Ove Hoegh-Guldberg Centre for Marine Studies, University of Queensland, St Lucia, 4067, QLD, Australia E-mail: oveh@uq.edu.au
- The consequences of a changing atmosphere for coral reefs are serious and are already occurring. With periods of abnormally warm seas and carbon dioxide concentrations above any seen for the last 400,000 years, corals are facing conditions that greatly exceed the environmental envelope to which they are adapted. Evidence that this is occurring is seen in the increasing frequency and severity of mass bleaching events, in which corals loose their brown symbionts and turn white. Increasingly, these events are resulting in large-scale mortalities, a consequence of longer and more intense warming during the summer months. The impact of increasing sea temperatures is being compounded by the changing aragonite saturation state of seawater (Ω−arag). Oceanic Ω−arag will continue to decrease over the next 50 years as CO2 builds up the atmosphere. Decreased Ω−arag directly reduces the calcifying ability of organisms such as corals, eventually making their skeletons fragile. While corals and coral reefs are unlikely to undergo complete extinction in geological time, the prognosis for reefs and the people that depend on them is not bright in the shorter term (tens to hundreds of years). Analysis using projected sea temperatures and the current sensitivity of corals to elevated sea temperatures indicates that conditions will soon greatly exceed the thermal thresholds of corals and their symbionts, with the consequence that major mortalities will be commonplace by 2030 and thermal stress events will rise above any experienced by corals in the past 50 years if not several thousand years. Given the importance of coral reefs to billion dollar economies and an estimated 100 million people worldwide, action must be taken to reduce these impacts.
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