The Great Barrier Reef, stretching over 2,100 kilometers along the Queensland coast and covering approximately 350,000 square kilometers, stands as the planet’s most expansive coral reef system. This system supports an extraordinary diversity of marine life, including over 1,600 species of fish and 600 types of coral. Its existence provides habitats and supports industries like tourism and fishing that contribute billions to the national economy. However, this natural wonder now faces significant challenges from a rapidly changing global climate.
Impact of Rising Ocean Temperatures
Corals are animals that live in a close partnership with tiny algae called zooxanthellae. These algae reside within the coral tissues and provide the coral with most of its energy through photosynthesis. When sea surface temperatures rise, corals experience stress. This thermal stress causes the coral to expel the zooxanthellae from their tissues.
The expulsion of these algae leaves the coral’s white calcium carbonate skeleton exposed, known as coral bleaching. While a bleached coral is not immediately dead, it has lost its primary food source and becomes susceptible to starvation and disease. The Great Barrier Reef has experienced eight mass bleaching events since 1979, with widespread events in 1998 and 2002. More recently, the reef suffered severe bleaching in 2016 and 2017, leading to the death of nearly a third of its corals in some areas.
Subsequent mass bleaching events occurred in 2020, 2022, 2024, and 2025, making it six events since 2016. The 2022 event was particularly concerning as it occurred during a La Niña summer, highlighting the increasing baseline ocean temperatures driven by climate change. These frequent and intense marine heatwaves reduce shallow water coral reefs by up to 50%. While corals can recover if temperatures drop, the diminishing intervals between these events hinder their ability to fully heal.
Threat of Ocean Acidification
Beyond rising temperatures, the Great Barrier Reef contends with another significant threat: ocean acidification. The world’s oceans absorb approximately 25% of carbon dioxide (CO2) emissions. Since the late 18th century, the ocean has absorbed about 30% of human-generated carbon, causing a measurable decrease in its pH level. This process occurs as dissolved CO2 reacts with seawater to form carbonic acid.
This chemical shift directly impacts corals that rely on calcium carbonate to build their skeletons. Specifically, increased acidity reduces the concentration of carbonate ions, particularly aragonite, the form of calcium carbonate corals use. When aragonite levels drop too low, corals find it difficult to grow skeletons, and existing structures can even begin to dissolve faster than they can be built.
A study revealed that ocean acidification has caused a 13% decline in the skeletal density of massive Porites corals on the Great Barrier Reef since 1950, weakening the reef’s foundation. This thinning of coral skeletons, similar to osteoporosis in human bones, compromises the reef’s structural integrity and makes it more vulnerable to physical damage. Increasing atmospheric carbon dioxide levels can reduce coral growth rates by an estimated 9% to 56% due to the reduced availability of carbonate ions.
Influence of Extreme Weather Patterns
Climate change also intensifies extreme weather events. Tropical cyclones, increasingly frequent and intense, cause immense physical destruction. Between 2004 and 2018, ten cyclones of category three or higher crossed the Great Barrier Reef. Their powerful winds generate massive waves that can shatter delicate coral structures, scour the seabed, and dislodge entire coral colonies, turning reef sections into rubble.
Recovery from such physical damage can take years to decades; for example, Cyclone Yasi in 2011 affected approximately 90,000 square kilometers, or 15% of the Marine Park. Repeated severe storms, like those in 2009, 2011, 2015, and 2017, prevent full healing. Such extensive damage affects the resilience of reefs.
Furthermore, extreme rainfall events lead to substantial land-based runoff entering the reef waters. Tropical Cyclone Jasper in December 2023 discharged an estimated 20,000 gigaliters of freshwater, equivalent to about 40 Sydney Harbours, into the northern Great Barrier Reef. This runoff carries large quantities of suspended sediment, nutrients, and pollutants, including agricultural pesticides.
Sediment reduces water clarity, blocking sunlight, and can smother coral organisms. Increased nutrient inputs, such as nitrogen and phosphorus, stimulate algal growth on reefs, contributing to outbreaks of coral disease. These nutrient surges can also fuel outbreaks of crown-of-thorns starfish and promote the growth of macroalgae. The impact of these pollutants can extend up to 20 to 30 kilometers offshore during large flood events.
Reef Resilience and Future Outlook
Despite these challenges, the Great Barrier Reef possesses a natural capacity for resilience. The future health of the reef is closely tied to global efforts to reduce greenhouse gas emissions, as the frequency of severe events currently outpaces the reef’s recovery processes. Recognizing this, management and scientific initiatives are underway to bolster the reef’s ability to adapt.
The Reef Restoration and Adaptation Program (RRAP) leads various projects aimed at enhancing the reef’s capacity to withstand warming. Scientists are investigating the genetic makeup of corals with greater heat tolerance, conducting stress tests to identify resilient traits. This research informs strategies such as selective cross-breeding of corals and conditioning them to gradual temperature increases.
Large-scale coral aquaculture is also being developed to grow and reintroduce corals to the reef. Other interventions include providing corals with special probiotics or diets to boost their health. Traditional Owners of the Great Barrier Reef are also actively involved, leading actions to protect their Sea Country, integrating traditional ecological knowledge with Western science.
Alongside these advanced interventions, efforts continue to improve local water quality by reducing land-based runoff of sediments and nutrients. Programs like the Reef 2050 Water Quality Improvement Plan aim for significant reductions in pollutants, such as a 60% reduction in dissolved inorganic nitrogen loads by 2025. While these targeted actions are important, the long-term survival of the Great Barrier Reef ultimately relies on comprehensive global action to address the root causes of climate change.