Another day, another extreme weather event with a one-in-a-something chance. This time, ocean surface temperatures are breaking all records, resulting in marine heatwaves stretching from New Zealand to the Galápagos Islands to the British Isles. The chance of this unprecedented heat? One-in-256,000, according to Brian McNoldy, a hurricane researcher at the University of Miami.

Thousands of fish have washed up on the coast of Texas, killed by the ocean’s low oxygen levels (a result of rising temperatures). They aren’t the only ones, and they won’t be the last: Thomas Smith, a professor of environmental geography at the London School of Economics, told the Washington Post that if the heatwaves surrounding Ireland and the UK continue, they could lead to the mass death of kelp, seagrass, oysters and fish. 

Then, depending on the direction of prevailing winds, Smith said, they will lead to “a more turbulent atmosphere, and the associated storms and heavy rainfall”. Or, if the winds don’t blow, “the surrounding heat has the potential to form a heat dome that might exacerbate summer heatwaves”.

Just as increased atmospheric heat results in higher sea temperatures, ocean warming influences the whole weather system. Warmer surface waters can lead to stronger, wetter storms and hurricanes, and conversely to droughts and heatwaves. But what happens below the ocean’s surface can be even worse. 

Dan Smale, the lead scientist of a 2019 study into marine heatwaves, compared them to wildfires, destroying kelp and seagrasses as a fire does forests—and in turn wiping out a major vehicle for carbon sequestration. Toxic algal blooms follow, like smoke after fire, consuming precious oxygen and poisoning everything up the food chain. Rising ocean temperatures also cause coral bleaching. 

Put all this together, and the foundation of many of the world’s marine ecosystems is put at risk, threatening a food source that more than 3 billion people depend on.

Explaining the changes in heat is complex, but the scientific consensus is that the major causes are El Niño and climate change. The warm El Niño phase of the El Niño-Southern Oscillation can raise Pacific Ocean temperatures by as much as 2°C, resulting in cascading weather system effects across the globe. Climate change, meanwhile, now forms the base upon which natural weather variation is founded: ocean warming due to greenhouse gases is reportedly almost 1°C.

The last record for ocean heat was in 2016, an El Niño year. Then, the Pacific was racked by a three-year-long marine heatwave that killed more than 100 million cod and a million seabirds. Most of the years since have been La Niña, leading to cooler Pacific waters that have masked the subsequent years of warming. 

With the return of El Niño, climate change reveals itself again in full force and, with it, the crimes of those pushing ahead with massive fossil fuel projects. When these “carbon bombs” detonate, they will hit the world’s oceans, reefs and marine ecosystems like wildfire.