Think about a pot of hot water vs. one that's boiling. The surface of the water in the boiling pot is not smooth, it has peaks and troughs. The level of some of those troughs will be lower than the level of the water in a less energetic pot.
The situation with global warming is similar, although much less simple (pots of water don't have seasons, among other things.) More energy in the atmosphere and the Earth's crust translates into more extreme temperature fluctuations, both up and down. However, the average will inevitably be higher. Basic physics doesn't allow for anything else.