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.