The ocean became a crime scene this week after environmental activists ensnared dead and bleached coral in yellow police tape.

Divers affiliated with the climate advocacy organization 350.org cordoned off colonies of sickly coral near Australia’s Great Barrier Reef, Somoa and the Bay of Bengal’s Andaman Islands.

Huge swaths of coral in those areas are struggling to survive as ocean temperatures rise, in part because of global warming.

SEE ALSO: Watch how coral bleaching happens in warming waters

The culprit for the crime, activists claimed, is U.S. oil and gas producer Exxon Mobil.

The Texas-based energy giant is facing criticism for funding climate denying think tanks in recent decades — a move critics say created public confusion about the realities of climate change and thwarted global efforts to limit greenhouse gas emissions.

Image: bioquest studios

Image: bioquest studios

Donning dive suits and snorkel masks, underwater activists this week held signs emblazoned with #ExxonKnew, referring to investigative reporting that revealed Exxon executives knew about the existential threat of climate change half a century ago, but buried that knowledge to protect the company’s bottom line.

“We’re not going to let them get away with murder,” Bill McKibben, the founder of 350.org and a prominent climate activist, said Wednesday in an op-ed in the Guardian about the coral crime scene.

“We need to remember that there’s nothing natural about this horror. It was caused, as so many crimes are, by greed,” McKibben wrote.

In an interview with Mashable this week, McKibben said the ongoing coral bleaching event in the Pacific Ocean is “one of the things that’s been more powerful and affecting for me so far this year.”

“‘Sobering’ is too sober a word,” he said by phone.

Lauren Kerr, a spokeswoman for Exxon, said allegations that the oil giant was to blame for killing coral reefs, and claims that the company deliberately misled the public on climate change, were “ridiculous.”

“To suggest that Exxon Mobil had reached definitive conclusions, decades before the world’s experts and while climate science was in an early stage of development, is not credible,” Kerr told Mashable in an email.

Suzanne McCarron, Exxon’s vice president of public and government affairs, described the #ExxonKnew campaign as an “inaccurate” portrayal of the company’s nearly 40-year history of climate research.

“It’s ironic that there is a movement under way to demonize our corporation as somehow denying the existence of climate change when we have acknowledged those risks for years,” McCarron wrote Wednesday in an op-ed in the Houston Chronicle.

Coral reefs at risk

The demonstrations highlight the growing threat to the world’s reefs, which are suffering the longest-ever global coral bleaching event on record.

Coral bleaching happens when coral expel the symbiotic algae that live in their tissue and give them color and nutrients. Increased water temperatures, pollution and other stressors can cause this action, which turns coral white or pale and makes them more vulnerable to disease or death.

Bleached corals can recover if ocean waters cool or pollution subsides, but they can die if the stressors last too long. When that happens, it’s not just the coral that suffer: Reefs provide food and shelter for up much as one-fourth of all ocean species.

More than half the reefs worldwide have already been hit twice by the ongoing coral bleaching event, according to satellite observations by the National Oceanic and Atmospheric Administration (NOAA).

At Australia’s Great Barrier Reef, up to 50 percent of previously healthy sections in the north have bleached and died, scientists in Australia said in May. Even higher mortality was observed around small islands in the central Pacific.

NOAA scientists warned in June that the Florida Keys, Puerto Rico, the U.S. Virgin Islands, Guam, Hawaii and the Northern Mariana Islands all stand a significant chance of enduring damaging levels of coral bleaching through the end of 2016.

Coral bleaching near the Maldives, May 10, 2015. Image: UIG via Getty Images

“After more than two years of coral bleaching, the ongoing global coral bleaching event is still in full swing,” Mark Eakin, who coordinates NOAA’s Coral Reef Watch program, said in a statement released by 350.org.

The most severe spot for coral bleaching is near the equator in the central Pacific, in the heart of the El Niño zone, where warmer-than-average ocean waters tend to develop and persist.

An intense El Niño event only recently subsided, and likely helped set off the bleaching event in many areas.

Prior to this event, the only two previous observed global coral bleaching episodes were tied to an El Niño.

But Eakin said this ongoing bleaching event — the worst yet — is largely due to human-caused global warming, which has raised ocean temperatures enough so that even a weak El Niño can set off serious bleaching.

Image: Bioquest studios

As countries burn oil, coal and natural gas for energy, and as industrial farmers and loggers eliminate forests, heat-trapping emissions of greenhouse gases have soared in recent decades. The oceans have absorbed the vast majority of that heat — about 93 percent — putting unprecedented stress on coral and other species that cannot withstand warmer waters.

“The problem of widespread coral bleaching is a new one, brought about by global warming in the 1980s and continuing to grow stronger and return more frequently,” Eakin said in the statement.

“Unless we can get atmospheric [carbon dioxide] levels back down to those that are healthy for coral reefs, we are going to lose one of the most diverse and valuable ecosystems on Earth,” he said.

Mashable science editor Andrew Freedman contributed reporting to this story.