be far-reaching. As seawater becomes less salty, it also becomes less dense. Fresher water tends to sit atop saltier, denser water, intensifying stratification and reducing vertical mixing. This mixing is critical for transporting nutrients from deeper layers to the sunlit surface, where plankton and seagrass form the base of the marine food web.
Reduced mixing could therefore limit nutrient supply to surface waters, threatening marine biodiversity. At the same time, weaker vertical exchange may trap heat near the surface, compounding thermal stress on already vulnerable ecosystems.
Beyond regional impacts, scientists warn that continued freshening could influence broader ocean circulation patterns, potentially carrying altered water masses toward the Atlantic and affecting climate systems far beyond Australia’s shores.
The findings add to mounting evidence that climate change is not only warming the oceans, but fundamentally reshaping their physical structure with consequences that ripple through ecosystems and across continents.