Here’s some data, and the question is: what’s the trend rate at present (in the year 2016)?
Here, for your edification, is the rate over time:
The rate during 2016 is +3 mm/yr.
So no, it’s not a trick question. The trick is what the unaware do innocently, and deniers often do not-so-innocently, to make you think the present trend isn’t what it is. They’ll compute the “30-year trend” (or whatever) by fitting a straight line to 30 years’ data, giving this:
Then they’ll tell you that the trend rate in the year 2016 is zero.
That is a reasonable way to estimate the average trend rate over the 30-year period from 1987 through 2016, possibly not the best way but at least reasonable. The average time of that span is 2001.5. Hence that’s an estimate of the trend rate at 2001.5. I’ll plot that estimate as a red dot, along with the actual trend over time as a black line, here:
In this case it turns out the estimate is even correct — at 2001.5.
Now I’ll plot that trend estimate as a blue dot, along with the actual trend over time as a black line, in the way deniers often want you to believe:
That estimate of the trend at 2016 is not correct. They’re plotting the estimated rate from 15 years ago as though it were the estimated rate today.
They do it most often to claim that there’s “no acceleration” of sea level rise. If you fit straight lines over sliding time windows, you’ll get estimated rates at the centers of the time windows — but they’ll plot them at the ends of the time windows to make it seem the final estimate is current. They also generally don’t mention that the methodology doesn’t even give an estimate for the current rate.
Perhaps a more realistic example will help. Let’s take three stations from Florida: Fernandina Beach, Key West, and Pensacola. Here are their data series (with the annual cycle removed) from the Permanent Service for Mean Sea Level:
There are a lot of things wrong with Steve Case’s graph, but most important is that he’s plotted the linear trend estimates for 30-year time spans at the end time, not the mean time, giving the false impression that the final estimate is “current.” It’s not.
There are other problems too. For instance, his estimates start at 1951. To get a 30-year trend estimate ending at 1951, you’d need data starting no later than 1922. But the data for Pensacola don’t start until May of 1923. Then there’s Fernandina Beach, with no data from July 1925 through October of 1938, a 13-year period of missing data. That hardly makes for an optimal estimate of the “30-year trend,” but he makes no mention of the missing data, just refers to the three stations as “three long running Florida tide gauges.” But all told, those are minor quibbles compared to plotting rate estimates 15 years later than they apply.
We can ameliorate the missing data problem by combining the stations to form a single composite, if they’re properly aligned. I get this:
Now we have a continuous record for Florida from June 1897 through December 2015. We can also smooth the data (using a lowess smooth) thus:
We can compute the linear trend rate for overlapping 30-year time spans, and if we plot it as Steve Case did (at the end times) it’s here:
Of course, he only shows the results from 1951 onward, which I’ve enclosed in the red box.
Plotting the estimates at their correct times, we can see that the most recent estimate is really for 2001, not 2016:
There are mathematical methods for estimating the trend rate all the way to the limit of the data, and here’s the result of one of them shown as a red line atop the moving-30-year-windows estimates:
Now we can see two important things. First, there has indeed been acceleration of sea level rise (and deceleration too, although acceleration is most recent). Second, the most recent trend estimate is the highest.
That rather undermines Steve Case’s support for one of the central themes of the original post: that an LA Times article (about Donald Trump’s climate denial and Florida’s sea level rise problem) is invalid because there’s been no acceleration of sea level rise.
But the bizarre thing about the whole post is that it doesn’t really depend on acceleration of sea level rise. Sea level rise, with or without acceleration, is already a serious problem in south Florida, and Donald Trump’s denial of the human origin of sea level rise is a slap in the face to the people who are already paying a huge price for the problems it brings.
But, deniers seem to fall back on the “no acceleration” argument every time sea level is discussed. Never mind that they focus on a tiny region of the globe to increase the noise level, never mind that the issue at hand may be a huge problem even in the absence of acceleration, never mind that they’re simply wrong about “no acceleration.” It’s a talking point, the choir they preach to lap it up, and that’s good enough for them.
As for the main post, there are many, very serious, problems with it, but that’s a topic for another blog post. Soon.
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