Australia’s Great Barrier Reef has been hit by a major coral bleaching event likely caused by heat stress built up over the summer at the biologically diverse site, a government agency has said.

The Great Barrier Reef Marine Park Authority (GBRMPA), which monitors the reef’s health, said on Friday that the damage was “widespread” in at least two-thirds of the areas surveyed at the UNESCO World Heritage Site.

The reef, a living organism, stretches some 2,300km (1,429 miles) off the northeastern coast of Australia and is considered one of the most species-rich habitats on earth. It is home to hundreds of coral, 1,500 fish species and 4,000 different molluscs.

Coral reefs are an important part of marine life as they house more than a quarter of all sea life on Earth, provide protection for coastal communities, and are natural carbon sinks. Climate change is the biggest threat to their survival because of their sensitivity to heat.

Bleaching causes corals to lose the colourful algae living in their tissues, turning them white. Corals can survive a bleaching event although it can stunt their growth and affect reproduction.

Prolonged, or intense heat could cause coral deaths.

“The results are consistent with what we have seen with above average sea surface temperatures across the Marine Park for an extended period of time,” Roger Beeden, chief scientist at GBRMPA, said in a statement.

“While the area’s aerial surveys are showing that the coral bleaching is extensive in the shallow water areas, we will need in-water surveys to confirm the severity of the coral bleaching and also what the depth range is.”

“We know the biggest threat to coral reefs worldwide is climate change. The Great Barrier Reef is no exception,” environment minister Tanya Plibersek said in a statement. “We need to act on climate change. We need to protect our special places and the plants and animals that call them home.”

The bleaching event is similar to reports on reefs around the world due to elevated sea surface temperatures primarily driven by climate change but amplified by the effects of the El Nino phenomenon, which usually results in warmer ocean waters, the agency added.

Earlier this week, the United States National Oceanic and Atmospheric Administration said the world was on the verge of a fourth mass coral bleaching event which could see wide swaths of tropical reefs die, including parts of the Great Barrier Reef.

The World Wide Fund for Nature (WWF) said in a statement that the bleaching event, which is one of several in recent years, proved climate change was putting the reef under “tremendous pressure”.

“WWF is very concerned that this bleaching event is unfolding in an area where corals have not been previously exposed to these extreme temperatures. Unless we see a significant drop off in temperatures in the next few weeks, the risk of significant coral mortality is high,” said Richard Leck, WWF-Australia’s head of oceans.

The Great Barrier Reef saw mass bleaching events in 1998, 2002, 2016, 2017, 2020 and 2022, according to the Australian Institute of Marine Science.

The United Nations’ cultural and scientific agency (UNESCO) has proposed to put the Great Barrier Reef on an endangered list but Australia has lobbied intensively to block that move for years.

The reef is a major tourist attraction for the country, contributing about $6 billion Australian dollars (US$4bn) to the economy, and providing 64,000 jobs. A UNESCO endangered listing could mean it loses this heritage status, and could make it less attractive to tourists.