It's happening. It's been happening. It's happening faster than ever now.
Satellite laser ranging revealed ocean mass growth as the main driver of rising seas. Melting land ice now dominates sea-level change. The increase in global mean sea level (GMSL) is a key signal of climate change. Researchers at The Hong Kong Polytechnic University (PolyU) have applied advanced
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The increase in global mean sea level (GMSL) is a key signal of climate change. Researchers at The Hong Kong Polytechnic University (PolyU) have applied advanced space-based geodetic methods to produce the first accurate 30-year record (1993–2022) of global ocean mass change, also known as barystatic sea level. Their analysis shows that changes in ocean mass play the leading role in driving sea-level rise.
The study also found that GMSL has been rising at an average of about 3.3 millimeters per year, with a clear acceleration over time, underscoring the worsening impacts of climate change. The results were published in the Proceedings of the National Academy of Sciences.
Two main processes are responsible for GMSL rise: the thermal expansion of seawater, as the oceans absorb roughly 90% of the excess heat from Earth’s climate system, and the growth in ocean mass caused by freshwater input from melting land ice. Continuous monitoring of ocean mass change is therefore critical to understanding current sea-level trends.
Altimetry expert Vinca Rosmorduc explains sea level rise and why it is so important to monitor.
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“Generally, when we talk about sea level rise, we think of three main contributors. The first is that the ocean is warming, and because warmer water takes up more space than cooler water, the level of a warmer ocean is higher than that of a cooler ocean. This is known as thermal expansion. It is the same thing that happens to mercury in a thermometer – when the surrounding air heats up, the mercury heats up and expands, so it reaches a higher number on the scale.
“The second contributor is the melting of glaciers, which is when frozen water on land melts and flows into the ocean. It is important to keep in mind that melting glaciers contribute to sea level rise but melting sea ice has almost no impact on sea level. This is because sea ice, which floats on the ocean, displaces the same volume in its frozen state as it does in its liquid state. So, whether sea ice remains frozen or melts, the sea level stays almost the same.
“It is a different story when it comes to the water in glaciers. When frozen, that water stays locked on land. But when a glacier melts, that water ends up in the ocean, causing the sea level to rise.
“The third contributor, which has a very small impact globally but can have a noticeable one locally, has to do with the fact that freshwater is less dense than saltwater. Because glaciers and sea ice are mainly composed of freshwater, melting glaciers and melting sea ice contributes more freshwater to the ocean, raising the sea level.
“What you can see in this map is that although the overall trend is that the sea level is rising, it does not do so homogenously across the globe. The ocean is not static – it is moving all the time. Some variations in sea level are caused by ocean currents and El Niño- and La Niña-related events, which impact the temperature of the ocean and sea level unevenly. So, the 30 years of data that we show here is long-term but not enough to compensate for all the variations.
...“When people hear that the sea level is rising just a few millimetres per year, some do not think much of it. But looking back at sea level rise over the past 30 years, it amounts to something like 10 centimetres. And because sea level is not just rising steadily but actually accelerating, we might have an additional 10 centimetres of rise in the next 15 years. Even if these increases still sound small, they put more areas at risk of severe coastal flooding. Even if we were to stop emitting every kind of greenhouse gas right now, the sea level would continue to rise for many more years.
