An 'extreme' heat wave has hit the seas around the UK and Ireland. Here's what's going on

One of the most severe marine heat waves on the planet is taking place in the shallow seas around the U.K. and Ireland. That's according to the U.S. National Oceanic and Atmospheric Administration (NOAA), which has labeled this a "Category 4" heat wave. Rarely used outside of the tropics, a cat 4 heat wave means "extreme" heat.