What Causes Coral Bleaching At The Great Barrier Reef?

What causes coral bleaching at the Great Barrier Reef?

Context

Scientists have warned that the Great Barrier Reef will face a critical period of heat stress over the coming weeks, following the most widespread coral bleaching the natural world has ever endured.
Warming ocean temperatures, a sign of climate change, is associated with the deteriorating health of the Reef. The Great Barrier Reef Marine Park, which spreads across a length of over 2,300 km and is roughly the size of Italy, is home to about 3,000 coral reefs, 600 continental islands, 1,625 type of fish, 133 varieties of shark and rays and 600 types of soft and hard corals.

According to the National Oceanic and Atmospheric Administration (NOAA), when corals are stressed by changes in conditions such as temperature, light or nutrients, they expel the algae living in their tissue, causing them to turn white, hence bleached.
Coral bleaching does not mean the corals are dead, but make them vulnerable, hence increasing their mortality. Warm ocean temperatures are one condition that could lead to coral bleaching. For instance, in 2005, the US lost half of its coral reefs in the Caribbean in one year due to a massive bleaching event.
Even so, NOAA says that not all bleaching events are due to warmer temperatures. In January 2010, cold water temperatures in the Florida Keys caused a coral bleaching event that resulted in some coral deaths.

The Great Barrier Reef, which covers an area of 344,400 sq km makes up roughly 10 per cent of the world’s coral reef ecosystems. Today, the reef is a Marine Park and World Heritage Area and supports a range of activities and contributes over AUD $5.6 billion each year to the Australian economy and is also responsible for creating over 70,000 jobs.
In the 2019 Outlook Report prepared by the Australian government, it said that climate change was the greatest threat to the Reef. Other threats included coastal development, land-based run-off and direct human use, such as activities like illegal fishing.
Significantly, coral bleaching events such as the ones that took place in 2016 and 2017 have had severe impacts on the Reef, causing changes in its ecosystem. However, some parts of the reef that escaped the impact of the bleaching and cyclones still remain in good condition, the report said.
The 2016 bleaching caused by intense heat exposure affected the northern third region of the Reef, while the 2017 bleaching affected the central region.
As per the latest Reef health update, while sea surface temperatures have remained relatively stable this week (until March 5, 2020), the temperature is still above normal for this time of the year. As of March 3, most of the marine park varies between 0.5 degree-1.5 degree Celsius above normal. In some inshore areas, the temperature is 2.5-3 degrees Celsius warmer.

Source;IE