Guest post by David Middleton
Featured image borrowed from USGS.
As promised, here is the sequel to “Five Pacific islands vanish from sight as sea levels rise.” The thoroughly ridiculous New Scientist article is based on this paper:
Simon Albert, Javier X Leon, Alistair R Grinham, John A Church, Badin R Gibbes and Colin D Woodroffe. Interactions between sea-level rise and wave exposure on reef island dynamics in the Solomon Islands. , ,
Apart from some obligatory babbling about IPCC sea level rise projections, the paper is actually very good.
At least eleven islands across the northern Solomon Islands have either totally disappeared over recent decades or are currently experiencing severe erosion. However, islands in the more sheltered Roviana area of the southern Solomon Islands did not experience significant coastal recession.
To date, the responses of the islands of Melanesia to sea-level rise have been poorly studied, a recent review of island change studies in the Pacific indicated ‘little evidence of heightened erosion or reduction in island size‘ (McLean and Kench 2015). These islands were located across the Central Pacific in areas with 1–5 mm yr−1 sea-level rise where net accretion occurred on most islands and only small (<1 ha) islands were actively eroding (McLean and Kench 2015).
Relative sea-level rise can also be the result of tectonics, the Solomon Islands are in a particularly tectonically active part of the globe with the convergence of the Pacific Plate, Solomon Arc block and Australian Plate causing localised crustal deformations (Tregoning et al 1998) manifesting as either island subsidence or uplift (Taylor et al 2008). Whilst the Isabel study site is considered to be in a more tectonically benign area, without active volcanoes, the Roviana site experienced an 8.1 megathrust earthquake in 2007 which led to the reef islands of Roviana subsiding by up to 60 cm (Taylor et al 2008). Island subsidence can compound sea-level rise rates and make these tectonically active islands particularly vulnerable under accelerated sea-level rise scenarios. However, the apparent resilience of islands in Roviana to subsidence, despite observed increases in coral cover on adjacent reefs attributed to deeper water (Saunders et al 2015), and only 1 (±1.4) mm yr−1 of vertical tectonic uplift measured in Honiara since 2008 (Jia et al 2015), suggests subsidence is not the primary driver of coastal erosion observed in this study.
The rapidly eroding islands identified in Choiseul and Isabel are all exposed to northerly swell and wind events that at times can be severe (Smithers and Hoeke 2014), with twenty-fold higher wave energy at Isabel compared to Roviana (figure 5). The largest net erosion since 1947 occurred on Sogomou, Hetaheta and Kale Islands on Isabel, causing erosive displacement in a south-south-westerly direction.
Albert et al., 2016 clearly demonstrate that this quote from the New Scientist article is a lie:
Going, going, gone. Five of the Solomon Islands have been swallowed whole by rising sea levels, offering a glimpse into the future of other low-lying nations.
The islands, to the extent that they were islands were exposed to intense erosional forces for decades, if not centuries. They eroded away. Sedimentary geology depends on lots of things eroding away. Without erosion, there wouldn’t be much in the way of clastic deposition (like sandstone).
If “five of the Solomon Islands [had] been swallowed whole by rising sea levels, it would be impossible for “islands in the more sheltered Roviana area of the southern Solomon Islands” to “not experience significant coastal recession.” Secular sea level rise doesn’t care about shelter, waves and storms do. However, Albert et al., 2016 assert that sea level in the Solomon Islands is rising at 7 mm/yr and that this is likely to accelerate. Dr. Albert Parker has previously demonstrated that the 7 mm/yr claim is “cherry picking,” based on tide gauge records from Honiara, Guadalcanal.
The 7 mm/yr claim is based on the time 1994-2014 time frame. This rate is more or less corroborated by the satellite data (1993-2016). The tide gauge data date back to 1974. Since the sunken Solomon Islands began sinking in 1947 (or earlier) and a fair bit of the sinking took place between 1947 and 1960, I decided to look for a sea level reconstruction for that period. I searched NOAA’s paleoclimate database While I didn’t find a reconstruction for the Solomon Islands, I did find one from the west coast of Australia (Zinke et al., 2014).
While Fremantle is a long way from Guadalcanal and technically in a different ocean, the patterns of sea level change since 1993 are very similar.
The Fremantle reconstruction exhibits no secular trend from 1795 to 2009…
If we overlay the data from Table 1 in Albert et al., 2016 onto the Zinke et al., 2014 sea level reconstruction, we can see that the sunken islands of Guadalcanal were sinking without the assistance of a secular rise in sea level…
The instrumental data may exhibit a higher dynamic range of amplitude than the reconstructed data; however sea level hasn’t been doing anything differently over the past 20 years than it was doing over the prior 200 years…
Proxy data always have lower resolution than instrumental data. The Earth is a big low-pass filter and whenever you apply a low-pass filter, you attenuate amplitude. Disregard of this very basic bit of signal processing theory is the foundation of most hockey sticks.
Albert,S., J. X. Leon, A. R. Grinham, J. A. Church, B. R. Gibbes and C. D. Woodroffe (2016), Interactions between sea-level rise and wave exposure on reef island dynamics in the Solomon Islands, Environmental Research Letters 11:054011.
Holgate, Simon J. , Andrew Matthews, Philip L. Woodworth, Lesley J. Rickards, Mark E. Tamisiea, Elizabeth Bradshaw, Peter R. Foden, Kathleen M. Gordon, Svetlana Jevrejeva, and Jeff Pugh (2013) New Data Systems and Products at the Permanent Service for Mean Sea Level. Journal of Coastal Research: Volume 29, Issue 3: pp. 493 – 504. doi:10.2112/JCOASTRES-D-12-00175.1. Permanent Service for Mean Sea Level (PSMSL), 2016, “Tide Gauge Data”, Retrieved 09 May 2016 from http://www.psmsl.org/data/obtaining/. Via NOAA
Nerem, R. S., D. Chambers, C. Choe, and G. T. Mitchum. “Estimating Mean Sea Level Change from the TOPEX and Jason Altimeter Missions.” Marine Geodesy 33, no. 1 supp 1 (2010): 435.
Zinke, J., A. Rountrey, M. Feng, S.P. Xie, D. Dissard, K. Rankenburg, J. Lough, and M.T. McCulloch. 2014. Corals record long-term Leeuwin Current variability including Ningaloo Niño/Niña since 1795. Nature Communications, 5, 3607. doi: 10.1038/ncomms4607