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

Tuva-who?

So my last blog was just a brief introduction to the overall topic. I’m now going to talk more about the dangers faced by low-lying island nations. The IPCC (Intergovernmental Panel on Climate Change) stated that sea level rise will affect both coastal ecosystems and economies, and even lead to shifts in islands’ species compositions as a whole. Island nations are at high risk as they, obviously, are completely surrounded by water, meaning a larger proportion of their populations live near a coast. Much of their livelihoods may also revolve around marine-based activities, so sea level rise would affect their economies negatively. Damage to vital resources could also occur, for example contamination of fresh water supplies.

Some of the most vulnerable islands that could potentially inundate if sea level rises are: 

· - The Marshall Islands and Kiribati (two thirds of these islands could submerge if sea level rises by 80cm)
· - The Maldives (85% of the capital could submerge if sea level rises 90cm)
· - Tuvalu, Tonga, The Federated States of Micronesia to name a few, if sea level rises by over a metre

Click here to view a small, interesting article (with pictures!) about which nations face the most threat and what they are trying to do. Many of the islands look like complete paradise - blue skies, clear waters, soft sand... just what you need really. But looks can be deceiving, and sea level rise and climate change will change all this in a few decades, it seems. 

A consideration for some of these island nations is whether they have the capability to cope with the upcoming sea level rise – for e.g. some may not have enough physical space, or higher ground for their inhabitants to retreat to if required. 

Tuvalu, for example, can already feel the effects of sea level rise, with regular inundation of areas of its nine islands every year. The highest point in the country is just 5 metres, the average height overall being 2 metres. 

This article from March earlier this year, tells us how Kiribati may need to buy Fijian land for some of its inhabitants to migrate to as sea levels rise, and the same issue is being faced by other similar countries. 

The video clip below is from the UN, highlighting the effects of sea level rise on Tuvalu and Kiribati, and shows the president of Kiribati’s viewpoint on the matter: 

I shall end this post with some food for thought - yet another concern that emerges from sea level rise, is that of sovereignty, identity, and loss of culture. Losing their land will mean losing their national resources, and losing a place the islanders can essentially call ‘home’. So where can these people actually go? And what will become of their cultures and languages in the future? The past has questioned climate change as a possible cause for the complete collapse of certain civilizations (for example, those of Easter Island). Could this be seen as something similar? Or will the current islanders be able to save themselves before it's too late?