Climate change creates several unique challenges that can impact the long-term future of the Great Barrier Reef, including more frequent severe weather events, rising ocean temperatures, ocean acidification, and rising sea levels.
Ocean plastic and other marine debris pollute this already fragile ecosystem, endangering marine life and stressing corals. Plastic can abrade coral or tear its skin, allowing pathogens such as bacteria and other microorganisms to enter and a potentially deadly infection to start.
Corals (which are animals) have microscopic marine algae (plants called zooxanthellae) living inside their tissue. Corals have a symbiotic relationship with the algae; the coral provides shelter for the algae and the algae provide oxygen, glucose, and amino acids that the coral needs to survive. The algae also give corals their color.
When high water temperatures put corals are under stress, they expel the zooxanthellae. This expulsion causes the Reef to turn a ghostly white in a process scientists call coral bleaching. A bleaching event doesn’t necessarily kill coral, though it can if the algae loss is prolonged and the stress isn’t alleviated. Coral that survives a bleaching event becomes weaker, struggles to grow and reproduce, and becomes more vulnerable to disease.
Tropical cyclones have become more intense, more frequent, and can physically damage or destroy the Reef structure. They can also reduce coral cover, alter species diversity, and affect reef productivity. The low pressure they generate can even decrease sea levels and leave some corals exposed.
Severe weather and extreme rainfall can also cause more freshwater and sediment to travel further away from the coast and onto the Reef. Freshwater changes the ocean’s pH and decreases salinity, which puts the corals under stress and can lead to bleaching events. Increased turbidity (more sediment in the water) can also block sunlight, affect coral growth, and limit shelter for other sea creatures who live on the Reef.
Coral reefs have a natural ability to withstand and recover quickly from the physical damage caused by storms and natural disasters. However, outside stressors like changing pH, salinity, and nutrient levels can weaken the Reef and increase the time it needs to recover. More frequent severe weather events increase damage while reducing the time corals have to recover.
Hard corals are reef-building corals. Massive reef structures like the Great Barrier Reef sit on a foundation made up of millions of hard corals. Their exoskeletons are almost stonelike because they’re composed of calcium carbonate, just like the shells of many mollusks. In order to build their exoskeletons, coral polyps must extract calcium from seawater—a process that requires a specific pH balance.
However, the ocean has absorbed about 30% of the additional carbon dioxide humans have introduced to the atmosphere since the late 1700s. Carbon dioxide reacts with seawater to form carbonic acid, which decreases the pH of seawater and changes the ocean’s chemistry. With fewer carbonate ions, it becomes more difficult for hard corals to grow and reinforce their exoskeletons. Weaker exoskeletons are more vulnerable to fracture during severe weather events.
The average global temperature has increased by almost 0.8°C since the 1880s. Around 90% of that excess heat is being absorbed by the ocean. Corals can experience stress after a one- or two-degree shift in temperature; prolonged heat stress increases the likelihood of mass coral bleaching, which leaves the corals more vulnerable to other threats.
The rising ocean temperatures and increased acidity associated with climate change are just two factors impacting water quality in and around the Great Barrier Reef. Coastal development and land-based runoff from agricultural, industrial, and urban land uses are increasing the amount of sediment, nutrients, and contaminants entering the coastal waters the Reef calls home.
Nutrient runoff has been linked to outbreaks of crown-of-thorns (COT) starfish, which prey on hard coral species. In small numbers, these echinoderms benefit the Reef by feeding on dominant hard coral species and maintaining biodiversity. However, population outbreaks of these starfish can have a devastating impact.
Industrial fertilizers that leak into the ocean from mainland river systems can fuel enormous blooms of algae and phytoplankton, which COT starfish larvae feed on. Overfishing has also decimated their natural predators, which means more larvae are surviving into adulthood and preying on an unusually high number of hard corals. Crown-of-thorn starfish outbreaks can be especially destructive during coral bleaching events or when corals are already stressed.
Coral reefs are naturally resilient. By reducing threats and minimizing impacts, we can enable the Reef to recover naturally. We’re partnering with the Great Barrier Reef Foundation so they can bring together the best minds and technologies to help develop the knowledge and solutions we need to protect and restore the Great Barrier Reef for future generations.
The Great Barrier Reef Foundation is a nonprofit that’s bringing together the best minds and technologies from around the world to help solve the challenges facing the Great Barrier Reef so it can remain a source of wonder and fascination for generations to come.
The Great Barrier Reef Bracelet was released in May 2019 in partnership with Great Barrier Reef Foundation and is no longer available at this time.