Global warming causes sea level to rise in two ways. First, it melts ice sheets, glaciers, and sea ice. The melting ice flows into the ocean, increasing the total amount of water as well as sea level. Second, water expands as it heats. Therefore, a warmer ocean is a bigger ocean.
Since sea-level rise is one of the most certain and widely reported effects of climate change, climate deniers are keen to find evidence that it isn’t happening. They’ve seized on some admittedly confusing data in the last few years. Precise, satellite-based sea-level measurements began in 1993. Between then and approximately 2002, global mean sea level increased by an average of slightly more than three millimeters per year. However, in the ensuing decade, when climate change was supposed to be accelerating, global mean sea-level rise appeared to slow to slightly more than two millimeters per year—a deceleration of nearly 30 percent.
This kind of data is a dream for people who deny climate change. They use it in combination with the claim that global warming has “paused” in the last decade, arguing that measurements on the ground are inconsistent with climate change theory. Here’s noted climate change skeptic Judith Curry making that claim: “Once again, the emerging best explanations for the ‘pause’ in global surface temperatures and the slowdown in sea-level rise bring into question the explanations for the rise in both in the last quarter of the 20th century. And makes the 21st century of sea level rise projections seem like unjustified arm waving.”
This argument has always been badly flawed, because one of its pillars of evidence, the supposed “pause” in global warming, is a fantasy. Global temperature continues to increase rapidly. There is no pause.
Today, the second part of the two-pronged denier argument took a major hit. In a paper published in the journal Nature, a team of scientists based in Australia, the United Kingdom, and the United States showed that the widely reported slowdown in global sea-level rise was due to an error in instrumentation. When measuring sea-level change, previous researchers had failed to take into account that land also moves up and down.
When they corrected the sea-level measurements to reflect those “vertical land movements,” the authors found that the ocean rose more slowly in the mid-1990s than we previously thought. In fact, global mean sea-level rise may have been as low as 2.6 millimeters per year at the end of the last century, instead of a little over 3 millimeters as previously thought. If these data are correct, the ocean is rising faster today than it was when satellite measurements began in the 1990s—just like climate change science predicts.
Let me put that as simply as possible, so there’s no room for creative misinterpretation: Sea-level rise is accelerating. There is no disagreement between the theories and the measurements.
It’s important to keep in mind that sea level is a complicated phenomenon and incorporates many factors beyond air temperature. Natural changes in the atmosphere and ocean currents also play a role, so it wouldn’t be shocking if sea-level rise decelerated briefly. If that were to happen, it wouldn’t invalidate climate change theory.
But again, to be clear, that is not happening: Sea-level rise is accelerating. It is not decelerating.
Also, remember that long-term observations are key. In the past five millennia, scientists estimate that the sea has risen less than one millimeter per year on average, based on geological markers on coastlines. So, whether sea level rises 2.5 or 3.1 millimeters this year, it is rising much faster than it has in the past.
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