Third consecutive year of coral bleaching – a tipping point passed?

The summers of 2015 in the northern hemisphere and of 2015-2016 in the southern hemisphere were the hottest ever recorded and caused the worst coral bleaching on record.

The United States National Oceanic and Atmospheric Administration (NOAA) declared this to be the third global coral bleaching event, following similar events in 1998 and 2010. Last year, approximately 50% of Australia’s Great Barrier Reef was bleached, most severely in the northern part, where two-thirds of the corals have died. Globally, bleaching was worse in some locales (e.g. Kiribati) and affected some areas that had not bleached in the previous events (eg. the northern Great Barrier Reef).

This week it was reported by Australia’s Great Barrier Reef Marine Park Authority that there are signs of more bleaching to come in 2017. Warming oceans have been modelled by oceanographers at NOAA, and their forecast for this year is bleak. Many more reefs are predicted to bleach in the next few months.

Given that the latest global coral bleaching is now in its third consecutive year and has affected more reefs than any previous bleaching event, the question must be asked if anthropogenic climate change has exceeded a critical tipping point for coral reefs? Has human activity now warmed the oceans to a temperature whereby combinations of El Nino with other factors will cause widespread coral bleaching most years from now on? Is there any hope that coral reefs will survive?

The concept of tipping points for natural systems is well established in the scientific literature. An “ecological tipping point” has been defined by the Biodiversity Information System for Europe as a “situation in which an ecosystem experiences a shift to a new state, with significant changes to biodiversity and the services to people it underpins, at a regional or global scale”. Passing a tipping point means that a “threshold beyond which an abrupt shift of ecological state” has occurred, causing long-lasting changes that are hard to reverse.

The bleaching and death of vast areas of coral reefs in recent years — in the northern and southern hemispheres — seems to fulfil this definition. Dead coral reefs can recover if allowed enough time and depending on other environmental factors. But when the frequency of bleaching events exceeds the rate of recovery, a tipping point has passed.

Globally, coral reefs cover an area of around 250,000 km2 and, according to the United Nations World Ocean Assessment, are of major importance for 275 million people located in 79 countries who depend on reef-associated fisheries as their major source of animal protein. The ecosystem services provided by coral reefs have been collectively valued at USD $29 billion per year. This includes their value to tourism ($11.5 billion), fisheries ($6.5 billion) and coastal protection ($10.7 billion). The demise of reefs puts all this at risk and threatens the health and prosperity of people who depend on them.

Once the corals have died, they no longer grow vertically upwards. The reefs gradually will erode and become less effective in providing shoreline protection from wave action during storms. Dead corals not only lack the aesthetic appeal that is fundamental to reef tourism, they also do not sustain the same fish community. This results in reduced tourist activity and reduced income from fisheries that can threaten the livelihoods of local communities. Living coral reefs are important religious symbols for some communities and the loss of corals harms this important cultural aspect as well.