For decades, climate scientists have been able to directly measure sea-level rise employing tide gauges, satellites, and meticulous observations. They have also been able to pinpoint the individual drivers: melting glaciers, shrinking ice sheets, and the oceans warming and physically expanding as they absorb heat.
The issue was that when these drivers were summed, the resulting total didn’t quite align with the measured figures. The discrepancy was small, but significant enough to be uncomfortable in a field where accurate accounting is paramount.
A new international study, spearheaded by scientists from the Institute of Atmospheric Physics at the Chinese Academy of Sciences, has finally identified the problem. The findings were published in the journal Science Advances.
The headline number is this: Since 1960, global sea level has risen by an average of approximately 2 millimeters per year. This sounds almost trivial as stated, but it is not.
A staggering volume of water – 2 millimeters per year – gets added to the world’s oceans. The impact on coastlines, storm surges, and low-lying communities has accumulated stealthily over decades.
But a more recent figure deserves attention. Between 2005 and 2023, the rate of rise more than doubled, reaching nearly 4 millimeters per year. The oceans are not just rising. They are rising twice as fast as they were a generation ago.
The dominant factor driving the price increase since 1960 (accounting for 43 percent of the total rise) is what’s known as thermal expansion. As the ocean absorbs heat from a warming atmosphere, the water expands and takes up more space.
This sounds too simple to explain such a scale, but it’s the entirety of the global ocean getting warmer, and warm water occupying a greater volume. In the end, it all adds up quickly.
The rest is ice. Mountain glaciers account for 27 percent of sea-level rise, the Greenland ice sheet for 15 percent, and the Antarctic ice sheet for 12 percent. And the balance is shifting – since 1993, the accelerated melting of ice in Greenland and Antarctica has played an increasingly important role. As the study makes clear, this trend is not slowing down.
After 2015, the gap between observed sea-level rise and the sum of its known drivers became persistent enough to be a real problem. The new study sought to identify the sources of this mismatch, and it found several.
Satellite measurements used to track sea level had accumulated small errors over time that needed correction.
Methods for assessing the movement of the Earth’s surface at coastal tide gauge stations, which influence the interpretation of sea-level readings, needed improvement. Researchers also refined estimates of ice loss from Greenland and Antarctica by using better data.
“For years there was a frustrating gap between how much we observed the oceans were rising and what we could account for from the individual causes,” said co-author John Abraham of the University of St. Thomas. “This work shows that with better instruments, processes, and more intelligent analysis, that gap in knowledge can be filled. We can explain sea-level rise with greater confidence.”
Closing this gap makes projections more robust, which in turn improves the decisions made based on them.
This is what distinguishes sea-level rise from most other consequences of climate change: it does not stop when we stop emitting carbon dioxide. If we cut carbon emissions to zero tomorrow, temperatures would stabilize relatively quickly, but the sea level would stay put.
The ocean is vast, and the heat absorbed at the surface takes decades or centuries to distribute throughout the water column.
Ice sheets melt even more slowly – the ice in Greenland and Antarctica that is melting today will continue to melt long after the warming that caused it has stabilized.
We have locked in centuries of sea-level rise that can no longer be prevented by any policy measure. The only question is how much more we will add on top.
All of this, in part, makes the findings of this study bad news. Understanding something clearly is always better than not understanding it.
This study gives us a complete and accurate picture of what has been happening to the world’s oceans over the past six decades. It shows where the water is coming from, how the balance of causes has changed over time, and how the rate of sea-level rise has accelerated.
This is the foundation upon which everything else must be built. The oceans are rising faster, and now we finally have the confidence to say why.
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