Geomorphic Response of Coral Reef Landforms to Climate Change
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Pacific Science Inter-Congress Tahiti, 2-6 March 2009 Geomorphic Response of Coral Reef Landforms to Climate Change Paul Kench Small Islands Research Unit School of Geography, Geology & Environmental Science 1. INTRODUCTION Reef Island Vulnerability • Coral reef islands are considered the most vulnerable landforms on earth. • Climate change and climatic variability are expected to promote widespread flooding, erosion and destabilisation of small islands. Assertions of island vulnerability are seldom based on rigorous understanding of the environmental processes controlling island formation. 1. INTRODUCTION Fundamentals of island development 1. Reef Growth 4. Morphological Products 2. Sediment production 3. Physical processes ECO-MORPHODYNAMICS • Multiple drivers of change. • Mechanisms of change operate at different timescales. • Temporal scales of relevance to Islands. • Examples of island change at different timescales provide analogues for future morphological adjustment. 2A. RESOLVING ISLAND DYNAMICS The Geological Timescale – Island Formation Location Timeframe of Isld References formation Kiribati, Makin 2500 – present Woodroffe & Morrison 2001 Tuvalu, Fongafale 2000 – 1000 McLean & Hosking 1991 Fiji, Islands in Nadi Bay 2,000 – 1000 McKoy, Kennedy & Kench in press Torres Strait, Warraber Isld 3,000 – present Woodroffe et al., 2007 GBR, Lady Elliot Isld 3,200 – present Chivas et al. 1986 GBR, Bewick Isld 4,600 – 1,600 Kench et al (in press) GBR, multiple cays 4,380 - 3020 Stoddart et al., 1978 Cocos (Keeling) Isld 4,000 – 2,000 Woodroffe et al., 1999 Maldives, Dhakandhoo Isld 5,500 – 4,000 Kench, McLean, Nichol, 2005 Maldives, Hulhudhoo Isld 5,500 – 4,000 Kench, McLean, Nichol, 2005 Maldives, Thiladhoo Isld 4,500 – 3,000 Kench, McLean, Nichol, 2005 Maldives, Addu 5,280 - 3150 Woodroffe, 1993 2A. RESOLVING ISLAND DYNAMICS The Geological Timescale - Sea level, reef growth & island formation Radiocarbon Years B.P. 8000 6000 4000 2000 0 Bewick Warraber 2 Makin Present Sea level Fongafale3 0 Reef flat Island Formation Linear 3-Stage Model Formation E l e v a t i • Reef flat development 1 o 10 n Upward / at sea level a D reef growth e precursor to Island p t with SL h formation. ( m ) 20 2A. RESOLVING ISLAND DYNAMICS The Geological Timescale - Sea level, reef growth & island formation Reef Growth and Inferred Sea Level History, Maldives Higher sea level Alternate model: Island formation • Islands formed in latter stages of sea-level rise. Depth below msl (m) msl below Depth Inferred Sea level Kench et al. (2005) Geology Kench et al. (in press) Geology Age (years BP) 2A. RESOLVING ISLAND DYNAMICS The Geological Timescale - Sea level, reef growth & island formation Implications 1. Sea level fall and/or stabilisation are not necessary precursors to island formation. 2. Reef flat development at sea level is not a necessary precursor for island evolution. 3. Geological analogues show islands have experienced higher sea levels. 4. Island change may not be as sensitive to small fluctuations in sea level as has been commonly perceived. 2B. RESOLVING ISLAND DYNAMICS Medium-Term: Multi-decadal Scale Change • A number of studies have related changes in wind and wave climate to shifts in position of islands on their reef platforms. (Umbgrove, 1947; Verstappen, 1954; Stoddart, 1978; Flood, 1986) Picture • Few studies have monitored island change at temporal scales consistent with modern sea level Malamala Isld, Nadi Bay, Fiji monitoring. (Kench and Harvey, 2003) 2B. RESOLVING ISLAND DYNAMICS Medium-Term: Multi-decadal Scale Change Summary of Multi-decadal Changes in Island Area, Funafuti Atoll 1984-2003 • Remote sensing analysis of island Island Initial Isld Net Change Net change change reveals: Area (ha) (ha) (%) – Most exhibit little change (< 5%). Paava Island 1.48 0.15 + 10.0 – ~30% changed more than 5%. Fualifeke Island 6.85 -0.24 - 3.5 Mulitefala Island 2.33 0.02 + 0.8 • Over a 20 y period when sea level Amatuku 6.13 0.29 + 4.6 rise has been 2 mm/y ( Church et al. Fatato 5.11 0.44 + 8.6 2006 Global & Planetary Change ) there Funagongo 10.66 0.1 + 1.0 has been little or positive change Funamanu 2.99 0.84 + 28.2 in island area. Falefatu 3.23 0.43 + 13.3 Mateiko 4.25 0.26 + 6.1 Northern Tip Funamanu Isld, Luamotu 1.8 -0.06 - 3.3 Funafuti Atoll (2003) Funafara 22.95 0.83 + 3.6 Telele 8.83 0.04 + 0.5 Motungie 4.97 0.05 + 1.0 Avalau /Teafuafou 12.14 -0.25 - 2.1 Tengasu 0.68 -0.10 - 14.7 Tutanga 1.66 -0.06 - 3.6 Falaoingo 1.31 0.00 0.0 Source: Webb (2005) SOPAC 2C. RESOLVING ISLAND DYNAMICS Short-Term Process Controls on Island Change Hulhudhoo Islands can exhibit large fluctuations in shoreline position between seasons. m Suggests islands have differing susceptibility to changes in climate/oceanographic regime dependent on island/reef platform shape. February 2003 (NE) Kench and Brander (2006) Journal of Geophysical Research 2C. SHORT-TERM PROCESS CONTROLS ON ISLAND CHANGE Impact of Extreme Events • Have both accretion & erosion impacts on island planform. Funafuti atoll Cyclone Bebe Rubble bank Kench, Perry and Spencer (2009), after Bayliss-Smith (1988) 2C. SHORT-TERM PROCESS CONTROLS ON ISLAND CHANGE Impact of Extreme Events – Indian Ocean Tsunami • Inundated Maldivian reef islands promoting erosion of the vegetated island core 1% - 9% - (planform changes). • Wave overtopping promoted washover sedimentation on islands. Summary of tsunami-induced physical changes on Thiladhoo Island, Maldives Erosion Washover limit Kench et al. (2006) Geology 2C. SHORT-TERM PROCESS CONTROLS ON ISLAND CHANGE Impact of Extreme Events – Indian Ocean Tsunami • Washover deposits allow islands Multiple washover sand deposits Thiladhoo Island to build vertically in extreme events. Washover sand sheet, Thiladhoo Island 3. SUMMARY & CONCLUSIONS Reef islands are Dynamic rather than Vulnerable landforms • At geological timescales islands are robust and have persisted on reef platforms for 5,000 – 2,000 years. • At multi-decadal timescales islands are also robust exhibiting small positive and negative shifts in net island area - altering island location on reef platforms. • In the short-term (seasons – days) islands exhibit significant morphological change in response to extreme events and change in wave processes. • Islands exhibit a range of morphological responses including migration, erosion, accretion and vertical land building. Islands will persist on reef platforms into the future! 3. SUMMARY & CONCLUSIONS Reef Island Dynamics and Management Challenges • Field evidence suggests islands are robust at the timescale of relevance to planning (decades). • Short-term variations in island morphology must be placed in the context of long-term stability to avoid ad hoc reactionary management interventions. • The secret to long-term resilience of islands relies on geomorphic processes being unconstrained to redistribute sediment. • Challenge is to allow maintenance of natural processes and seek alternatives to conventional solutions..