Saturday, August 27, 2011

Changes in sea level

One of the downsides to having a PhD is that my relatives have no idea what it is I do. For some it seems that having a PhD means that I am a "scientist" with expertise in anything that could be lumped under the term science. My degree is actually in computer science and then I specialized in a narrow subset of that field. From there I went into industry to work on projects that are only vaguely related to my dissertation. My general point though is that I don't have any particular expertise in other scientific fields. I do have an interest and read some books intended for laymen, but that doesn't mean I'm an expert on those topics.

A number of my relatives have recently taken an interest in global warming and wanted my opinion on the matter because I was the "scientist" in the room. My general answer is to just point to the scientific consensus and state that I provisionally accept it without being aware of all of the details. The EPA provides a summary of evidence for climate change and in particular a page describing the current state of knowledge. This put me at odds with them suggesting that global warming is some sort of conspiracy and that there is no real evidence. I couldn't really tell whether they disagree that the earth is warming or whether they just disagree that human activity is causing or at least contributing to it. The only thing that really seemed consistent in their arguments was the certainty that they were right and that nothing needed to be done to rectify the situation. Of course, I lost the debate because my typical retort was "I don't know." I haven't spent much time looking into global warming so I'm not that well versed on the evidence to support it.

Anyway, there were a number of questions that came up and for my own curiosity I wanted to know the answer. For this particular post, the question I want to look at is: what do we know about changes in the sea level? This question comes up in the context of global warming because melting ice sheets lead to an increase in sea level. My relatives were interested in the doom and gloom reports, but the numbers that caught my eye were the rate of increase since the 1960s:
Global average sea level rose at an average rate of around 1.8 mm per year over 1961 to 2003 and at an average rate of about 3.1 mm per year from 1993 to 2003.
That is a pretty accurate measurement. Go to the ocean and look at the waves and tides. How would you accurately measure the average sea level? Now consider less obvious sources of problems such as evaporation and how much water gets stuck on land vs returned to the oceans from year to year. These days we use measurements from satellites to help improve the accuracy, but what is the tried and true technique for measuring the sea level?

The answer is the tide gauge. Tide gauges are cool because of the simplicity of the basic mechanism. It is essentially a big pipe with a hole below the sea level to allow water in. The pipe protects the water inside from all of the normal disturbances on the surface such as waves. As the name suggests it will still vary with the tides, but it allows fairly accurate measurement of the high and low tides. If you record this for a long enough time you could work out the average sea level for the location of the gauge as well as how this has changed over time. Place enough of these devices around the world and keep track of the measurements and you can figure out the average global sea level and if there are any discrepancies across the world.

The next obvious question is: with such a simple device how much tide history do we have? Humans have had the technology to make such a device for a long time, especially if you allow for manual measurement instead of various automated schemes that seem to have been developed in the mid to late 1800s with Kelvin's tide gauge. The Permanent Service for Mean Sea Level (PSMSL) has a page listing various long term records with the oldest being from Amsterdam starting in the year 1700. They also provide the data with sea level values relative to the NAP (a fixed reference level frequently used in Europe):

From this data set there is a clear rise in mean sea level starting in the 1800s. The PSMSL site also lists data from Stockholm over the period 1774 to 1984. Does it show the same trend? At first glance the answer is no:

What is going on? How could this record be so different? The explanation I found was from a paper Swedish Sea Level Series - A Climate Indicator and the reason is because the land is rising. Specifically this is known as post-glacial rebound (although glacial isostatic adjustment seems to be the preferred term now) and the land is slowly rising after being depressed by the weight of huge ice sheets that covered the region in a previous ice age. To figure out what the sea level has done we need to remove the trend caused by the rising land. Post-glacial rebound can vary from place to place, e.g., see the paper Measuring Postglacial Rebound with GPS and Absolute Gravity that looks at four sites in North America. That doesn't help me for determining the rate for Stockholm Sweden though. I couldn't find a good data set for showing what the rate of the land rise is, and the sources I read suggest it varies over time. The Swedish Sea Level Series paper mentioned earlier shows a trend, but it isn't clear to me what source they used for the rate. Various sources I have seen suggest the rate is less than 1cm per year now, one example is the description of the lake Mälaren. If I assume a rate of 5mm per year and generate a trend the output looks close to what the Swedish report is showing:

There is also a data provided for Liverpool. For this data set there are two versions: an annual MHW and adjusted MHW. For those that are not aware, MHW is the mean high water measurement. The adjustment is supposed to be described in the associated paper, but I could not find a version of the paper for free online so I don't know exactly what was done. I included both in the graph:

Both the annual and adjusted MHW show that the MHW level is rising. The adjusted value at a slower rate than the raw annual reading, in particular for older measurements. A 2008 paper uses these sources to show a trend in mean sea level from 1700 to 2000. The general trend is that mean sea level has been increasing for the last 200 years.

So what does this tell us? The three data sets being checked here are all from Europe and it is pretty clear that the sea level in that region has been rising for the last 200 years. It would be interesting to see what the longer term sea level has looked like. In particular, are there any long term cyclical trends that take place over thousands of years. Such data may exist, I looked at a handful of the top sources that came up when searching for information and as stated in the opening I am not an expert in this field. There are some interesting complications such as the post-glacial rebounding that can make it difficult to discern what is really happening. I should also point out that I did not look at the evidence for what was causing the rise in sea level. Of course, the general explanation is melting ice sheets as well as just the expansion of water due to warmer oceans. At some point I'll have to look at the evidence for the actual temperature changes, but most importantly (to me at least), it looks like climate change will be an interesting topic to explore.

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