| Imagine a slow-moving wildfire that swept around the globe, jumping from one country to the next, killing and weakening millions of trees in the richest and most spectacular forests on the planet. Such a spectacle would be unmissable. Smoke, flames and the suffering of habitats and wildlife would be traumatic and devastating. But when it happens underwater – as it has to the planet’s coral reefs over the past year – the impacts are not so obvious. Across the Atlantic, Pacific and Indian oceans, coral reefs have been turning bone white and, in some cases, dying from record-breaking ocean temperatures. Coral reefs matter because although they cover less than 1% of the ocean about a quarter of all marine species are found on them. They bring tourism cash, food and coastal protection for hundreds of millions of people. For anyone lucky enough to have snorkelled or dived on a healthy tropical coral reef, they are an overwhelming blaze of colour, life and beauty. Earlier this month, the US government’s National Oceanic and Atmospheric Administration (NOAA) declared a fourth planet-wide mass coral bleaching event was under way. An underwater wildfire striking reefs in the waters of more than 50 countries. The extent of the bleaching is likely to be the highest on record. The agency’s Coral Reef Watch (CRW) says already 54% of the ocean that contains coral reefs have seen levels of heat stress high enough to cause bleaching. CRW uses a measure known as “degree heating weeks” (DHWs) to assess the heat stress corals have been exposed to, and it’s worth understanding this metric. If a reef is exposed, for example, to temperatures 1C above the usual maximum for one week, then it has accumulated 1DHW. Coral Reef watch considers “bleaching level” heat stress starts at 4DHWs but the actual thermal limits differ from one species of coral to the next. CRW was forced earlier this year to add three new levels to its global warning system which used to top out at 8DHW but now goes to 20DHW and above. Australia’s Great Barrier Reef, the planet’s biggest coral reef system, which covers an area a little larger than the size of Italy, has also experienced its most widespread bleaching on record. Aerial surveys of 1,080 individual reefs across the GBR carried out by government scientists found that at least 10% of the corals on 73% of reefs had bleached. But in the southern section of the reef, most reefs had at least 61% of corals bleached. What does this mean for the corals? Bleaching is the process when corals expel the tiny symbiotic algae that give them most of their nutrients and colour. If bleaching is mild, corals can recover their symbionts, although they can be more susceptible to disease and their reproductive capabilities take a hit. Dead corals are quickly overrun by algae. Under the water in the south of the GBR, the scene is grim. Some places here have seen DHWs of about 15 – the highest ever recorded on the Great Barrier Reef. This week, journalist Joe Hinchliffe visited Heron Island, a tiny island about 45 miles off Australia’s east coast, with two renowned coral scientists, Prof Terry Hughes and Dr Selina Ward, for Guardian Australia. Hughes, like colleagues in Florida and across the Americas who witnessed extreme bleaching last year, was traumatised. “They said the bleaching was extensive and uniform. They didn’t say it was extensive, uniform and fucking awful,” Hughes said. “It’s a graveyard out there.” About 90% of the extra heat that humans are trapping around the planet – mostly by burning fossil fuels – is taken up by the oceans. Nowhere is that white heat more visible than across the planet’s coral reefs. Read more on our oceans: | 