From 1975 on, the global surface ocean had shown a pronounced, though wavering, warming trend. Starting in 2004, however, that warming seemed to stall. Researchers measuring the Earth's total energy budget - the balance of sunlight streaming in compared to the amount of light and heat leaving from the top of the atmosphere - saw that the planet was still holding on to more heat than it was letting out.
But with that energy not going into warming the surface ocean, a traditionally important energy sink, scientists weren't sure where it went. It became known as a case of "missing heat."