How climate change has pushed our oceans to the brink of catastrophe

This time last year, Dean Grubbs and his colleagues were celebrating a conservation success story. The star was the smalltooth sawfish, a type of large ray with a saw-like snout trimmed with tiny teeth. A victim of coastal development and bycatch, in 2003 it became the first sea fish to get US federal protection under the Endangered Species Act. By 2023, Florida’s population of sawfish, the last in the US, was on the rise. “We were excited. We were seeing the population start to recover,” says Grubbs, a marine ecologist at Florida State University.

Then disaster struck. In January, sawfish were spotted thrashing in the shallows, spinning in frenzied circles and turning up dead. This came after months of smaller fish showing similar behaviour. Suddenly, Grubbs and his team were spending their days pulling dead sawfish out of the water. Only after months of investigations and tests is a culprit emerging: ocean heat. A record-breaking heatwave brought “hot tub” water temperatures to Florida’s coast in 2023, triggering a chain reaction that looks to have decimated the fragile sawfish population.

This is just one salutary tale. Something isn’t right in the world’s oceans. From orange algal blooms in the North Sea and a boom in gelatinous Bombay duck fish off China to disappearing “bottom water” in the Antarctic, there is growing evidence that extreme temperatures are wreaking havoc in our waters. After years of the oceans acting as silent sinks for excess human-caused heat, they are starting to creak under pressure – and we are finally waking up to how worried we should be.

Around 90 per cent of the excess…