The water level of the Earth’s oceans does not increase by chance. Every millimeter has a cause. When sea levels rise, the reasons are buried in melting ice, warming seas, and the transfer of water from land to ocean.
Scientists now treat global mean sea level as one of the clearest signals of climate change. And that signal has become louder in the past 30 years.
Building 30-year record
Researchers at Hong Kong Polytechnic University (PolyU) have built the first 30-year record of global ocean mass change.
The study, led by Professor Jianli Chen and Dr. Yufeng Nie, used satellite laser ranging, or SLR, to track water added to the seas between 1993 and 2022.
Before now, reliable mass records began only in 2002, with the GRACE mission.
The results of the latest study, published in the Proceedings of the National Academy of Sciences, close that gap and strengthen confidence in how scientists monitor rising sea levels.
Ice melt and sea-level rise
Sea level rises for two main reasons. Warm water expands, and melting ice adds mass to the ocean. The PolyU record shows how much mass matters.
Between 1993 and 2022, global sea level rose by about 90 millimeters (3.5 inches).
Around 60 percent of that came from added ocean mass, not expansion. In the past two decades, the contribution from ice loss has accelerated, pushing the rate of rise from 3.2 to 3.6 millimeters (0.13 to 0.14 inches) per year.
Satellites track sea-level rise
SLR is not a new technology. It has been used for decades to measure satellite distances. But its data were once too coarse to track ocean mass.
The PolyU team solved that by applying forward modeling, a method that corrects distortions along land-ocean boundaries and accounts for geocenter motion.
The result is a continuous and reliable record that lines up with the GRACE observations, despite the older method’s limitations.
The weight of melting ice
Breaking down the numbers shows where the rise comes from. Greenland added about 0.60 millimeters (0,24 inches) of sea-level rise each year between 1993 and 2022. Glaciers contributed a similar amount. Antarctica added 0.40 millimeters (0.16 inches).
Changes in land water storage, such as reservoirs and groundwater use, added about 0.32 millimeters (0.012 inches).
Since the 2000s, Greenland has become the single largest source, while melting ice sheets and glaciers combined now account for about 85 percent of all ocean mass increase.
The study also picked up short-term swings. Sea levels dipped during the 2010–2011 La Niña and rose sharply during the 2015–2016 El Niño event.
GRACE captured these shifts with high precision, while SLR detected them more roughly. Even with noise, the agreement between the two systems builds trust in the results. The long-term signal remains unmistakable: ice melt drives today’s rising seas.
The budget closes
For years, scientists struggled to match observed sea-level rise with the sum of its causes. That gap created doubts.
The PolyU study shows the budget now closes. The combined effects of warming water and ocean mass increase line up with altimetry observations almost perfectly.
From 1993 to 2022, the numbers differ by less than 0.1 millimeters per year (0.0004 of an inch). That closure means researchers can now account for nearly every drop.
Climate and sea-level rise
“In recent decades, climate warming has led to accelerated land ice loss, which has played an increasingly dominant role in driving global sea-level rise,” said Professor Jianli Chen.
“Our research enables the direct quantification of global ocean mass increase and provides a comprehensive assessment of its long-term impact on sea-level budget. This offers crucial data for validating coupled climate models used to project future sea-level rise scenarios,” added Prof. Chen.
This point highlights the urgency of connecting observational science with climate models. Accurate data on ocean mass strengthens the ability to project how coastlines may change in decades to come.
“The research showed that the ocean-mass changes derived from SLR analysis align well with the total sea-level changes observed by satellite altimeters, after accounting for the effect of ocean thermal expansion. This demonstrates that the traditional SLR technique can now serve as a novel and powerful tool for long-term climate change studies,” noted Dr. Yufeng Nie.
What comes next
The work proves SLR still has value. It cannot match the fine detail of GRACE, but it extends the record back another decade and cross-checks existing data.
Improvements are possible. Adding other satellite systems and refining models could sharpen results even further. What matters most is the message: the oceans are rising faster, and melting ice is the main reason.
Sea-level rise no longer hides behind uncertainty. The numbers tell the same story across methods. Thermal expansion is steady, but ice loss is climbing.
The seas are rising, and they are rising faster than before. The message is clear, and it is one that humanity cannot afford to ignore.
The study is published in the journal Proceedings of the National Academy of Sciences.
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