Saturday, 7 July 2012

Extending the record - Cook Islands, Rarotonga

There's been a lot of discussion over the past few years about the so-called "vanishing islands" in the central and western Pacific. Much of is nonsense, on the one hand alarmists claiming the populations are doomed, that many islanders have relocated, are relocating, or will relocate anytime soon (take your pick, none of these claims are ever substantiated), on the other hand some AGW sceptics claim (without proof, there is none) that sea levels there haven't risen in the past, and by implication or statement, that they therefore won't rise much, if at all, in the future.

There's also a more sanguine, realistic, and balanced view of the situation, from those who actually scrutinise the data, and take the longer view. I'm one such who believes that data is truth, analysis is interpretation, and that the "longer view" is aided by longer datasets. The Australian National Tidal Centre's "South Pacific Sea Level and Climate Monitoring Project" does a great job measuring and archiving sea level and climate data for many Pacific islands - their data forms the foundation of my South Pacific Sea Level reference page (sidebar top). However, their record is relatively short, and can give an inflated view of sea level rise in many cases. For example, the chart for the southern Cook Islands island of Rarotonga shows a rate of 5.25 mm/year rise to end 2011. Many of the NTC gauges on the Pacific islands were completely new installations, but the one on Rarotonga replaced an earlier gauge which recorded from 1977. The NTC doesn''t archive data for the older gauge, but PSMSL does, along with a slightly out-of-date record (end 2011) for the new gauge (updated to April 2012 from NTC data), and here are both plotted together:

The trend lines show that short-term data can give very different impressions of trends, even more emphasised when the slightly adjusted (-40 mm) earlier data is spliced onto the start of the later:

 .... when a much lower trend than either emerges. I've another reconstruction under construction (ouch!) which will show an even more interesting past (and a possibly less than interesting future, that's a good thing!) for another island in the area.

Data sources:
Rarotonga 1977-2001
Rarotonga B 1993-2011
Rarotonga 1993-2011 (NTC)

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