Rise or Fall?

How is the sea level actually measured?

Sea level is measured in two ways –

(1) relative to the ocean floor (“relative sea level”) – tide gauges

(2) relative to Earth’s centre of mass (“absolute sea level”) – satellite altimetry

Over the 20^th century, sea level was primarily measured using tide gauge observations taken along coastlines and islands. Older data is slightly incomplete due to the uneven global distribution of tide gauges. They can also be affected by ground motions (tectonic, volcanic, land movement, etc), therefore leading to some inaccuracies. But technology today has progressed. Since the early 1990s, the introduction of satellite altimetry has revolutionised this field. For data from previous centuries/millennia, sea level has been deduced from proxy records. 

The past two millennia have, overall, seen a relatively constant sea level, with no major fluctuations as such. In fact, over the last almost 6000 years, mean sea level has risen at the almost unnoticeable rate of 1.4 millimeters per year (6 inches per century).  It was only around the early 19th century that a greater rise in mean sea level became apparent – this corresponds with the progression of the industrial era.

Satellite altimetry has been used to map regional variations in sea level throughout the world. For example, check out this map below, showing sea level trends from 1993 to 2009:

As you can see, in some areas (e.g. the western Pacific), the rates of sea level rise have been much higher than the global mean rate (3mm) over this time period, whereas in other areas (e.g. the eastern Pacific), the opposite has occurred and rates are slower than the global mean, or even negative. Most studies attribute this to non-uniform warming of the ocean waters.

The 2007 IPCC (International Panel on Climate Change) report predicted the global sea level to rise anywhere between 18cm-59cm from the 1990s to the 2090s. However, I think this range is broad and therefore quite ambiguous. Less optimistic studies have predicted the sea level to rise by 1m or more in the 21^st century, while some just say sea level rising to destructive levels is all a myth. The average annual rise has been around 3mm since 1990; yet this figure is just that – an average. Therefore we cannot really say what is going to happen in the coming years. Sea levels may be falling in some regions, but increasing in others. It is hard to know what exactly the future holds for us, and which parts of the world will be affected the most and the least. Guess we will just have to wait and see...?

References:

- Cazenave, A (2010) Contemporary Sea Level Rise, Annual Review of Marine Science, 2: p145-173

- Meyssignac, B et al (2012) Sea level: A review of present-day and recent-past changes and variability, Journal of Geodynamics, 58: p96-109

- Milne, G et al (2009) Identifying the causes of sea-level change, Nature Geoscience, 2: p471 - 478

- http://theresilientearth.com/?q=content/sea-level-shenanigans