365 Days of Climate Awareness 208 – Coral Islands and Atolls vs. Sea Level Rise

Virtually all coral islands and atolls are at severe risk from rising ocean levels. Most are less than 2m above mean sea level, and many, such as in the Marshall and Solomon Islands now, are inundated at especially high tides (“king” tides, which are spring tides, under a perigean moon—that is, when the moon is at or near perigee, its closest point to earth to the earth) and during storms. Global sea level is predicted, with glacial melt and warming water, to rise anywhere from 0.3 to well over one meter. Any rise of close to 1 m will render most coralline islands uninhabitable, since high tides and storms will regularly inundate them, likely destroying the delicate terrestrial ecosystems there and destabilizing what sediment there is.

Various IPCC sea level rise scenarios through 2100.

The threat of sea level rise to low-lying atolls and coral islands has already prompted many in the Marshall Islands and elsewhere to emigrate to islands or continents not in imminent danger. It has led some, such as the nation of Kiribati, to propose bringing in sediment to build the islands up, by dredging the lagoons.

One of the Solomon Islands at high tide.

Some scientists hold that increasing storm activity in the oceans will deposit more detritus (loose sediment) on the islands than before, overcoming sea level rise and leading to an increase in most coral islands’ and atolls’ size. Studies of some Pacific atolls have shown varying responses to rising ocean levels, with some islands shrinking and others growing larger. This phenomenon is known on barrier (sand) islands along the US coast, where shoreline transport by waves erodes sand from some island locations and deposits it elsewhere. Atolls in the deep ocean are not part of a linked sediment transport system like that, but accretion in some islands has been observed. More widespread evidence, however, is clear that higher sea levels will make human habitation on many of these low-lying islands dangerous or impossible.

Roi-Namur Atoll, Marshall Islands.

Global warming affects temperature and rain patterns in the tropics, as elsewhere—possible changes in ENSO are one huge potential example—but the differences are far more pronounced in the higher latitudes, both north and south. Heat transport from the equator toward the poles has increased as the world has warmed, resulting in much greater relative differences in the mid- to high latitudes, especially in the north. In the tropics and near tropics, and especially on the islands, the main threat from global warming lies in sea level rise. The next few posts will focus on particular examples of this throughout Oceania.

Tomorrow: proposed raising of Kiribati.

Be brave, and be well.