An interpreter at negotiations listens not only to the volume of the voice but also to shifts in topic. Climatologists have long listened only to volume: the average temperature. But if the average doesn't change, the weather can still become different. A new method compares not averages but how often different temperatures occur.
Scientists took temperature data from 1998 to 2012, split the period into two halves, and looked at how the distribution of temperatures changed at each point on the planet. The distribution is how often different values occur. They found two important changes. In the Pacific Ocean, hot days became rarer and cold days more frequent, even though the average temperature didn't change. In the Arctic, in the Barents and Kara Seas, the average temperature rose, but the key change was in the very shape of the distribution.
It is now clear that even without a change in the average, the climate can change significantly. This method will help test climate models and monitor extreme weather, especially where ice is melting.
The Wasserstein distance is sometimes called the "earth mover's distance" because it quantifies the minimal work needed to move a pile of earth from one shape to another. In climate, the "pile of earth" is the temperature distribution, and the "work" is the number of days by which extremes have shifted.