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Widespread coral bleaching at Ningaloo Reef off Western Australia's coast has deeply alarmed scientists and conservationists.

Photos captured by divers, , show severe bleaching at several sites along the reef, which runs for 260 kilometers off the state's northwest.

A severe marine heat wave in the Indian Ocean off WA has caused the coral bleaching. In some places, surface temperatures up to have been recorded.

Hotter temperatures aren't only happening at the ocean's surface—data indicates they also extend . Warm, deeper water can shut down the ocean's natural cooling process, putting corals at even greater risk of bleaching.

Counting the cost

The full extent of damage to Ningaloo won't be known until scientists conduct field surveys in coming months.

So far, bleaching has been documented at several sites, including Turquoise Bay, Coral Bay, Tantabiddi, and Bundegi (Exmouth Gulf).

Other sites such as Scott Reef, Ashmore Reef, the Rowley Shoals and Rottnest Island are also at risk.

Damage wrought by the heat wave extends beyond coral. More than 30,000 fish have died since the September onset.

The images below show the heat wave's progression. Temperatures from February last year are included for comparison.

The white circle shows the location of Ningaloo. Cooler temperatures are in blue and purple. Warmer temperatures are in yellow and orange.

The images show the heat wave reached Ningaloo in December last year and moved south in January. Temperatures fell slightly in February due to strong southerly winds. From March, temperatures are forecast to increase again.

A complex warming picture

According to and , hotter ocean temperatures off northern WA run several hundred meters deep.

This has been caused by developing . , influence ocean temperatures and weather patterns across the Pacific.

During La Niña, trade winds strengthen and push warm water westward. This intensifies two important ocean currents.

The first is the —which carries warm Pacific waters through the Indonesian seas and into the eastern Indian Ocean. The second is the , which picks up this warm water and takes it further south towards Perth.

This has led to a build-up of hotter water along the WA coastline.

La Niña is also affecting WA's reefs in other ways.

Some coral reefs are naturally cooled , colder water towards the surface. This process, which has been likened to an ocean's "air conditioner," can for reefs.

The process relies on ""—that is, layers of seawater that differ in temperature, salinity and density (or weight). Warmer, less dense water collects at the surface and colder, denser water falls to deeper levels.

La Niña conditions can suppress, or even shut down, this cooling effect in two ways.

First, it reduces the . This causes water to . Second, it increases water temperatures at depth.

All this means the water pumped to the surface isn't much cooler than temperatures at the surface.

For many reefs along the coast of WA, the suppression of this tidal cooling is probably contributing to worsening conditions, and more coral bleaching.

Most bleaching forecasts rely on sea surface temperatures. This means scientists may be underestimating the vulnerability of deeper reefs.

What's in store for Ningaloo and surrounds?

Looking ahead, the situation at Ningaloo and surrounding reefs remains critical.

quickly. This means theoretically, Ningaloo and other affected reefs may survive the summer.

But unfortunately, temperatures are rising again and the marine heat wave is expected to continue until April, as the image below shows.

Climate change is making marine heat waves more . It means reefs often don't have time to recover between destructive bleaching events.

All this is compounded by the general trend towards warmer oceans as the planet heats up.

Drastic action on climate change is needed now. If this alarming pattern continues, the world's reefs .