Are Modern Rates Of Sea Level Rise
Too Slow For Optimal Coral Growth?
Since the 20th century began, global sea levels have been rising at rates of about 1.7 – 1.8 mm/year, or about 0.17 to 0.18 of a meter (~7 inches) per century.
“Our estimated rates for the northern Mediterranean, a relatively small regional sea, are slightly lower than the global mean rate, + 1.7 ± 0.2 mm/year, recently published in the IPCC AR5 (Intergovernmental Panel on Climate Change 5th Assessment Report) … Our regional results, however, are in close agreement with the global mean rate, + 1.2 mm/year, published by Hay et al. (2015) which is currently being discussed by the oceanographic community.”
“From our reconstruction, we found that the Arctic mean sea level trend is around 1.5 mm +/- 0.3 mm/y for the period 1950 to 2010, between 68ºN and 82ºN. This value is in good agreement with the global mean trend of 1.8 +/- 0.3 mm/y over the same period as found by Church and White (2004).”
“Sea level change in the Indian Ocean is about 1.5 mm/year in the past sixty years or so, whereas the global sea level trends are a bit higher [1.7 mm/year].”
“Global averaged sea-level rise is estimated at about 1.7 ± 0.2 mm year−1 (Rhein et al. 2013), however, this global average rise ignores any local land movements. Church et al. (2006) and J. A. Church (2016; personal communication) suggest a long-term average rate of relative (ocean relative to land) sea-level rise of ∼1.3 mm year.”
“Global mean sea level change since 1900 is found to be 1.77 ± 0.38 mm year on average. … [T]he acceleration found for the global mean, +0.0042 ± 0.0092 mm year, is not significant“
In contrast, during the middle Holocene, sea levels rose at rates of 9.6 mm/yr (0.96 of a meter per century) during the 350 years between 6,850 to 6,500 years ago (Meltzner et al., 2017), and relative sea levels (RSL) were about 1 to 2 meters higher than present during that time. During the Early Holocene (~12,000 to 8,000 years ago), sea levels rose at rates of about 0.74 of a meter to to almost 1.1 meter per century (7.4 mm/yr to 10.9 mm/yr), which is about 5 to 6 times the modern rate (Khan et al., 2017).
Corals, thought to be biologically fragile and highly susceptible to abrupt sea level changes and high sea temperatures…survived these much higher rates of sea level rise from the geological past.
Scientists have apparently found that coral communities do not grow as well, but instead they “shut down” — even reaching very high mortality rates (85%) — when sea levels fall rapidly. Falling sea levels (and cooling) are suggested to be more lethal to corals than high-temperature bleaching events during El Niño years or rising sea levels (Eghbert et al., 2017).
These findings would not appear to support the current perspective that modern coral communities are threatened by “global” warming and rapidly rising sea levels.
Recent Rapid Sea Level Fall Induced Higher Coral Mortality Than Bleaching
“In September 2015, altimetry data show that sea level was at its lowest in the past 12 years [Indonesia], affecting corals living in the bathymetric range exposed to unusual emersion. In March 2016, Bunaken Island (North Sulawesi) displayed up to 85% mortality on reef flats dominated by Porites, Heliopora and Goniastrea corals with differential mortality rates by coral genus.”
“[R]apid sea level fall could be more important in the dynamics and resilience of Indonesian reef flat communities than previously thought. This study reports coral mortality in Indonesia after an El Niño-induced sea level fall. The fact that sea level fall, or extremely low tides, induces coral mortality is not new, but this study demonstrates that through rapid sea level fall, the 2015–2016 El Niño has impacted Indonesian shallow coral reefs well before high sea surface temperature could trigger any coral bleaching. Sea level fall appears as a major mortality factor for Bunaken Island in North Sulawesi, and altimetry suggests similar impact throughout Indonesia.”
Reefs ‘Turn Off’ (Stop Growing) When Sea Levels Fall And Seas Cool
“[I]t is generally accepted that relative sea level reached a maximum of 1–1.5 m above present mean sea level (pmsl) by ~7 ka [7,000 years ago] (Lewis et al., 2013).”
“Over the last few decades, the global decline of modern reefs has been linked to environmental and climatic changes in response to anthropogenic activities. However, recent geological and ecological research on fossil reefs in the Great Barrier Reef (GBR) and wider Indo-Pacific identified intervals of significant reef ‘turn-off’ in response to natural environmental forces earlier in their development during the mid- to late Holocene.”
“Increased upwelling, turbidity and cyclone activity in response to increased sea-surface temperature (SST’s), precipitation and El-Nino Southern Oscillation variability have been ruled out as possible mechanisms of reef turn-off for the mid-outer platform reefs. Rather, a fall (~0.5 m) in relative sea level at 4–3.5 ka is the most likely explanation for why reefs in the northern and southern regions turned off during this time.”
“Similar hiatuses in Holocene reef growth were identified in Japan from about 5.9 to 5.8 ka, 4.4 to 4.0 ka and from 3.3 to 3.2 ka. They were attributed to oscillating sea level and relatively cold sea-surface temperatures.”
Corals Survived Sea Level Rise Of 6 – 13 mm/yr During Middle Holocene – But ‘Killed’ When Sea Levels Fell Rapidly
“Half-metre sea-level fluctuations on centennial timescales from mid-Holocene corals of Southeast Asia … RSL [relative sea level] history between 6850 and 6500 cal years BP that includes two 0.6 m fluctuations, with rates of RSL change reaching 13±4 mm per year.”
“Here RSL rose to an initial peak of +1.9 m [above present] at 6,720 cal years BP, then fell rapidly to a lowstand of +1.3 m, remaining at about that level for ∼100 years, before rising to a second peak at +1.7 m shortly after 6,550 cal years BP. Around 6,480 cal years BP, RSL appears to have fallen again to +1.3 m before rising to a third peak at +1.6 m or higher. … The peak rate of RSL rise, averaged over a 20-year running time window over the period of study (∼6,850–6,500 cal years BP), is +9.6±4.2 mm per year (2σ); the peak rate of RSL fall is −12.6±4.2 mm per year.”
“The central dome of each microatoll grew during a period when RSL was high; RSL then fell rapidly, killing the upper portions of the corals; RSL then stabilized at a lower elevation, forming a series of low concentric annuli ∼0.6 m higher than present-day analogues; RSL [relative sea level] then rose ∼0.6 m in less than a century, allowing the coral to grow upward to 1.2 m higher than modern living corals.”
During The Early Holocene, Sea Levels Rose At Rates 5 – 6 Times Higher Than Today
“Only Suriname and Guyana [Caribbean] exhibited higher RSL [relative sea level] than present (82% probability), reaching a maximum height of ∼1 m [above present] at 5.2 ka [5,200 years ago].”
“Because of meltwater input, the rates of RSL change were highest during the early Holocene, with a maximum of 10.9 ± 0.6 m/ka [1.09 meters per century] in Suriname and Guyana and minimum of 7.4 ± 0.7 m/ka [0.74 meters per century] in south Florida from 12 to 8 ka [12,000 to 8,000 years ago].”