Warmer southerly waters, which are saltier and denser, flow north to cool and sink below waters at higher latitudes, releasing heat into the atmosphere.
Then, once it has sunk beneath the ocean, the water slowly drifts southward, heats up again, and the cycle repeats. But climate change is slowing this flow. Fresh water from melting ice sheets has made the water less dense and salty
This is the premise for the 2004 film The Day After Tomorrow, itself based on the 1999 book The Coming Global Superstorm (co-written by Coast to Coast AM radio host Art Bell). Not a new concern, I think the change for these guys is trying to put a timeline on it.