Climate change is the greatest threat to the Great Barrier Reef and coral reefs worldwide. It is caused, by global emissions of greenhouse gases, such as carbon dioxide from the burning of fossil fuels (coal, oil and natural gas), agriculture and land clearing. Not only has the current concentration of carbon dioxide in the atmosphere reached record levels in the last 800,000 years, but the climate is also changing at a rate unprecedented over decades to millennia. The rapid increase in greenhouse gas emissions has caused an estimated 1.0°C increase in global average temperature since pre-industrial times. The rising global temperature is causing an increase in sea temperature, which has a multitude of impacts, including destructive marine heatwaves.

Bleaching occurs when hotter water destroys the algae upon which the coral feeds, causing it to turn white. The IUCN moved the Reef’s status for the first time to “critical” and deteriorating on its conservation watchlist. Some of the activities which threaten it, such as fishing and coastal development, can be tackled by the management authorities, the organisation said. “Other pressures cannot be addressed at the site level, such as climate change, which is recognised as the greatest threat,” it stressed.

Climate change is also likely to increase the proportion of severe tropical cyclones and the frequency and severity of heavy rainfall events. Severe weather events have various impacts on the Reef — floodwaters can cause flood plumes and reduce the salinity of reefs, and cyclones can cause extensive damage to both individual corals and coral structures.

If the current rate of greenhouse gas emissions continues, the global average temperature will continue to increase rapidly, which will have further negative impacts on the Reef. A report by the Intergovernmental Panel on Climate Change advised that coral reefs worldwide are projected to decline by a further 70-90 per cent at a 1.5°C increase in temperature, with greater losses at a 2.0°C increase.

Water temperature moderates fish body temperature, which means warmer oceans can affect important biological processes of fish, including growth, reproduction, swimming ability and behaviour. Temperature limits can also affect the distribution and abundance of bait-fish aggregations. Some species are likely to expand their geographic ranges southward (or contract their migrations northward) as waters warm. Some fish respond well to high sea temperatures, as these temperatures can shorten incubation time, increase growth rates and improve swimming ability in juvenile fish. However, these benefits are limited to relatively minor temperature increase.

Climate change affects turtles, sea snakes and crocodiles because the environmental temperature controls the reptiles’ body temperatures (except for the leatherback turtle). Of all the marine reptiles on the Reef, turtles are the most vulnerable to climate change. The temperature of the sand, where eggs are laid, determines the sex of turtles. Air temperature and sea temperature increases will alter turtle breeding seasons and patterns, egg hatching success and the sex ratio of the populations.

Temperature is also an important factor for the estuarine crocodile, the only crocodile species on the Reef. Nesting periods, sex determination and the running and swimming speed of a hatchling are environmentally determined and will be influenced by changes in temperature. Seabirds are considered to be some of the most vulnerable species to climate change impacts. During frequent or intense El Niño/La Niña-Southern Oscillation events in tropical waters, seabirds have fewer breeding cycles, slowed chick development and reduced nesting success. This is because higher sea temperatures during such events affect the availability of food for seabirds.

A steady decline in most seabird species at Raine Island (the biggest seabird nesting colony in the Great Barrier Reef) has been recorded over the last 12 years. There is also evidence that climate change has driven the ranges of Australian seabirds further south. Water temperature partly determines the photosynthesis rates for seagrass — an important food source for dugongs and marine turtles. Temperature increases can reduce the efficiency of photosynthesis; however, the extent of this impact may depend on the species’ reliance on the light. Temperature also plays a role in seagrass flowering (and thus reproductive) patterns.

A healthy Reef is naturally resilient to disturbances, however, the rapid increase in sea temperature presents significant challenges for the Reef to adapt to a changing climate. More frequent and more intense bleaching allows less time for coral reefs to recover and adapt and reduces their ability to withstand other impacts such as disease. Loss of corals leads to a reduction in the fish and associated species they support and have major implications on the whole Reef ecosystem. These effects are likely to have far-reaching consequences for the Great Barrier Reef and its outstanding universal value as a World Heritage Area.

Progress towards safeguarding the Reef under a long-term sustainability plan until 2050 has been slow and it has not been possible to stop its deterioration, the report said. The turtle populations – including loggerhead, hawksbill and northern green – as well as the scalloped hammerhead shark, many seabird populations and possibly some dolphin species, are declining. Efforts to protect the Reef are increasing, however. HSBC and the Queensland government said in October they would buy “Reef Credits”, a tradable unit that quantifies and values the work undertaken to improve water quality flowing onto the Reef. Similar to the carbon offset market which incentivizes the reduction of carbon dioxide from the atmosphere, the scheme pays landholders for improved water quality.

By Jumana Jabeer