A regular big lie from climate deniers, in fact a huge lie from climate deniers, is when they deny that there’s been acceleration of sea level rise. Sea level acceleration is a fact.



Let’s start with what happened during the 20th century (before the year 2000). The most respected reconstruction of global sea level during that time based on the global network of tide gauges is from Church & White:

According to these data, the average rate of sea level rise from 1900 to 2000 is about 1.6 mm/yr. There are other reconstructions of course, and they lead to the same conclusion: that the average rate during the 20th century is less than 2 mm/yr.

Now let’s look at how sea level has changed since 1993, using altimetry from the satellites which give us global coverage (monthly averages with the seasonal cycle removed):

The average rate over the last 24 years is about 3.4 mm/yr. That’s about twice the average rate since 1880. It’s certainly over 3 mm/yr.

20th-century average rate: less than 2 mm/yr. 21st-century average rate: more than 3 mm/yr. Three is larger than two. Conclusion: the rate of sea level rise went up. That’s acceleration.

Some have urged caution estimating the trend of sea level rise using satellite data because the record only covers 24 years. They express concern about the lunar nodal cycle, an 18.61-year oscillation of the moon’s orbit, which causes a small change in mean sea level. Some, in particular those who want to help prepare for the risks, have issued this caution because they’re genuinely concerned with the uncertainty of rate estimates. Others, particularly “free-market think tanks” (translation: corporate profit over public good) do so for no other reason than to discredit the trend from satellite data.

But the argument is specious. The lunar nodal cycle causes an oscillation of mean sea level at a particular location, one which depends on latitude. Its global pattern generally follows the low-order spherical harmonics, so when sea level is raised at the equator it’s depressed at the poles and vice versa. The global effect is just about zero, because it doesn’t affect the total volume of water in the ocean, and that’s what global sea level is really about. You can find some of the details here.

We can detect acceleration in the Church & White sea level data itself. The average rate of 1.6 mm/yr is based on fitting a straight line to the data after 1900 and before 2000, but there’s pre-1900 and post-2000 data, and it turns out sea level isn’t actually following a straight line. We can fit a smooth curve instead, which allows for non-linear changes, and I’ll use a modified lowess smooth:

An extremely useful aspect of smoothing is that it enables us to estimate the rate of change through time, as well as the actual value. This particular smooth gives this estimate of the rate at which sea level has been rising:

The red line shows the sea level rate through time; the blue line shows the average level for comparison. The acceleration is obvious.

There was, however, a time span (from about 1940 to 1980) when sea level did not accelerate; if anything, the rate decreased slightly so sea level rise decelerated. I have often pointed out that one of the most interesting facts about sea level rise during the 20th and 21st centuries is that it shows both acceleration and deceleration. There’s no “inconsistency” to explain from this; models — both process models (which simulate the physics) and semi-empirical models (which are mathematical rather than “computer models”) reproduce this behavior. All this is utterly ignored by climate deniers, who simply argue that there’s no acceleration in a vain attempt to promote a “don’t do anything” agenda.

According to these data, the latest bout of acceleration covers about the last 50 years. So, I decided to look for it in individual tide gauge records by isolating the data since 1965 and fitting a quadratic function of time; the quadratic coefficient gives an estimate of the acceleration. I insisted that any record included have data in least 40 of the years since 1965.

Tide gauge data tend to be quite noisy, so for many of them a 50-year span is insufficient to detect acceleration even if present. Nonethess, we can use the ensemble of estimated quadratic coefficients to get at least some insight into which are showing acceleration (positive coefficients) or deceleration (negative coefficients). The results won’t be perfect of course, but perhaps they’ll give us some perspective.

Here’s a map showing gray cirlces for each tide gauge station with sufficient data, larger circles for larger coefficients (either positive or negative), with red circles indicating statistically significant acceleration, blue circles significant deceleration (autocorrelation correction was included in the significance test):

Stations with significant acceleration outnumber those with significant decleration, 74-to-11.

Here’s the same map, but will all stations coded red for acceleration (positive) and blue for deceleration (negative) whether statistically significant or not:

What’s most interesting is where the stations showing possible deceleration are located. They’re concentrated at high latitudes, in particular at parts of northern Europe and North America at which the melting of glaciers has been pronounced enough to cause local deceleration because of the reduced gravity from smaller modern glaciers. It’s truly fascinating that even these locations showing decelerating sea level rise are most likely because of global warming: melting glaciers, less gravity, lower local sea level.

It’s also quite interesting that so many more stations show signs of acceleration than deceleration, especially when one requires statistical significance. The evidence is strong: sea level rise is truly accelerating, as evident in global reconstructions from tide gauges, in the analysis of recent data from individual tide gauges, and in the satellite data.

As if its present rate and acceleration weren’t bad enough, the best science indicates more sea level acceleration to come. Both physical and mathematical models suggest a range of from 2 to 6 feet by the year 2100. Unfortunately the latest IPCC report isn’t up-to-date with the latest science; one hopes the next will reflect the overall opinion of the community of genuine sea level experts, that the problem has gone beyond “worrisome” and become critical. Coastal regions everywhere should be preparing for what’s to come, and the uncertainties in how much and how fast those changes will happen is not a valid reason to minimize preparation; a wise society realizes that uncertainty is not your friend.

But of course “free-market think tanks” (translation: corporate profit over public good) will continue to dispute this, will continue to discredit any data which shows how bad the problem is while touting any data which they can present in some way to make their “don’t worry be happy” case. Personally, I’d rather be prepared for what’s really ahead. Living in a fantasy world where sea level rise will only be an annoyance, is fine for the super-rich. For the rest of humanity, believing such fantasy will only make the inevitable worse.

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