The following is an excerpt from an unpublished thesis paper on “Atoll Habitability Thresholds” by M. Stege, Columbia University M.A. in
Climate and Society Program Class of 2014. Above image courtesy Schmidt 2010.
An atoll is a ring of reef surrounding a central lagoon.
There are often islands or sand cays interspersed within the reef. Sand cays and vegetated islets are all that
remain visible above water level during high, tide, and represent the only
potentially habitable areas on an atoll. It was Charles Darwin who, while observing
these unique landforms near the western coast of South America, first theorized
how tiny polyps could form barrier reefs grown atop subsiding volcanic edifices
(Darwin 1842: 109). Since then reef core and radiocarbon evidence from multiple
atoll sites have confirmed Darwin’s theory, and expanded on the important role
that sea-level fluctuations due to tectonic- and glacial-controlled forces have
played in reef development. (Woodroffe and Webber 2014: 193) In other words,
atolls are barrier reefs grown atop both
subsiding volcanic edifices dating back tens of millions of years and episodic coral reef growth and
discontinuity relative to sea level along millennial timescales. Note that This
discussion of atolls does not address the handful of ‘emergent atolls’ and
islands that have been lifted above sea level by tectonic forces. Only
‘subsiding atolls’ are discussed.
The last episode of coral reef growth and discontinuity (Figure
3, inset B) is uniformly evidenced among atolls starting around 8,000-10,000 BP
with the inundation and submergence of present day atoll platforms due to sea
level rise during the Last Interglacial. Coral growth appears to have lagged
behind rising sea level during this time, until eustatic sea level stabilized
at the ‘mid-Holocene highstand’ around 5,000-6,000 BP (Woodroffe and Webber
2014: 241-243). A period of explosive ‘catch up’ growth followed, during which
different reef structures ‘caught up’ to sea level at different times and where
margins of the atoll more exposed to wind-driven seawater nutrients generally
grew fastest. Sea levels remained stable at the mid-Holocene highstand between
3,000-5,000 BP, a period marked in atolls today by the development of
conglomerate platforms which formed after vigorous reef growth had ceased and
lateral reef consolidation flourished. (Dickinson 2009: 4-8) These conglomerate
platforms are the fossilization of reef, sediment, and boulder deposits, all
cemented together to form a durable bedrock on which many of an atoll’s sand
cays and islets are today “pinned.” (Dickinson 2009: 7) Excluding major storms these
foundations make those pinned atolls resistant to wave damage, and therefore
stable over the last few thousand years. As would be expected, these pinned
islands are consistently most prevalent on the windward margins of an
individual atoll. In lieu of projected sea level rise,
Dickinson refers to ‘crossover dates’ when these conglomerate platforms start
to become largely awash, as they were during the mid-Holocene highstand.
Looking towards the future, these conglomerate
foundations that underpin the stable Pacific atolls are predicted to be become
largely swept away by rising seas at some ‘crossover’ point, leading to the
atolls being vulnerable to fair-weather wave attack. (Dickinson 2009: 7) When
these crossover points occur, atoll erosion processes will become greatly accelerated,
signaling a return to the atoll’s previously submerged and uninhabitable state
a number of millennia ago. It is worth noting that around 2,000-3,000 BP a
gradual ‘post-mid-Holocene sea level decline’ of about 0.8 meters occurred This
was caused by oceanic siphoning wherein seawater was required to infill
collapsed submarine arches and deepening continental shelves created by
geological glacial mass loss (Miltrovica and Peltier 1991, 2002). Near consensus within
the scientific community further holds with “very high confidence” that by the
end of this century global sea levels will have risen by at least the observed global average since 1900 of about 0.24
meters plus ocean thermal expansion so closer to 0.62 meters (IPCC 2013). This is a conservative estimate of sea level rise which
excludes escalations from glacial and ice sheet melt, which would raise the
estimate to 1.2 meters (IPCC 2013), and even higher beyond the 21st
century. (Joughin et al 2014, Rignot et al 2014) All this on top of several
meters of natural sea level variability due to fair-weather ocean dynamics leaves little doubt that coral atolls will ultimately become
uninhabitable due to sea level rise.
These “irrefutable, as well as shocking” realities of
climate change, as pronounced by US Secretary of State John Kerry in his
address to the 2013 Pacific Islands Forum, necessitate a coordinated and
scalable response.
