Warm surface water travels northwards in the Atlantic ocean, raising the temperature of western Europe. After giving up its heat to the overlying atmosphere, some of this water sinks and returns southwards at depth; the remainder flows south in the overlying surface layer (Fig. 1). Model simulations suggest that this system of currents, coined the Atlantic meridional overturning circulation (MOC), has slowed slightly since the mid-nineties1. Predictive models suggest that this trend is set to continue over the next century2. Writing in Nature Geoscience, Lozier and colleagues3 use historical records of pressure, temperature and salinity in the North Atlantic to show that during the latter half of the twentieth century, the overturning circulation weakened in subtropical waters, but strengthened in subpolar waters.