There seems to be some interest in global sea level as estimated by tide gauges. In particular, we have recently been directed to a graph of the data from Church & White, which is the most reputable of the available choices. Alas, the graph we were directed to only shows their data up to the end of 1992. What, you may wonder, happens after that?



The data extend for another 21 years, to the end of 2013. And here it is:

How, you may ponder, does this compare to what the satellites report after 1993? Like this:

How fast, you may ask, is the estimated trend since 1993 according to these tide-gauge-based data? It’s 3.56 mm/yr, even larger than the estimated rate from satellite data:

Do bear in mind that the difference (from the satellite-based estimate) isn’t statistically significant. Essentially, they agree (certainly within their uncertainty limits).

The original graph easily gives the visual impression of following a straight line since 1930. But, as I’ve cautioned so often, “looks like” isn’t a realiable way to determine results from data. It’s a great way to get ideas and clues, but history is littered with the carcasses of ideas that are wrong but were believed because of “looks like.”

One way to improve the “look” is to fit a straight line (by least squares or other methods), then subtract that fit from the data to see what’s left over. These are called “residuals,” and show a clearer picture of what the data are doing besides that straight-line increase. Here they are:

The pattern of acceleration and deceleration is now evident. Especially prominent is the recent upswing, a sign of the acceleration that has happened recently.

If we smooth the data, a lot depends on the time scale for smoothing. Too short a time scale will tell us about the ubiquitous wiggles, which is not what we’re interested in. We want to know about the persistent trends. In my opinion, a good choice for these data result in this smoothed estimate:

This also produces an estimate of the rate of sea level rise:

It’s rather apparent that the rate has gone both up and down (both acceleration and deceleration of sea level), but that recently we’ve seen a sizeable increase in how fast the ocean is rising.

To some, it may seem puzzling that those in denial of the danger of sea level rise focus so strongly on denying the existence of acceleration. Claims of its absence are sometimes based on fitting a quadratic curve when such is not an appropriate choice and falling back on the simpleton’s approach, but are more often based on linking to a graph from NOAA and saying “See!”

But even without acceleration, the present rate is real trouble. Miami is already spending about half a billion dollars to deal with the flooding they’re already getting from sea level rise. Another foot will bring yet more damage, and threatens the economic viability of somewhere in the neighborhood of a trillion dollars of prime real estate development.

We expect to see considerably more acceleration this century. This is suggested by empirical models (such as those of Vermeer and Rahmstorf), on “process models” (which emulate the physical process), and on something we like to call “laws of physics.”

Of this we can be certain: deniers will continue to declare “no acceleration” based on shoddy analysis or, more often, none at all. Meanwhile, the sea will continue to rise.

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