On the 8th of October 2018 the UN Intergovernmental Panel on Climate Change (the IPCC) published a long-anticipated report on the range effects of global warming by the end of this century within a temperature range of 1.5-2.0 ⁰C. The report flooded media headlines for weeks with one of the key messages it drove home; nations need to set climate change mitigation targets and meet them – and they need to be quick about it. The report forecasts alarming predictions should we fail to limit the warming of the planet to 1.5 ⁰C; in the oceans, even at 1.5 ⁰C of warming we could lose up to 70% of tropical coral reefs.
At 2.0 ⁰C of warming we could lose them all.
Unless one of the 275 million people living within 30km of a reef it can be difficult to truly understand the far-reaching significance of reefs, but occupying less than 0.1% of the ocean floor warm water corals are host nearly 25% of all ocean biodiversity. This makes the reefs ideal habitats for fisheries; in tropical countries and island nations coral reefs account for up to 12% of all fish caught. Reefs also act as breakwaters protecting shorelines from destructive tropical cyclones and storm surges. Lastly reefs bring the benefits of tourism to over 100 countries, as visitors are attracted to the beauty of such iconic marine environments, which in turn generates employment and economic benefits.
But these fragile ecosystems that have remained relatively stable for thousands of years are now finding themselves increasingly threatened under the intense fire of anthropogenic stressors. Local disturbances to corals caused by humans include unsustainable coastal development, pollution, nutrient enrichment from aquaculture and sewage effluence, and the overexploitation of coral fisheries; species are being harvested at a rate that exceeds their rate to replenish and reproduce, which decreases the species richness of the reef and begins to fray complex food webs. Yet any corals that display a resilience towards these stressors are then faced with the synergy of ocean warming and ocean acidification as consequences of the climate change caused by the spike in carbon dioxide in the atmosphere from the burning of fossil fuels as humans civilisations continue to industrialise.
As high concentrations of atmospheric carbon dioxide contribute the warming green house effect, it is the oceans that then absorb much of the excess heat trapped in the atmosphere. However, even small increases in sea temperatures can lead to mass coral ‘bleaching’. In periods of unfavorable temperatures corals expel the photosynthetic algae (zooxanthellae) they harvest in their tissues that are their primary source of nutrients and sustenance; without it the coral tissues become transparent exposing white skeletons of calcium carbonate underneath. Ultimately the corals starve and die. In 1998, one mass bleaching event lead to the death of 16% of all the corals on Earth.
The oceans are not only sinks for excess heat but also for the carbon dioxide itself as its pumped into the atmosphere. Carbon dioxide reacts with sea water to produce carbonic acid which decreases the Ph and increases the acidity of the water which suddenly makes manufacturing calcium carbonate skeletons an incredibly difficult task for corals. This difficulty is further exacerbated as carbonic acid reacts with carbonate ions in the water to form bicarbonate ions meaning the carbonate ions become less available to corals who would otherwise combine them with calcium ions. As the corals growth rate is stunted and their calcite structures are weakened, they become only more susceptible to temperature related stress and disease. The disheartening result is the possibility that corals will shift from a state of reef building to net dissolution by the end of this century.
At present over half of the planet’s reefs are under medium or high risk of degradation; a clear indicator that we must either begin to undertake drastic actions to conserve the reefs in their abundance, or face the near future and a sobering reality of virtually lifeless coastal seas.
(figures and statistics taken from 2018 IPCC report)
Thanks for reading! 😊 ~ Layla Sklar .