Tide gauges measure local sea level, the difference between sea surface height (SSH) and the height of the land. If the sea surface gets higher — what we usually think of as “sea level rise” — local sea level won’t necessarily go up because the land itself might also be getting higher. Although tide gauges are generally fixed to solid rock (the more solid the better), the land itself can still move up or down, a process called vertical land movement (VLM).
One of the main causes of VLM is glacial isostatic adjustment. When the last ice age ended, the great ice sheets covering much of north America and northern Europe melted away and their immense mass no longer pressed down on those areas. They have since rebounded, but the rebound proceeds at a “glacial” pace (pun intended). More useful is the fact that on time scales of the last few centuries they have done so at a constant rate. This means that the rate of vertical land movement due to glacial isostatic rebound has been roughly constant, faster at some locations and slower (even negative) at others, but not changing from year to year fast enough to worry about. If we can figure out what that constant rate is at a given location, we can compensate for it and get a good estimate of how fast the sea surface height itself is changing (which is what we’re really interested in, on a global scale).
There are factors which can change local VLM on shorter time scales (years to decades), such as pumping groundwater and/or oil out of the earth, or sediment accumulating, or earthquakes. But the main influence on large scales is glacial isostatic adjustment. To correct for it we need to know its rate, but that’s not so easy. We can use models based on geophysics to estimate how it varies from place to place — for instance, the regions where the great ice sheets used to be are rebounding upward fastest. Lately some places have attached GPS (global positioning system) sensors to their tide gauges for direct measurement, but they haven’t been in place long enough to make estimates of VLM very precise. All in all, measuring or estimating VLM is tricky business.
Yet estimates of global sea level depend on them. To combine and average data from different locations, we need to remove the influence of local VLM or we’ll be averaging differences in VLM rather than the common changes of sea surface height. Much research has focused on allowing for VLM, but its uncertainty remains a sticky issue. It really is tricky business.
Although there are over a thousand tide gauges around the world which contribute their data to the study of sea level, many of them don’t have data stretching very far back in time. But some do; I looked at the data available from PSMSL (the Permanent Service for Mean Sea Level) and found 125 tide gauges with at least 80 years’ data which include at least some data as modern as 1970. My goal is to align and average these long records to get a global sea level estimate.
But what to do about VLM? Estimates of their rates are available, but I’ve tried a new idea. Instead of estimating it from a model, or from a brief record of GPS data (which is only available for a fraction of stations anyway), I decided to let the tide gauge data itself tell me the rate of VLM at a given place — or at least, how different it is from other locations.
Specifically, for each station I estimated the rate during a “reference period” from 1950 through 1989. Then I subtracted that trend from the entire record, so the revised data will necessarily have a rate of zero for that time period. Finally, I’m prepared to align and average the revised records to form a global composite. The idea is that they should all have roughly the same rate of rise for sea surface height during the “reference period” so subtracting their different local rates will cancel out that rate plus their local rate of vertical land movement. No model or GPS needed.
The aligned revised records will of course be “tilted” by the actual global rate during the reference period. To get actual global sea level that rate should be restored, so I took the aligned composite values and added back an estimate of the global rate during the reference period, 1.6 mm/yr. That’s the rate indicated by the global composite from Church & White (which I consider the best), so the final result should be a new estimate of global sea level, based on tide gauge stations with at least 80 years of data.
Where are the stations with long records? There are a few scattered about the globe, but most of them are in Europe and North America:
It’s worth noting that far north stations (especially in Europe) show very strong glacial isostatic adjustment, rebounding strongly from the disappearance of the great ice sheets, so a large fraction of these stations can be expected to show large upward VLM, leading to strong underestimates of how the sea surface height is rising. This emphasizes the need to correct for VLM when using such a limited, and geographically biased, sample.
If I align and average but don’t correct for VLM with the 125 long-record stations, I get this:
But when I apply my new method of correcting for VLM I get this:
That’s different! It emphasizes just how strongly the result is affected by the different values of VLM at different locations.
I smoothed the new estimate (with a modified lowess smooth), and for your edification I’ll compare it to two other global sea level estimates, from Church and White (shown in blue) and from Jevrejeva et al. (shown in green):
My new estimate agrees exceptionally well with the data from Church & White, although at least in part that’s because it’s constrained to have the same average rate of increase during the reference period 1950 through 1989. Mine shows less sea level rise during the 1930-1950 period, but overall agrees quite well during the wole period covered by both. The Jevrejeva data, however, are in considerable disagreement with my own estimate (as well as that of Church and White) prior to 1950, yet do agree with my estimate pretty well during the very early time span from 1800 to 1850.
One of the virtues of the smooth is that it enables a reasonable estimate of how the rate of sea level rise has changed over time. Using a 30-year time scale, I get this:
This belies claims that decades ago, sea level rise showed a rate comparable to today’s. It also shows a present rate in reasonable agreement with the satellite data, considering that these are roughly 30-year rates, while the satellite data only cover about 25 years and sea level has been accelerating during that period.
My new reconstruction also illustrates yet again (as do all good estimates in fact) that sea level has shown both acceleration and deceleration over the last couple of centuries. One of climate deniers’ favorite claims is that sea level hasn’t accelerated at all, so its rise will be modest this century. Such claims are terribly misleading for at least two reasons: first, that the sea level rise we’ve seen already (let alone what’s to come) isn’t modest and certainly isn’t harmless, it’s costing millions from increased flooding during storm surge and flooding even without storms (“sunny-day” flooding); second, that their basic premise (no acceleration) is just plain wrong.
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