Geology Lesson for the Friends of Science (‘cause we love them)
The Friends of Science submission on the Government of Canada’s CO2 reduction plan, which we featured a couple of weeks back, contains - among the usual lies, misconceptions, zombie arguments and irrelevancies - this statement on the effects of sea-level rise on atoll islands (Page 15):
“The islands are mostly comprised of coral debris eroded from encircling reefs that is pushed up onto the islands by winds and waves. As the sea level rises, more sand and coral debris accumulates on the islands with the result that the islands rise with the sea level. The coral itself continues to grow upwards to match the sea level rise. Island atolls can grow much faster than recent rates of sea level rise. Sea level rise does not endanger low-lying coral islands.”
In support of this model of atoll island evolution they cited recent work by Webb & Kench, who found that of 27 islands on four different atolls they examined in air photos and satellite images covering periods of 19 to 61 years, only four had decreased in area while 11 had increased and the rest had experienced no significant change either way. This is an interesting result, but to see if it really does support our Friends’ contention (or even Webb & Kench’s own more cautious conclusion that it “suggests that reef islands are geomorphically resilient landforms") we need to look more closely at the origin and construction of atoll islands and their histories over a longer timescale.
Coral atolls owe their present form to the postglacial rise in sea level. They originally developed as thickening accumulations of coral on extinct marine volcanoes that slowly sank due to thermal subsidence of their underlying lithosphere. During the last glaciation when sea level was c. 120 m below present, and during earlier glaciations, these accumulations formed emergent limestone plateaus. Solution and erosion resulting from this emergence gave the plateaus saucer-shaped plans, with centres a few tens of metres lower than high-standing ‘ramparts’ around their edges (e.g., Purdy & Winterer, 2001). When rising postglacial seas overtopped these plateaus 8000-9000 years ago, coral again began to grow on them and their inherited saucer-shaped plans were maintained, the lagoons, despite accumulating coral, remaining up to a few tens of m below the rims.
Growth of coral on the atoll rims was maintained until the ‘mid-Holocene highstand’ of 4000-2000 years ago, when sea level in the atoll province reached an elevation 1~2.5 m higher than at present, subsequently declining due to slow isostatic adjustments of the crust to the redistribution of the load of the melted ice (e.g., Mitrovica & Milne, 2002; Peltier, 2002; Dickinson, 2009). The paleoreef flats of cemented coral, resistant to wave attack, which remained above the now-lower sea-level then formed the stable cores for accumulations of unconsolidated surficial sediment emplaced by storm washover to create what Dickinson refers to as ‘pinned islets’. Short-timescale changes in wind and wave action can be expected to redistribute sediment around these in minor ways; but around transient sand cays not underlain by emergent paleoreef material (‘unpinned islets&rsquo, more extreme morphologic changes are not unlikely. Dickinson (2009) and others have pointed out that Pacific atolls only became inhabited after sea-level had stabilized below the mid-Holocene highstand, i.e., after ~2000 years ago. The majority of atoll islets with permanent populations are pinned islets.
The important date for the stability of pinned islets will be when high-tide level approaches the elevation of the top of the emergent paleo-reef flat, leaving the unconsolidated sediment above exposed to wave attack during storms and times of high sea-level during, e.g., the warm-water phases of ENSO cycles. Growth of coral keeping pace with sea-level rise will not counter this, as coral of course only grows underwater – islets currently pinned to emergent paleoreef remnants will not regain their foundations. Dickinson (2009) discusses the range of times at which this is predicted to happen under current sea-level rise projections. His Table 1 shows ranges of 50-150 years.
Webb & Kench did not distinguish between pinned and unpinned islets, but their database includes examples of both and it would seem important to compare the short-term behaviour of the two types and to not only consider change in islet area but also change in shape and position. As they point out (Section 5.2):
“The net changes in island area mask larger gross changes in island planform configuration and location on the reef platform that have occurred over the time periods of analysis. For example, on Fualifeke in Funafuti (Fig. 5B), the eastern half of the island has migrated south indicating up to 30% of island materials have been reworked over the 19 yr window of analysis.”
Fualifeke is an unpinned islet (see Fig. 1 of Dickinson, 2009) and not surprisingly is unpopulated. Even though its net area has decreased only slightly over the study period, people would doubtless have found it inconvenient to have had 30% of their island shift around under their feet in less than 20 years. The five islets with the largest % areal increases in the Webb & Kench study (see their Table 2), Funamanu and Falefatu on Funafuti atoll (Tuvalu) and Betio, Bairiki and Nanikai on Tarawa atoll (Kiribati) are pinned islets, as are Amatuku (+4.6%), Fatato (+8.6%) and Mateiko (+6.1%) (all on Funafuti). The only unpinned islet in their study to have experienced an areal increase greater than 3% (Webb & Kench’s nominal limit for ‘significant’ change) is Paava (Funafuti) (+10%). This is on the leeward side of the atoll, well placed to accumulate sediment on its oceanward side as observed, and at just 1.48 ha initial area is the third smallest of the 27 examined.
It might be tempting to conclude from this that pinned and unpinned islets behave differently, pinned ones predominantly increasing in area; but the significance of the changes to the Tarawa group must be questioned, as they are heavily populated and have undergone considerable anthropogenic modification including reclamation. It is more informative to focus on the unpinned islets, as this is what the pinned ones will evolve into in the coming decades. Webb & Kench’s Table 2 shows similar numbers of small positive and negative changes (neglecting one large negative, -14.7% to tiny (0.68 ha) Tengasu on Funafuti) to these islets over the 20-yr timespan of the imagery examined (their Table 1).
From all of this, a number of conclusions seem evident:
- There are islets and islets; pinned and unpinned.
- Any study of the morphologic evolution of islets should distinguish between the two types.
- The present elevation of atoll islets is due to a drop in sea-level after the mid-Holocene highstand, not to coral growth during rising sea-level.
- The larger, populated, pinned islets are underlain by an erosion-resistant core of paleoreef limestone created by coral growth before the post mid-Holocene sea-level drop.
- Pinned islets will become unpinned in coming decades as rising sea-level overtops their paleoreef cores; then they will behave like unpinned islets.
- Current data indicate both increases and decreases in areas of unpinned islets with increasing sea-level and background natural changes, but the number of observations is small and their timescale is short; and in some cases, small net area changes belie significant morphological changes due to large-scale sediment reworking.
- There is nothing in recent or earlier studies to indicate that extant atoll islets will grow vertically to keep pace with rising sea-levels after their paleoreef cores are overtopped.
- As usual, the Friends of Science are full of shit.