Climate Change

Climate change is the greatest threat to the Great Barrier Reef and coral reefs worldwide. The rapid increase in greenhouse gas emissions since the pre-industrial era has led to an estimated 1.0°C increase in global average temperature. global temperature increases lead to warmer seawater, which can have a variety of effects, including damaging ocean heat waves.
Not only are atmospheric carbon dioxide concentrations currently at record levels for the past 800,000 years, but the climate is changing at a rate not seen in decades to millennia.

Since 1985, the Great Barrier Reef has lost more than half of its reefs to these hazards, with two-thirds of this loss occurring since 1998.A March 2016 report showed that a third global bleaching event triggered by ocean warming could wipe out the entire north-central reef, accounting for more than 40% of the Great Barrier Reef. 

Ocean acidification
is having a major impact on climate change in the Great Barrier Reef ecosystem. Acidification occurs because the ocean acts as a carbon sink, absorbing carbon dioxide from the atmosphere. This lowers the pH (a measure of acidity or alkalinity) of the ocean over an extended period of time, thus changing the chemical composition of the ocean. Since pre-industrial times, the oceans have absorbed around 30% of anthropogenic CO2 emissions. The pH of the ocean surface has decreased by 0.1 compared to pre-industrial times, to 8.1. This corresponds to a 26% increase in acidity. Coral reef development is thought to have ceased at a pH of 7.8.

Storms and cyclones
Climate change-induced changes in weather patterns affecting the frequency, intensity or distribution of these disturbances will have a significant impact.
According to the State of the Climate 2014 report, on average, fewer tropical cyclones are expected in the Australian region. However, it is expected that the proportion of intense cyclones will increase. 48% of coral losses recorded by the Australian Institute of Marine Science’s long-term monitoring program between 1985 and 2012 were caused by cyclones. 10 cyclones of category 3 or greater passed through the Great Barrier Reef Marine Park between 2004 and 2018. The southern half of the region was the most severely affected, causing significant damage to reef habitats.
(information from Great Barrier Reef Marine Park Authority)