Sea Level: Rising Fast

Here’s the graph from NOAA of sea level at Pensacola, FL (tide gauge data)

Some might actually look at that and think “steady rise,” but the readers of this blog would probably think otherwise. Here’s my graph of the same data:

Let’s “reduce” the data in a very simple way — one so simple that it’s hard to argue with — let’s split the observed time span into 5-year intervals, and compute an average for each.

Then, let’s just graph the averages:

I think this graph gives a much better “visual impression” of what’s been happening to sea level at Pensacola.

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The most obvious feature is the rapid increase in the last 10 years; the 2017-2022 average is almost 120 mm higher than the 2007-2012 average. With 10 years between their average times, that suggests a rate of sea level rise of 12 mm/yr sustained for an entire decade.

There is a faster rise earlier — the 1972-1977 average rises almost 70 mm above the 1967-1972 average in only five years — but is immediately followed by a 42 mm drop over the next (1977-1982) average.

There is also a sustained rise in the 1930s and 1940s, increasing by nearly 122 mm from the pre-1927 average to the 1947-1952 average. It then proceeds more slowly, with ups and downs but only reaching another 88 mm in the next 60 years, a rate not much above 1 mm/yr.

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That particular “visual impression” turns out to be correct. To estimate the time-varying rate of sea level rise, I fit my usual two exploratory models: a lowess smooth, and a PLF (Piece-wise Linear Fit). That of course requires me to choose the “time scale” for both, and I’ve settled on a rougly 10-year time scale for good time resolution without letting the high noise level drive things crazy. Here’s the lowess smooth in red and the PLF in black:

Here’s what they say about the rate of sea level rise:

The red line is the estimated rate from the lowess smooth, with pink shading showing its uncertainty range. The blue line is the rate estimated from the PLF, with light blue uncertainty range, and really represents the estimated average rate during each (roughly 10-year) interval.

The most obvious feature is very rapid sea level rise during the last decade.

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That particular feature shows in the same analysis applied to data from Wilmington, NC:

And from Apalachicola, FL:

And in St. Petersburg, FL:

And in Charleston, SC:

This is what’s really happening with sea level rise on the U.S. Atlantic coast south of Cape Hatteras, and the Gulf coast of Florida. The rates of sea level rise are startling, and many of those areas also have to struggle with land subsidence, which makes the problem worse because it causes relative sea level rise even when the sea surface height isn’t changing.

Naturally one wonders, how is it possible for so much of the U.S. east coast to show such rapid levels of sea level rise? Will they be sustained? I doubt it.

My best guess for the reason — and I’m not an expert on this subject — is that either: changes in the AMOC (Atlantic Meridional Overturning Circulation) have altered ocean currents to push more water on the U.S. south-east Atlantic coast and the Gulf of Mexico; or ocean temperatures in those regions have risen much faster than globally, causing exaggerated thermal expansion in that area. Or, both.

But whatever the reason, rising rapidly it is.

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