The oceans are a critical component of the climate system, storing roughly 1000 times as much heat, and 50 times as much carbon, as the atmosphere. In this talk, Prof Marshall will discuss the challenges of predicting the evolution of a complex system that is grossly under-sampled and spans a bewildering range of scales in both space and time. These challenges will be illustrated through the important but over-sensationalised problem of how the Gulf Stream may change over the next century and impacts on European climate.