How do we measure Sea Level Change?

In the last post I made some assertions that global mean sea level (GMSL) is rising, but making me ask myself how do we know this? Physically measuring the rate of sea level change (SLC) is not a simple process, and as such there are different methods for doing so:

Tide Gauges

In terms of directly measuring SLC there have been approximately 300 years of coverage in an initially very limited spatial area. Tide gauges were the first form of SLC measurement, and work quite simply by constantly measuring the water level at a fixed geographical point. The first tide gauge was constructed in Amsterdam in 1700, and across Europe most major ports had them by the end of the 18^th Century.

Tide gauge stations with >40 year records. Source

Current global coverage. Source

However as the maps above show, tide gauges have lacked global spatial coverage, with a considerable Northern Hemisphere bias until the last 40 years or so.

Therefore for long term SLC they lack consistency but if you choose the most accurate and consistent gauges can still find a reasonable fit for recent SLC. This new paper from Thompson et al., (2016) shows that even when using the best quality tide gauge records it remains difficult to validate the recent observed sea level trends due to systemic underestimation of melt from 20^th century ice cap loss. This is likely due to local trends at each tide gauge station for example melting of ice caps from different parts of the world produce a variable sea level trend due to differing distribution of the meltwater across the globe. 

Meltwater impact on regional SLC from Greenland (above) and Antarctica (below). Source

These local signals imprint on the global signal and therefore tide gauges records need to be selected very carefully in estimating global SLC. As you can see from some tide gauge data I collated (below), there is much noise in the data but most show an overall trend of slow rise through the 20^th Century. 

Self- collated SLC at sites from across the world for the last 100 years. Source

The attachment of tide gauges to land means vertical land motion is another complicating factor, therefore has to be accounted for when doing reconstructions. This is a complicated process requiring data from models that is being constantly updated and therefore past sea level trends may have been poorly estimated due to issues with vertical land motion. This explains the relative sea level fall at Stockholm. Fingerprinting the difference between local/regional and global sea level trends appears to be crucial in accurately working out the SLC signal. 

Satellite Altimetry

Satellite altimetry is the more recent, more accurate form of measuring SLC. It uses satellites (with the catchy names of JASON and TOPEX…) to measure the height of the sea surface compared to a reference and therefore you can look at its change over time to get GMSL change. 

How satellite altimetry works. Source

However you still need to consider ocean basin volume changes and changes in pressure that lead to differing regional levels. This technology is only approximately 25 years old so gives a shorter term record but will be most used to assess future SLC, considering the revolution it has led to in the understanding of SLC since its inception (Milne et al., 2009). To compare GMSL change for the differing instrumental eras gives an indication of the recent acceleration in GMSL, from tide gauges 1900-2012 it is 1.7mm/yr, from altimetry 1993-2010 3.2mm/yr.  Does this reflect differing accuracy or a clear acceleration is SLC in the late 20^th Century? These questions are critical to estimating future SLC and the potential magnitude of its effect. Altimetry appears limited in measuring local changes and therefore a combination of the two major methods (tide gauges for local, altimetry for global) appears to often be the best for working out recent SLC and looking towards the future.

Another way of measuring SLC is to work out the magnitude of differing contributions to SLC. Therefore the next couple of blogs will look at the major contributions to 20^th Century SLC. Steric changes of ocean water, ice melt, changes in terrestrial water storage and groundwater depletion all contibute to SLC. These will be discussed in the coming posts...