An iceberg floating near Disko Bay near Ilulissat, Greenland. Image credit: NASA/Saskia Madlener

An iceberg floating near Disko Bay near Ilulissat, Greenland. Image credit: NASA/Saskia Madlener

The question: Melt ice cubes in a glass of water, and the water level will not change. Can the same be said for ice floating in the ocean?

The answer: There is a common misconception that sea level change comes only from ice attached to land, and not from floating sea ice. Although that is mostly true, it turns out that there is an effect, even if it is minor.

An often-overlooked ingredient makes a significant difference: saltiness. Various studies show that because floating ice is made of fresh water, it actually increases sea level slightly when it melts into the salty sea – unlike what happens in your water glass.

A floating object, like an iceberg or other sea ice, displaces its own weight in water. But fresh water is less dense than salt water. So, when floating ice melts and becomes liquid, it takes up more volume than the seawater it displaced when it was ice, raising sea level. This has about 3% the effect of grounded ice-melt and raises sea level.

Floating Ice Figure
Ice melting in a glass of water does not change the density, while ice melting in the ocean decreases the density of the salt water, raising the sea level slightly. Note: magnitude has been exaggerated for artistic effect

This process becomes important when accounting for the total sea level rise, said Gavin Schmidt, the director of NASA’s Goddard Institute for Space Studies.

“Obviously, if you’ve got a huge glacier that falls into the sea, that raises sea level more than a floating ice shelf melting does,” he said. “But it’s not zero, and the reason why it’s not zero speaks to the complexities of the system.”

Fresh water flowing into the ocean also can have broader implications.

“The effect of water going into the ocean or fresh water going into the ocean is not just on sea level,” he said. “It also changes the stratification; it also changes potentially the carbon dioxide uptake and the temperature trends.”

Why it’s important: While their effects might be minimal, these floating bodies of ice still contributed about .04 inches (1.1 millimeters) to sea level between 1994 and 2017 and are important to understanding these changes accurately.

“If you only looked at the grounded ice change, you’d actually be underestimating the effect of the freshwater addition by about half,” Schmidt said. “So, understanding and keeping track of the floating ice bit is important for those direct effects and then also those small sea level effects.”

These factors should be included in the broader discussion of sea level, he said, and he hopes new climate models will reflect the details of ocean-ice shelf interactions, particularly in Antarctica.

“It’s one more thing that we need to monitor,” he said. “We need to get a better understanding of what’s happening under ice shelves. Currently, there are no global climate models that include these effects, or that can predict these effects. And that should be fixed.”