Does the melting ice make the water less salty?

That’s a great question, and the simple answer is ‘yes’, melting ice definitely makes the ocean less salty (or fresher, as we oceanographers say). What that means for the ocean depends on what type of ice melts, and where it melts, but the most important thing to understand is that salty water is heavier and will tend to sink, but fresh water is lighter and will tend to stay at the surface. This matters because sinking ocean water takes heat and carbon from the atmosphere to the deep ocean, and reduces the amount of climate change that we all deal with. So, by freshening the ocean surface and making it lighter, melting ice makes it difficult for the ocean to reduce the impacts of climate change.

In polar regions the biggest source of freshening by far is sea ice; this is frozen sea water that makes the ocean salty when it freezes, but fresher when it melts. Just in the Antarctic, an area of ocean two and half times the size of Australia freezes over in autumn and then melts in spring – every single year. This natural freshening/salting process helps to mix a lot of water between the surface and the deep ocean, and is very important both for our climate and for the marine life near Antarctica.

Another source of freshening, that is smaller than sea ice but getting bigger all the time, is melt water from the Antarctic ice sheets – this is ice on land that has formed from thousands of years of snowfall. As that ice melts into the ocean it causes the sea level to rise up, but it also freshens the surface and reduces that up-down pump of sea water. Scientists are still working to fully understand how this will affect the climate – it’s a new and fascinating part of my research.