Since I am not an employee of the Florida state government under their science-denying governor Rick Scott, I’m allowed to say the following phrases: “climate change” and “global warming.”
I’m also able to mention those topics when discussing sea level rise. In the hope that I can annoy Florida’s science-denying governor Rick Scott by flying in the face of his denial of science itself, I’ll print the truth for all Floridians (and others) to see: sea level rise is being caused by man-made climate change, also known as global warming.
There are 11 tide gauge stations in Florida with data extenting up to (and beyond) the year 2010. Their data are freely available from multiple sources, including PSMSL (the Permanent Service for Mean Sea Level).
In order to combine the data into a single estimate of sea level changes in the vicinity of the state of Florida, we need to align them because they’re not all on the same baseline. I introduced a method to do so some time back. If we align and combine the data from these 11 stations, we obtain this:
These are anomalies, meaning that the annual cycle has already been removed so we can concentrate on the changes over time rather than just the seasonal fluctuations.
One of the complications inherent in monthly sea level data is that it is strongly autocorrelated. There are ways to deal with this, but we can simplify greatly, eliminating most of the autocorrelation, by working with annual averages rather than monthly. Those look like this:
Note that I’ve plotted a solid blue line to illustrate the best-fit straight line by least-squares regression. The estimated rate of sea level rise over the entire time span turns out to be 2.05 +/- 0.16 mm/yr.
But perhaps the rate isn’t constant over time. We can fit a smooth curve, which I’ll do by my favorite method (modifed lowess smooth), giving this:
The smooth departs from the straight line, notably. This suggests that indeed the rate hasn’t been constant over time. Although fitting a smooth isn’t the same as a proper statistical test of whether a change has occurred, it is certainly suggestive.
Yet those who deny the danger posed by sea level rise have a nasty habit of stating “no acceleration” while providing no evidence to back that up, and quoting rates of sea level rise based on straight-line fits to very long spans of time.
What would we find, you might wonder, if we fit straight lines to the same data set but instead of starting way back before the year 1900, we started at a later year? We expect such estimates will be less precise because they’re based on less data, but more relevant because what we’re most interested is not the average rate over the last 120 years, we want the rate now.
Let’s find out. I fit a straight line to the annual average sea level in Florida, starting with every possible start year from 1900 to the year 2000, ending with the final year. I also computed the uncertainty range. Here’s a graph of the estimated rate (as a dot) with error bars showing the 95% confidence limits:
The horizontal red line shows the estimated rate from 1900 to the present. Note that starting at more recent years gives an estimated rate considerably higher than that. Considerably. More to the point, the 95% confidence interval excludes that older, smaller rate. This isn’t just suggestive, it’s actual evidence that the rate now is higher than it has been, in fact it’s higher than the overall rate. There’s a word for that kind of behavior: acceleration.
There are other, better ways to test for such patterns, some of which are illustrated in this post about temperature time series. Yes, I’ve done many of those tests too. Yes, I’ve done them using monthly data and proper allowance for autocorrelation rather than just using annual averages. Yes, the rate of sea level rise around Florida has shown acceleration.
It has also shown deceleration. In fact it shows a pattern of both slowdowns and speedups, just as global sea level does. By golly, there’s even published research about that. Before anyone protests that this invalidates climate models and/or theories of sea level change on which future projections are based, save yourself the embarrassment. Because it doesn’t.
As a matter of fact, the models which match best to observed data are semi-empirical models, which also happen to be the models which suggest the greatest sea level rise in the near future. If they’re correct, these models which best fit the data, then before the end of this century we are … how shall one say? … screwed.
But, those in denial will continue to shout “no acceleration” without any evidence to back that up, and continue to quote rates since 1900 or even earlier as though they were estimates of the present rate.
They’ll also make many more misleading statements, usually out of ignorance but sometimes out of dishonesty. For instance, they may trivialize the impact of sea level rises that seem small, just an inch or a few, without being aware (or admitting) that ain’t so. Even a tiny rise in sea level will, when a storm hits and the storm surge inundates the area, make the ocean creep further inland by vastly greater distances. Vastly. If you want details and evidence, there’s research about that too.
They’ll even proclaim that as sea level rises, we can just raise roads and bridges and other things, as though that’s a cheap and easy way to deal with sea level rise. Unfortunately, it doesn’t work that way. The amount of sea level rise that is expected this century is huge — it could be an entire meter or more — and will put rather large swaths of land underwater, because sea level is accelerating and will continue to do so. Unless, of course, ice melt and thermal expansion cease to be the result of temperature increase, i.e., the laws of physics cease to apply.
One would hope that they will pay attention to actual evidence, accept the results of valid statistics over “doesn’t look like it to me!”, and give the safety and prosperity of future generations a bit more priority than the profit margin of Florida’s science-denying governor Rick Scott and the uber-rich he serves. Because he sure ain’t serving the people of the state of Florida.