Since 1993 we’ve been monitoring sea level with satellites. The overall rise is obvious (it was obvious even before 1993), and we now know that the rate of rise is faster than it has been in a very very long time. But one might wonder, what has it done lately?
I retrieved global sea level data from NASA, and if you’re a regular reader you know I love graphs:
A lot of the wiggling around (and there is a lot of that!) is due to the annual seasonal cycle, and there’s also a cycle that’s related to the satellites’ orbits. If we want a clearer picture of how the sea level trend is progressing, we should remove those; fortunately NASA also provides smoothed sea level anomaly (I’ll even include a straight-line fit, which estimates the average rate for these last 24 years or so: rising at 3.4 mm/yr).
It’s good to know the overall average rate, and it’s obvious that even with the seasonal and orbital cycles removed it still wiggles around — a lot. But has it done anything else trend-wise? Is there any departure from that simple straight line which we can have some confidence in, some valid evidence that it’s not just random wiggling around that looks like something meaningful but isn’t really?
Some look at these data in an attempt to find something, anything, they can cherry-pick to claim that either global warming’s effect on sea level isn’t happening, or that we should look at it as “no problem.” A classic example happened nearly 10 years ago, when Danish climate “skeptic” Bjorn Lomborg wrote this in the U.K. newspaper The Guardian:
“Over the past two years, sea levels have not increased at all — actually, they show a slight drop. Should we not be told that this is much better than expected?”
Let me answer that question: No, Bjorn, we should not be told a lie.
I say that because sea level is so “noisy” (meaning, it wiggles around so much) that even with a steady trend, a 2-year period can easily show no rise because the noise happens to be going down enough to cancel the trend continuing up. No, Bjorn, this was not “much better than expected” — episodes like this are expected from time to time on a regular basis. Roll the dice often enough, sooner or later you’re gonna get snake-eyes.
Poor Bjorn. He was roundly (and rightly) ridiculed for his comment, perhaps most pointedly years later in a graphic from Greg Laden’s blog when Lomborg spouted more nonsense about sea level:
You’d think that would be the end of it. Who would be dumb enough to try the same trick?
Anthony Watts, that’s who. The identical strategy was “Bjorn-again” when Watts gave us a wonderful post revealing what he calls a “pause” — at least as far as global warming is concerned. He declares a “pause” in sea level rise by doing exactly the same thing Bjorn Lomborg did.
Exposing nonsense, like that from Lomborg and Watts, can be amusing. But it doesn’t answer the question: what has sea level rise been up to, on the global scale, lately?
I prefer not to use “smoothed” data for analysis; if you’re into statistics you might know that it dramatically increases the autocorrelation of the data, which makes analysis a whole helluva lot trickier. So, I removed the seasonal and orbital cycles myself in order to generate anomaly values without that extra autocorrelation. And here it is:
Then I went looking for patterns (other than the straight-line increase) that could be claimed to mean something, not just suggestive-looking noise. I started by fitting a smooth to these data, not to analyze the result but just to get ideas by illustrating what changes might have occured. Here’s the smooth in a form I find rather attractive:
It looks like maybe, just maybe, it slowed down a wee bit around 2005 and sped up again about 2011. So I returned to the original data and tried modelling it with not one, but three straight line segments. I found the “turning point times” which fit best, and tested their statistical significance. Lo and behold, they turn out to be real and the model fit looks like this:
How much did it slow down, and how much speed up? We can use both the three-lines model and the original smooth I applied to estimate the rate of sea level change and its uncertainty (three-line model in blue, smooth in red):
Bear in mind that these estimates aren’t of instantaneous rate; the blue lines represent the average over many years for the time segments they cover, and the red line also represents an average over many years as our “window” on the data moves through time.
One final note: I will admit doubt that these results indicate actual trend changes, for several reasons. First, the autocorrelation in the original data (smoothed or not) is fierce, so the analysis is far from easy. Second, there are other factors which can cause temporary ups and down that last for years, such as the el Nino oscillation and the movement of water from ocean to land and back (often when extreme floods inundate the land, the water eventually finding its way back to the sea).
To sum up: I regard the changes indicated by the three-lines model to be real, but I’m not certain they reflect long-term trend, they could be a manifestastion of the kind of noise that lasts a few years — leading to apparent trend — but never really gets anywhere because it’s not really part of the trend.
Perhaps we’ve already seen recent strong acceleration in sea level rise, but the aforementioned caveats mean we should wait for further data before stating firm conclusions. But the danger — the extreme danger and cost from sea level rise — means that we should absolutely not wait for further data to begin getting ready for what’s to come, and taking steps to make it as painless as possible.
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