Climate change: Something strange is happening in the Pacific and we must find out why

FOR years, climate models have predicted that as greenhouse gas emissions rise, ocean waters will warm. For the most part, they have been correct. Yet in a patch of the Pacific Ocean, the opposite is happening. Stretching west from the coast of Ecuador for thousands of kilometres lies a tentacle of water that has been cooling for the past 30 years. Why is this swathe of the eastern Pacific defying our predictions? Welcome to the mystery of the cold tongue.

This isn’t just an academic puzzle. Pedro DiNezio at the University of Colorado Boulder calls it “the most important unanswered question in climate science”. The trouble is that not knowing why this cooling is happening means we also don’t know when it will stop, or whether it will suddenly flip over into warming. This has global implications. The future of the cold tongue could determine whether California is gripped by permanent drought or Australia by ever-deadlier wildfires. It influences the intensity of monsoon season in India and the chances of famine in the Horn of Africa. It could even alter the extent of climate change globally by tweaking how sensitive Earth’s atmosphere is to rising greenhouse gas emissions.

Given all this, it isn’t surprising that climate scientists are trying to find out what is going on with increasing urgency. Like any good mystery, this is a tale of intrigue, confusion and competing theories. We haven’t quite…