Threat posed to islands in the Western Pacific

If you were to type into Google, ‘sea level threat’ it asks whether you want to search for ‘sea level threat in Tuvalu’ as the first option in a drop-down list. Why is this? Similarly, if you were asked to name a place that you thought was at risk of rising sea levels you might well name an island, such as Tuvalu or Kiribati, in the Western Pacific. Increasingly the media has focused on these islands where it is claimed climate change is real and impacting heavily on peoples’ lives.

Figure 1: Western Pacific Islands

The region, containing a number of small, low-lying, and densely populated islands in the tropical Western Pacific is often thought of as one of the most vulnerable places in the world to future changes in sea level (Nicholls and Cazenave 2010). Part of the problem is that average land elevation on this islands rarely exceeds 3 metres above sea level. Nicholls et al. (2007) argue that a combination of natural stresses and human activities could combine with sea level rise to pose an even greater threat to life on these volcanic archipelagos made up of islands and atolls. This includes ground movement caused by tectonic activity and volcanism, extreme climatic events such as storm surges, subsistence as a result of water or oil extraction, and increased pressure on the land due to urbanisation.

While average global sea level measured from tidal gauges since 1950 pointed to an increase of around 1.7 mm/year, my previous blog highlighted two important factors that need consideration (Church and White 2006). Firstly, new satellite altimeter readings have found that this rate is closer to 3.3 mm/year for the period since 1993 (Ablain et al. 2006). In addition to this, sea level is not uniform around the world meaning global averages hide extreme rises (and falls). A further issue is that of timescale as measurements can be obscured by annual or even seasonal fluctuations. Satellite altimeter measurements of the western tropical Pacific region have shown an increase of about 3 or 4 times the global average between 1993 and 2010 but this must be taken with caution due to the relatively short time period examined (Becker et al. 2012).

Becker et al. (2012) and Church et al. (2006) have both investigated changes in sea level in the Pacific Island region which meant overcoming problematic data sets and using a number of different types of measurement data, as well as removing factors that caused short-term fluctuations. While tidal gauge records go back much further in time than satellite observations, they contain gaps and also lack the accuracy of satellite records as they are fixed to the seabed thus relying on it not rising or sinking. To counter this effect, GPS data was used to account for vertical land movement making way for sea level change reconstructions to be formed. This found that ‘ground subsidence increases the [total] climate-related sea level rise by about 10%’ (Becker et al. 2012: 97).

Figure 2. Becker et al. 2012 (Funafuti is an island of Tuvalu)

A further consideration that I have touched upon is that of seasonal or annual variation. This is especially relevant for the Pacific as it is this region that is heavily influenced by El Niño-Southern Oscillation (ENSO) events. This is because La Niña causes a tilting of the thermocline which results in water piling up in the tropical western Pacific while El Niño reduces this tilt meaning water levels out over the Pacific (Church et al. 2006). Consequently, during La Niña, sea levels around Tuvalu rise and in an El Niño phase they fall. The magnitude of sea level variability due to ENSO events was found to be around 20-30 cm above or below average levels in the Tuvalu region (Becker et al. 2012). They therefore have an enormous impact on inter-annual variability as it results in between 40-60 times the annual sea level rise here.

There are two key reasons why ENSO effects were examined. Firstly to explore the magnitude of change caused by them and secondly, to explore how much sea level is altering in this particular region when short-term fluctuations are removed. When this is done, and new models of combined data have been used to reconstruct sea level models, long-term changes can be calculated. Becker et al. (2012) found that sea level rose at a rate of 5.1 (±0.7) mm/year during the period 1950-2009 at Tuvalu. This is a gigantic rise and is exactly 3 times as large as the global average figure mentioned above. Thus it is clear to see now why there is so much media coverage of the impacts of climate change impacting on these low-lying islands and their people. Future posts will look at their perspectives and responses to these changes.


References

Ablain, M., A. Cazenave, G. Valladeau and S. Guinehut (2009) ‘A new assessment of the error budget of global mean sea level rate estimated by satellite altimetry over 1993–2008’, Ocean Science, 5, 193-201.

Becker, M., B. Meyssignac, C. Letetrel, W. Llovel, A. Cazenave and T. Delcroix (2012) ‘Sea level variations at tropical Pacific islands since 1950’, Global and Planetary Change, 80–81, 85–98.

Church, J. A. and N. J. White (2006) ‘A 20th century acceleration in global sea-level rise’, Geophysical Research Letters, 33.

Church, J. A., N. J. White and J. R. Hunter (2006) ‘Sea-level rise at tropical Pacific and Indian Ocean islands’, Global and Planetary Change, 53, 155–168.

Nicholls, R. J. and A. Cazenave (2010) ‘Sea-level rise and its impact on coastal zones’, Science, 328, 5985, 1517-1520.

Nicholls, R. J., P. P. Wong, V. R. Burkett, J. O. Codignotto, J. E. Hay, R. F. McLean, S. Ragoonaden and C. D. Woodroffe (2007) ‘Coastal systems and low-lying areas’, Climate Change 2007: impacts, adaptation and vulnerability. Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 315–356.