England has another possible explanation for why the IPCC models could be getting future La Niña-like conditions wrong. As the world warms and the Greenland ice sheet melts, its fresh cold water is expected to slow down a dominant conveyor belt of ocean currents: the Atlantic Meridional Overturning Circulation (AMOC). Scientists mostly agree that the AMOC current has slowed down in recent decades4, but don’t agree on why, or how much it will slow in future.

In a study published in Nature Climate Change on 6 June5, England and his colleagues model how an AMOC collapse would leave an excess of heat in the tropical South Atlantic, which would trigger a series of air-pressure changes that ultimately strengthen the Pacific trade winds. These winds push warm water to the west, thus creating more La Niña-like conditions. But England says that the current IPCC models don’t reflect this trend because they don’t include the complex interactions between ice-sheet melt, freshwater injections, ocean currents and atmospheric circulation. “We keep adding bells and whistles to these models. But we need to add in the ice sheets,” he says.

Michael Mann, a climatologist at Pennsylvania State University in State College, has also argued that climate change will both slow the AMOC and create more La Niña-like conditions. He says the study shows how these two factors can reinforce each other. Getting the models to better reflect what’s going on in the ocean, says Seager, “remains a very active research topic”.