Looking to the past to see the future.

The debate over the future of low lying coral island nations is difficult to resolve without detailed scientific study of land area changes in the vain of Webb and Kench’s (2010) investigation. And even with these studies, they do little to reassure against future more rapid rises in sea level which have been predicted by the IPCC. The above IPCC (2007) figure clearly shows an increasing rate of sea level rise.

By 2100 sea level may have risen by half a meter, such a rapid rise in level is unprecedented in the estimates of recent past and throughout the Instrument record. To assess whether low lying coral islands will be able to cope this high rate the paleohistoric record of coral growth must be analyzed. The island chain of Hawaii is surrounded by many deeply submerged reefs which were once shallow reefs and low lying islands, they have been focused upon by many scientific investigations as these submerged reefs may indicate the future for present day low lying coral islands. One reef in particular looked at by Webster et al. (2004) is now at -150 meters below sea level, and has been radiocarbon dated (calibrated) to have been submerged between 15.2 and 14.7 ka (Moore and Fornari, 1984). It has therefore been suggested by Webster et al. (2004) that this coral island was submerged during the last deglaciation, specifically the first (MWP-1A) of two rapid sea level rises; MWP-1A (14.2–13.8 ka) and MWP-1B (11.5–11.1 ka). The delay of 0.5 ka is due to the lag in time for a the coral structure to be submerged below a critical 30-40 meter depth, once this occurs the type of coral which will grow will shift from Porites coral (reef building) to deep water coralline algal growth (static) (Webster et al., 2004). Only at this layer in preserved coral can it be indicated that the structure was submerged.

 The MWP-1A and MWP-1B periods are parts of the Holocene transgression where it has been estimated that sea level rose on average of 14 mm/year between 15 ka and 6 ka (Bloom, 1971; Chappell, 1974; Adey, 1978), however during short periods of the transgression such as MWP-1A this rate of sea level rise reached 40-50mm/year (Peltier, 2002). Webster et al. (2004) then goes further, referencing studies which suggest that coral reefs can not accrete at rates faster than 10-20 mm/year (Neumann and Macintyre, 1985; Montaggioni et al., 1997), and therefore widely indicating that during very rapid sea level rise events such as MWP-1A coral structures are at risk of becoming submerged.

This has wide ranging implications on the near term future of low lying coral islands and there supporting reef ecosystems which are critical to the well-being of coral island inhabitants (Pernetta, 1992). More specifically, an emissions scenario based projection of sea level rise is detailed in the IPCC Summary for Policymakers (2007) and is shown below:

(all above ranges are in cm)

 If the worst case of the A1F1 emissions scenario, which represents a more global, economically focused future, heavily dependent on fossil fuels is correct and by 2099 sea level is at 59 cm above today then the rate will still be only around 5.5-6mm per year, way below Peltier’s (2020) MWP-1A rise and even far below the max accretion rates of 10-20mm/year suggested by Neumann and Macintyre (1985) and Montaggioni et al. (1997).

This would appear to stamp paleohistories mark upon the current sea level rises as ‘safe’ however the IPCC widely admits that the certainty of its models is low, clearly including in the above scenario table ‘Model-based range excluding future rapid dynamical changes in ice flow’, therefore rates could rise at higher rates than 6mm/year. Furthermore the security of an island nation should be longer than to 2100. Future posts will attempt to assess the security of these nations to the next millennium and detail the role underlying tectonic activity on the pacific islands.

References:

Adey, W. 1978. Coral reef morphogenesis: A multidimensional model. Science. 202. 831-837.

Bloom, A. 1971. Glacial-eustatic and isostatic controls of sea level since the last glaciation. In: Turekian K (ed) The late cenozoic glacial ages. Yale University Press. 355-379.

Chappell, J. 1974. relationship between sea levels, oxygen 18 variations and orbital perturbations during the last 250,000 years. Nature. 252. 199-202.
Montaggioni, L.F., Cabioch, G., Camoin, G.F., Bard, E., Faure, G., Dejardin, P., and Recy, J., 1997. A 14,000 year continuous record of reef growth in a mid-Pacific island: Geology.  25. 555–559.

Moore, J.G., and Fornari, D.J., 1984. Drowned reefs as indicators of the rate of subsidence of the Island of Hawaii: Geology. 92. 752–759.

Neumann, A.C., and Macintyre, I.G., 1985. Reef response to sea level rise: Keep-up, catch-up or give-up: Fifth International Coral Reef Congress, Tahiti, Proceedings. 3. 105–109.

Pernetta, J.C. 1992. Impacts of Climate Change and Sea -Level Rise on Small Islands States: National and International Responses.

Webster, J.M., Clague, A.D., Riker-Coleman, K., Gallup, C., Braga, J.C., Potts, D., Moore, J.G., Winterer, E.L., Paull, C.K. 2004. Drowning of the 150 m reef off Hawaii: A casualty of Global meltwater pulse 1A? Geology. 32 (3). 249-252.