Think about a glass of ice water. This might sound unusual at first but stay with me.
Even though the ice cubes melt, the water level doesn’t change much because the total amount of water is already in the glass. But if you heat the water, the water level rises because warmer water expands. It’s a principle known as thermal expansion.
Now add more ice cubes to the glass. The glass will eventually overflow because more water has been added.
That idea of adding more water has been the conventional wisdom behind sea level rise. Two-thirds of sea level rise has been attributed to melting ice sheets and glaciers (adding more water to the oceans), while the remaining one-third has been linked to thermal expansion (warming of seawater).
This thinking had to change, however, when researchers reviewed data from 2024.
“The rise we saw in 2024 was higher than we expected,” said Josh Willis, a sea level researcher as NASA’s Jet Propulsion Laboratory in Southern California, in an article from NASA. “Every year is a little bit different, but what’s clear is that the ocean continues to rise, and the rate of rise is getting faster and faster.”
According to NASA’s article, ocean levels grew by an expansion rate of 0.23 inches (0.59 centimeters) in 2024. That’s more than the expected 0.17 inches (0.43 centimeters). Scientists say the reason for greater rise in sea level was flipped from conventional wisdom. Instead of just one-third, two-thirds of sea level rise in 2024 resulted from thermal expansion.
Ocean levels have been monitored by several satellites since 1993. Since measurements began, the annual rate of sea level rise has more than doubled. Global sea level has gone up by 4 inches (10 centimeters) since 1993.
“The warming of Earth is primarily due to accumulation of heat-trapping greenhouse gases, and more than 90 percent of this trapped heat is absorbed by the oceans,” explains NASA. “As this heat is absorbed, ocean temperatures rise and water expands.”
Researchers say there are several ways that heat makes its way to the oceans. The first is how the ocean is layered.
Seawater arranges itself into layers by ocean temperature and density. Warmer water floats on top of cooler water, which is denser. Heat from the surface moves slowly into deeper water.
But high winds can stir up the surface and speed up the mixing. Changes in large ocean currents can mix the waters as well. The massive movement of water during El Niño, in which large pools of warm water in the western Pacific Ocean move into the central and eastern Pacific, also result in vertical movements of heat within the ocean.
“With 2024 as the warmest year on record, Earth’s expanding oceans are following suit, reaching their highest levels in three decades,” said Nadya Vinogradova Shiffer, head of physical oceanography programs and the Integrated Earth System Observatory at NASA Headquarters in Washington.
Scientists will be watching sea levels closely to see how they change in the years to come.
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