But the researchers tried a different approach: they calculated the temperature preferences of fish species – confusingly, they called this the mean temperature of catch – and then they analysed the annual haul of 990 species across 52 large marine ecosystems between 1970 and 2006.
They accounted for possible confounding factors (overfishing being one of them) and then came up with a “fish thermometer”, on the argument that, just as changes in the pattern of tree growth rings would expose the climate history of a forest, so changes in the pattern of fish catches would tell them something about ocean temperatures.
Their new scale of measurement revealed that overall, oceans were warming at a rate of 0.19°C per decade, and in the non-tropical regions even faster: at 0.23°C per decade.