A comprehensive 30-year analysis of global ocean data has confirmed that the increasing mass of the world's oceans, primarily due to melting land ice, is the dominant factor behind rising sea levels. Researchers at The Hong Kong Polytechnic University found that from 1993 to 2022, the global mean sea level rose by approximately 90 millimeters, with an accelerating trend that highlights the growing impact of climate change.
The study, published in the Proceedings of the National Academy of Sciences, utilized an innovative technique to analyze satellite laser ranging data, providing the first direct and continuous record of ocean mass changes over three decades. This new dataset clarifies the critical role of melting glaciers and ice sheets in the ongoing rise of our seas.
Key Takeaways
- Global mean sea level rose by approximately 90 mm between 1993 and 2022.
- Increased ocean mass from melting land ice accounted for about 60% of this rise.
- The rate of sea-level rise is accelerating, averaging about 3.3 millimeters per year over the period.
- A new modeling technique allowed researchers to use satellite laser ranging data for this long-term analysis.
Understanding the Causes of Rising Seas
Global sea-level rise is driven by two main processes. The first is thermal expansion, where seawater expands as it absorbs excess heat from the atmosphere. The oceans have absorbed roughly 90% of the extra heat trapped by greenhouse gases, causing the water to take up more space.
The second, and increasingly dominant, process is the addition of new water to the oceans. This is known as barystatic sea-level change and results from the melting of land-based ice, including polar ice sheets in Greenland and Antarctica, as well as mountain glaciers around the world.
Distinguishing Mass from Volume
While thermal expansion increases the ocean's volume without changing its mass, melting land ice adds new mass in the form of freshwater. Accurately measuring this change in mass is crucial for understanding the direct contribution of ice loss to sea-level rise and for validating climate models.
According to the new research, the contribution from melting ice has become the primary driver. The study found that since approximately 2005, the rapid increase in global ocean mass has been the main reason for the accelerated rise in sea levels.
A Technological Breakthrough in Measurement
Until now, long-term projections of sea-level rise have largely relied on data from satellite altimetry, which measures the height of the sea surface. Direct measurements of ocean mass change from satellite gravimetry were only available since the launch of the Gravity Recovery and Climate Experiment (GRACE) mission in 2002.
The research team, led by Prof. Jianli Chen, developed a method to use a much older technology: Satellite Laser Ranging (SLR). SLR works by measuring the time it takes for laser pulses to travel from ground stations to satellites and back, providing precise distance measurements.
Overcoming Past Limitations
Historically, SLR was not considered suitable for measuring ocean mass changes directly. This was due to several limitations:
- A small number of dedicated satellites and ground stations.
- High orbital altitudes, which meant SLR could only detect very large-scale gravitational changes.
- Low spatial resolution of the resulting data.
To overcome these issues, the PolyU team implemented an innovative forward modelling technique. This approach integrates detailed geographical information about ocean and land boundaries into the analysis, effectively sharpening the resolution of the SLR data and allowing for accurate, long-term estimates of global ocean mass.
Key Statistics from the Study
- Total Rise (1993-2022): ~90 mm
- Average Annual Rise: ~3.3 mm/year
- Contribution from Ocean Mass Increase: ~60%
- Contribution from Land Ice Melt to Mass Increase: >80%
The Dominant Role of Melting Ice
The study's findings paint a clear picture of a changing planet. The accelerated melting of land ice, particularly from the Greenland ice sheet, is the largest contributor to the increase in ocean mass. Over the entire 30-year period, meltwater from polar ice sheets and mountain glaciers was responsible for more than 80% of the total gain in global ocean mass.
This direct quantification of ocean mass change provides a critical benchmark for climate science. It offers a new, independent dataset that can be used to verify and improve the climate models that scientists rely on to project future sea-level scenarios.
"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 Prof. 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 new record confirms that the problem is not only ongoing but also worsening. The acceleration in sea-level rise points to a feedback loop where a warmer climate leads to more ice melt, which in turn contributes to further changes in the Earth's systems.
Implications for Future Climate Research
The ability to use SLR data for this purpose is a significant advancement. It effectively extends the reliable record of ocean mass change back by nearly a decade, creating a continuous 30-year timeline that aligns with modern satellite altimetry records.
Dr. Yufeng Nie, the lead author of the study, highlighted the importance of this alignment. "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," he explained.
This consistency demonstrates that the long-established SLR technique can now be considered a powerful new tool for long-term climate studies. It provides scientists with another layer of evidence to track one of the most critical indicators of global climate change.
As the world grapples with mitigating the effects of a warming planet, accurate, long-term data on the sources of sea-level rise is essential for policymakers, coastal communities, and scientists working to predict and adapt to future changes.
