Instability and breakdown of the coral–algae symbiosis upon exceedence of the interglacial pCO2 threshold (>260 ppmv): the “missing” Earth-System feedback mechanism

Abstract

Changes in the atmospheric partial pressure of CO2 (pCO2) leads to predictable impacts on the surface ocean carbonate system. Here, the importance of atmospheric pCO2 <260 ppmv is established for the optimum performance (and stability) of the algal endosymbiosis employed by a key suite of tropical reef-building coral species. Violation of this symbiotic threshold is revealed as a prerequisite for major historical reef extinction events, glacial–interglacial feedback climate cycles, and the modern decline of coral reef ecosystems. Indeed, it is concluded that this symbiotic threshold enacts a fundamental feedback mechanism needed to explain the characteristic dynamics (and drivers) of the coupled land–ocean–atmosphere carbon cycle of the Earth System since the mid-Miocene, some 25 million yr ago.