Do Darwin points and tectonics hold the answer?

To better asses the future of coral islands within the context of sea level rise an understanding of the mechanics of coral island development is necessary. Charles Darwin although famous for his work on evolution during the 1836 Beagle voyage, also solved the puzzle of atoll formation. R.W.Grigg (1982) references a note Darwin wrote about Tahiti and Moorea “Hence if we imagine such an Island after long successive intervals, to subside a few feet in a manner similar but with a movement opposite to the continent of S.  America; the coral would be continued upwards, rising from the foundation of the encircling reef. In time, the central land would sink beneath the level of the sea and disappear but the coral would have completed its circular wall. Should we not then have a coral Island? Under this view we must look at a Lagoon Island as a monument raised by myriads of tiny architects to mark the spot where a former land lies buried in the depths of the ocean.”

Figure 1 below visualizes this process. Source: The Australian.

However the investigation of academic material by this blog has clearly shown that it is possible for there to be stages after (3.) in Figure 1 above, whereby the Atoll becomes submerged. This submergence has become apart of the de facto rhetoric of the environmentalist movement, suggesting that sea level rise caused by global warming is the sole answer to all atoll submergence. However Grigg (1982) suggested another reason for this submergence is due to underlying tectonics, he studied the Hawaiian Archipelago to test his assertion of the existence of a ‘Darwin Point’. A Darwin point is a latitudinal boundary at which due to the subduction of the underlying plate and unfavorable environmental conditions for Coral to continue to calcify coral islands become submerged. Grigg (1982) detailed the Darwin point in the following sketch shown below in Figure 2.

For the Hawaiian Archipelago Grigg (1982) put the point at 28 degrees North as his results had suggested that at this point calcification rates were not high enough to cope with isostatic and eustatic sea level rise.

Grigg (1982) goes further, suggesting that these Darwin points will occur at other latitudes in different oceans and geographic locations depending on regional climate and tectonic pasts. This could therefore account for a proportion of the 14% of Pacific islands found to have lost land area over recent decades by Webb and Kench (2010). The argument for a large scale loss of low lying coral islands due specifically to global warming induced sea level rise without a role of other mechanisms seems to have been undermined yet again by examination of the scientific evidence.

However to some extent it still appears that human induced sea level rise has a role to play in the submergence of coral islands. By increasing the rate of sea level rise this ‘Darwin Point’ could move progressively south, threatening more coral islands, sooner than otherwise would of been the case. Therefore in the next post, i will attempt to make informed suggestions as to the actions these threatened human populations should take to  mitigate negative impacts.

References:

Grigg, R.W. 1982. Darwin Point: A Thershold for Atoll Formation. Coral Reefs. 1. 29-34.

Webb and Kench. 2010. doi: 10.1016/j.gloplacha.2010.05.003

Security into the next millennia?

Last weeks blog post through analysis of paleohistoric records stamped a ‘safe’ label on the current and near term predicted rates of sea level rise to 2100. Even under the most extreme future sea level rise scenarios rates were 4-14 mm/year under the 10-20mm/year possible accretion rates supported by Neumann and Macintyre (1985) and Montaggioni et al. (1997). However the security of an island nation should be analyzed further than 90 years into the future if its economy is to be able to draw investment from overseas and from its own people. Gillet et al. 2011 produced a paper based around computer simulations of global warming to the year 3000, thereby allowing the future of low lying island nations to be assessed.

In this paper one key and possible scenario is focused upon, that being a complete halt of carbon dioxide emissions from the year 2100 onwards to the year 3000 and its effects on the Earth to this date (ZE2100). The other is an unrealistic scenario where carbon dioxide emissions halted in 2010 to 3000 (ZE2010). The following figure (1) has been taken from this study showing the movements of this gas between 2000 and 3000:

From the graph (b) after carbon dioxide levels rapidly drop for a hundred years they only slowly fall down to around 550ppm by the year 3000, meaning that 55% of the pre industrial rise in carbon dioxide will still be present in the earths atmosphere by the year 3000. The impacts of this carbon dioxide are simulated by a high resolution 3rd Generation Atmospheric General Circulation Model (AGCM3) and and advanced Ocean General Circulation Model (OGCM3.5). The simulation for the ZE2100 scenario showed that the thermal expansion of water would cause sea levels to rise by 1 meter by 3000 (as shown below in figure 2), and the collapse of the West Antarctic Ice Shelf would contribute a rise in sea level of 3-4 meters. The following figure details the climatic response to carbon dioxide levels to the year 3000:

The above warming is a result of carbon dioxide levels and as has been previously shown directly effects sea level.

Therefore a cumulative rise in sea levels to the year 3000 could be 5 meters, this would equate to a rate of just over 5mm/year rise averaged out from 2011-3000. Below the average predicted by the IPCC to 2100 of 6mm/year under the worst case scenario, and again below the 10-20mm/year rates of accretion possible by coral islands set by Neumann and Macintyre (1985) and Montaggioni et al. (1997). This does not thought give a ‘safe’ stamp to these islands to the year 3000, as we have seen with the Holocene transgression where rates averaged 14mm/year at certain times such as the Bolling-allerod MWP-1A rates can reach 40/50mm/year (Peltier, 2002) causing low lying coral islands to be submerged. If similar rapid meltwater pulses occurred between now and the year 3000, rates may once again outstrip accretion rates. Furthermore it may be unrealistic to expect the world to have transferred to a completely carbon free economy by 2100, causing carbon dioxide to continue to be released, and ultimately increasing predicted rates of sea level rise to the year 3000.

The findings of this post and last weeks post would appear to undermine those who suggest low lying coral islands are under a current threat from eustatic sea level rise. The next post the role of underlying tectonics and isostatic sea level rise will be analyzed to see if it can answer any of the questions remaining surrounding this debate, namely ‘why are a limited number of islands being submerged?’.

References:

Gillett, N.P., Arora, V.K., Zickfeld, K., Marshal, S.J., Merryfield, W.J. 2011. Ongoing climate change following a complete cessation of carbon dioxide emissions. Nature Geoscience. DOI: 10.1038/NGEO1047

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.

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.