Florida Flood

A new paper (press release here) about sea level rise in southeast Florida concludes that locally, sea level rise has accelerated recently. What particularly caught my eye was the magnitude; from the abstract:

The average pre-2006 rate is 3 ± 2 mm/yr, similar to the global long-term rate of SLR, whereas after 2006 the average rate of SLR in Southeast Florida rose to 9 ± 4 mm/yr.

I must confess, I’m somewhat skeptical.

Naturally I went to get some data myself; there are a number of tide gauge stations in the area:

The green arrow marks the station at Virginia Key, which (at 20 years) is the longest record which is also in the Miami Beach area. Some of the records don’t cover the time of interest, and a couple are outside the region of interest (Naples and Ft. Myers aren’t southeast Florida); I ended up with 7 stations which might inform how sea level has changed in the area.

This is a place where the annual cycle is pronounced, so that had to be removed. Then the station records had to be aligned. When I do so, the individual stations line up nicely:

Now we can form a composite estimate of the sea level in southeast Florida, and smooth that data for a better image of what the trend looks like:

There has indeed been a recent uptick, you can certainly call it an acceleration, but I doubt that it’s the kind of acceleration in the long-term trend that will persist. After all, we’ve seen upticks like this before which didn’t last (late 1940s, early 1970s). It’s the nature of local sea level to show those ups and downs — and global sea level can, too, given the short-term impact of things like el Niño, and its consequent shifting of water from continents to ocean and back.

I’m also highly skeptical about their identification of 2006 as the time at which the sea level trend changed. I can fit a continuous piecewise linear trend to the data with its trend break at 2006, but I can also find a better trend break time by changepoint analysis, which suggests 2011:

This is borne out by looking, not at monthly data, but annual averages:

The pre-2011 rise rate is 2.4 mm/yr, while the post-2011 rate is a whopping 19.2 mm/yr! We certainly don’t expect that kind of sea level rise to continue in the near future, in southeast Florida or anywhere else (at least, not unless something drastic happens like the breakup of the ice sheets). As for later this century …

Despite my misgivings about some of the stated numerical results, the problem of flooding in Miami Beach is undeniable. I estimated the ocean height due to sea level trend and tidal variations alone (removing the weather influence of storms, wind, rain), and got this for the Virginia Key location:

The red line, marking the flood level (due to tide alone), is a crude approximation but it gives the right idea. Starting around 2006, southeast Florida entered the regime in which flooding not only can, but will occur at high tide in October. As the sea has continued to rise, the amount by which high tide exceeds this (crude) threshhold has gotten bigger:

The key question of course is: what will the future bring? Tidal variation suggests 2016 will be another bad year for flooding in southeast Florida, 2017 won’t be so bad, then the bad years will resume. Sea level fluctuation is an open question in the short term; it may actually decline (oh so slightly) in the next year or two. But in the long term, the sea will continue to rise and make the problem worse, far worse, and extraordinarily worse. As for the rate at which that happens, it may be faster in southeast Florida (with weakening of the ocean’s overturning circulation) but it’s too early to tell, and I very much doubt the stated 9 mm/yr rise rate is the proper forecast.

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