Coral Reefs as Deglacial Sea Level Archives: Constraining Potential Rates of Sea Level Rise in the next 75 years

Abstract:Coral reefs offer valuable archives of past sea-level changes, especially during the last deglaciation (~20,000 to 6,000 years ago). These archives are increasingly used to constrain potential rates of sea-level rise in the 21st century. Coral reefs are excellent sea level archives, because corals grow cemented in situ on the sea floor and thrive close to sea level, in well-defined water depth zones where light penetration is at its optimum, also referred to the euphotic zone. Coral growth rates are, therefore, sensitive to rates of sea level rise, having to keep up with the euphotic zone. Coral accumulation archives sea-level stability, steady rise, or even sudden jump. Coral reef terraces and fossil coral microatolls preserve stepwise sea- level positions. Contrarily, times of sea level fall are commonly recorded by clear reef exposure and death. As clear advantages, corals can be dated by radiocarbon and even with higher precision by U/Th. Corals, being reliable recorders of sea level rises, become, therefore, good analogs for ice sheet behavior. In mid-deglaciation, for instance 14,500 years ago, coral archives show that sea level rose ~14–18 meters in less than 500 years, corresponding to rates of rise possibly reaching 40–50 mm/year or several meters per century; such a deglacial meltwater pulse had to correspond to dramatic partial ice sheet collapses. Because global temperatures are continuously getting warmer in the context of a steadily increasing greenhouse gas emission, sea level will continue, not only to be rising in the future, but the rates of sea level rise are expected to increase in the next 75 years. In the extreme scenario of non-abated rates of greenhouse gas emission, often referred to as the business as usual scenario, a global warming of 2 to 4 degree Celsius would not only trigger the partial melting of ice sheets in Greenland and Antarctica, but most likely enhance the risks of their partial collapse, generating global sea level rise of as much as 2.4 m above the year 2000 level. Coral deglacial paleo-records clearly illustrate that extreme sea level rise in this century (even > 2 m/century) is physically possible if major ice sheet partial collapses occur in Greenland and West Antarctica.