The role of historical coral bleaching – can corals survive in a warmer world?

Coral reefs, one of the world’s most ecologically and socioeconomically important ecosystems, are in crisis due to anthropogenic climate change that threatens the future survival of corals. Prior to this research, limited information about incidences of coral bleaching existed before the observational record (which began in the late 1970s). This is a problem because previously published future predictions of coral bleaching are based only on observational and satellite (1982 onwards) records, which span ~ 40 years. At present, where observational time series are short; it is thus difficult to detect whether coral can acclimate or adapt under increased temperatures. The overall aim of this research was to determine whether corals can survive in a warmer world using historic bleaching records to understand if corals acclimate and / or adapt. Lack of widespread sampling has limited previous coral core research; therefore the thesis aim was achieved by investigating: i) the bleaching history of the Maldives through coral core analysis of past growth rates (Chapter 3) ii) whether Sr/Ca, the most common paleothermometer, can be used to reconstruct sea surface temperature in the Maldives, for use in coral bleaching reconstructions (Chapter 4), and iii) future global projections of mass coral bleaching under Intergovernmental Panel on Climate Change (IPCC) climate scenarios based upon reconstructed historical bleaching events (Chapter 5). This is the first research to incorporate reconstructed bleaching events into future trajectories under predicted global increased temperatures during the coming century. By expanding bleaching records for the Maldives, growth rates of Maldivian coral cores show evidence of coral bleaching since 1985, two years prior to the first observational record. Furthermore, indirect evidence of either local adaptation or acclimatisation is presented based on the growth – temperature relationship at the reef scale. Additionally, the findings within this thesis highlight the lack of reproducibility of the commonly used palaeothermometer, Sr/Ca, in the Maldives, where only three (out of twelve) cores displayed a significant relationship between Sr/Ca and SST, and Sr/Ca derived SSTs were offset by up to 1 °C. Currently a reliance on records of observed bleaching exists globally and the full picture of the bleaching history within an area may not thus be representative. This research also presents evidence of bleaching at the global scale since 1582 and demonstrated a relationship between historical bleaching events and SST, which was then used to predict future bleaching events under IPCC climate scenarios. The findings within this thesis show that using long records provide more conservative bleaching trajectories compared to models based on shorter satellite datasets because the baseline and resultant trends from which projections are made are longer and often more gradual. For example, the results presented from this research show that under the highest emissions IPCC scenario (8.5) 25 % of corals are predicted to bleach within this century, based on slow growing Porites sp., compared to the previously published prediction based on 40-years’ worth satellite data that 100 % of global reefs will bleach by 2100. Overall this thesis shows that corals have shown evidence of possible adaptation / acclimatisation to increased temperature across different spatial scales: globally (1552 – 2016), at the realm scale (Central Indo-Pacific: 1572 – 2013; Western Indo-Pacific: 1905 – 2016 and Tropical Atlantic: 1552 - 2016) and regionally (Maldives; 1916 – 2016). This implies evidence of plasticity in their physiology until their thermal tolerance limits are reache