Atmospheric temperatures still swing between “cold” and “hot” years, even as they now trend warmer on average. This is because of natural variation in the climate system, as heat cycles between the atmosphere and the ocean through phenomena such as El Niño. Heat records are more likely to be broken when natural variation runs in the same direction as global warming. When the two are out of sync, it is more likely that temperatures will be average or even a little below average. Therefore, the rising trend in the ratio of record highs to record lows is clearest when looking over decades.