Reducing water salinity triggers a domino effect throughout the marine food chain by threatening plankton and seagrass.
A vast expanse of the South Indian Ocean, off the west coast of Australia, is become less salty at the fastest rate ever recorded across the Southern Hemisphere, according to new research.
Scientists say this change, driven by climate change in winds and ocean currents, could disrupt global climate systems and threaten marine ecosystems, says .
The , led by researchers at the University of Colorado at Boulder and published in the journal Nature Climate Change, shows that rising global temperatures over the past six decades have reshaped key wind patterns in the Indian and Pacific Oceans. These altered winds are direct ocean currents that channel increasing amounts of freshwater into the South Indian Ocean, rapidly reducing surface salinity in waters that were previously among the saltiest in the region.
Ocean salinity plays a crucial role in regulating the formation of water layers, the way heat is transported around the planet, and the way nutrients move upward to sustain marine life. Less salty water is lighter and tends to stay on saltier and denser waterstrengthening stratification and reducing vertical mixing. This process limits the exchange of heat and nutrients between the surface and deep layers, with potential consequences for climate regulation and ocean ecosystems.
Much of the freshwater entering the region originates from the Indo-Pacific freshwater basin, a vast tropical region stretching from the eastern Indian Ocean to the western Pacific, where heavy rainfall keeps surface waters relatively dilute. This basin connects to the global “treadmill” of ocean circulation, formally known as thermohaline circulationwhich transports heat and salt between ocean basins. Even small changes in salinity can influence density-driven sinking and dispersal.
Researchers estimate that the area of salty surface water off southwestern Australia has decreased by around 30% in the last 60 years. The team found that local precipitation alone does not explain this trend. Instead, climate-driven changes in surface winds appear to be redirecting freshwater from the tropics to the southern Indian Ocean.
Scientists warn that this increase in salinity could have far-reaching impacts. Reducing mixing can limit the supply of nutrients to surface waters, threatening plankton and seagrasses that form the basis of marine food chains.
