Coral reefs are one of the ecosystems most sensitive to global warming. In recent years, there have been a number of mass bleaching events in warm-water corals, primarily caused by prolonged high ocean temperatures. Under prolonged heat stress, the corals expel the tiny colorful algae that live in their tissues — known as zooxanthellae — leaving behind a white skeleton. The algae provide the corals with energy through photosynthesis. Without them, corals can slowly starve. Although corals can regain their zooxanthellae when conditions become more favorable, prolonged heat stress can wipe out coral communities on entire reefs.
Over the past 40 years, mass bleaching events have become five times more frequent. The map below from the IPCC’s Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) shows how coral reefs around the world were affected in 2015–16. The shading on the map shows the annual maximum “degree heating week” (DHW) in 2015 and 2016. DHW is a measure of cumulative heat stress that describes how much heat has accumulated in an area over the past 12 weeks by adding all temperatures that are 1°C above the maximum summer mean. Dots mark reefs that experienced severe (purple), moderate (purple), and no significant (white) bleaching.
Thermal stress is not the only threat to coral reefs. They are also threatened by other factors, including overfishing, destructive fishing practices, sedimentation associated with sea level rise, nutrient runoff from land, storm damage, ocean acidification, and changes in ocean circulation.
Why are coral reefs a tipping point?
Ecologically intact reefs can recover from events such as hurricanes or even severe bleaching. But if they lack herbivores, they cannot recover because fast-growing tropical macroalgae can quickly takeover dead coral skeletons, potentially preventing them from being recolonised by corals and leaving them in an altered, albeit stable, state. This means that overfishing is a major problem. A 2007 study confirms this and explains that herbivores play an important role in promoting reef resilience and recovery by removing algae. Therefore, they are critical to returning a reef to a coral-dominated state when disturbance has occurred. The study emphasizes that the key is to have a healthy number of herbivores that can graze the algae and prevent it from becoming established. Once the algae are large, it is unlikely that they will be eaten. So it is naive to assume that coral reefs will recover once algae are established.
The recovery of a coral reef depends on the corals having sufficient time to reestablish themselves. A healthy reef can recover within 10–15 years after a bleaching event. However, many reefs have shown little or no recovery after severe bleaching events. In this work, severe bleaching events have now been shown to recur in less than six years. This is far too fast, as 10–15 years is the best case scenario for coral reef recovery.
Where is the tipping point of coral reefs?
Since coral reefs take 10–15 years to recover at best, and mass bleaching events are likely to recur every 6 years or faster, it is likely that we have already passed the tipping point. This study suggests that with a warming of 1.5°C to 2°C compared to pre-industrial levels, approximately 90% to 99% of the remaining coral reefs are at risk. The researchers even point out that their results are likely “rather conservative” because they only consider the effects of rising CO2 levels and not other stressors on coral reefs. This IPCC report paints a similar picture. A 2020 study published in Nature Communications suggests that once tipped, Caribbean coral reefs could collapse within 15 years. Severe bleaching is already occurring around the world, and the recent global coral bleaching of 2014–17 was devastating for many reefs around the world. For example, the Great Barrier Reef lost half of its coral in just two years.
What would happen if we crossed the tipping point?
The loss of coral reefs would be devastating for ecosystems, economies and people. According to the International Union for Conservation of Nature (IUCN), reefs host more than 25% of all marine fish species, despite covering less than 0.1% of the ocean floor. Coral reefs also directly support over 500 million people worldwide, who rely on them for daily subsistence, mostly in poor countries. Additionally, natural barriers such as coral reefs help dissipate wave energy, reducing the risk of flooding.