Damage to the Great Barrier Reef due to global warming has compromised the capacity of corals to recover, according to scientists. New research published on 3 April in Nature shows that after the mass bleaching events of 2016 and 2017, coral reefs are struggling to recover and the number of new corals settling on the Great Barrier Reef has declined by 89 per cent (1).
The Great Barrier Reef, located off of the North-East coast of Australia, is made up of nearly 3000 individual reefs spanning over 2,300 kilometres. The reef ecosystem is home to numerous marine species, including 1,500 species of fish and 4,000 types of mollusc.
Many corals reproduce during the hot summer months by releasing millions of eggs and sperm which travel to the surface of the water and combine to form gametes. Some of these eventually become larvae, which settle in new places forming new coral colonies ― a process known as coral “recruitment”.
The recruitment process is essential for replenishing coral stocks. However, warming waters are making it much harder for corals to reproduce and replenish the massive losses recently seen in adult coral. Global warming also seems to be dramatically altering the mix of corals with some impacted worse than others.
How does global warming impact coral reefs?
In the last 20 years, the Great Barrier Reef experienced four bleaching events owing to global warming, 1998, 2002, and back-to-back in 2016 and 2017. Gaps between extreme bleaching events are getting shorter (2). Under a “business-as-usual scenario” for greenhouse gas emissions, climate models predict that bleaching will occur twice per decade from 2035, and annually after 2044.
The increasingly shorter timeframes between these extreme events mean corals simply don’t have time to recover. Sadly, the mass death of adult coral caused by the recent unprecedented back-to-back bleaching events of 2016 and 2017 led to a dramatic drop in replenishment.
To estimate how many corals survived along the reef following the extreme heat of 2018 and how many new corals were produced to replace those damaged, the researchers measured differences in the numbers of larvae during so-called spawning events between 2018 and previous years. They placed thousands of sticky panels along the reef, spanning almost 3000 km, to collect the gametes.
Based on their findings, the researchers discovered a massive 93 per cent drop in the replenishment of Acropora, a dominant species of branching and table coral, in 2018 compared to previous years.
The low resistance of adult corals ― so-called adult broodstock ― to repeated bleaching events are inextricably linked to an impaired recovery capacity. In other words, fewer baby corals are produced, and certainly not enough to replace the adult corals that have died.
The reefs could recover in five to ten years, scientists say, but only if no more mass bleaching events occur during that time ― which is probably unlikely. The only way to solve the problem is to reduce global warming is by stopping greenhouse gas emissions as quickly as possible.
(1) Hughes, T.P. et al. Global warming impairs stock–recruitment dynamics of corals. Nature (2019). DOI: 10.1038/s41586-019-1081-y
(2) Hughes, T.P. et al. Spatial and temporal patterns of mass bleaching of corals in the Anthropocene. Science (2018). DOI: 10.1126/science.aan8048