An interpreter at a negotiation hears not only the volume of the voice but also the shift in topic. Climatologists have long listened only to volume: the average temperature. If it rises, the planet is warming. But behind the average hide events that matter more than the trend: heat waves, cold snaps, sudden changes. In the first decade of the 21st century, warming slowed, and average trends showed almost no change, even though shifts were occurring in the system. The new method, Wasserstein Stability Analysis (WSA), compares not averages but entire weather distributions.

A distribution is how often different temperature values occur. WSA measures how much the entire shape of the distribution has changed, not just its center. To do this, the authors took surface air temperature data from the ERA5 reanalysis (weather reconstructed from models and observations) from 1998 to 2012, split the period into two halves, and built distributions for each point on the planet. Then they applied the Wasserstein distance, sometimes called the 'earth mover's distance': it shows how much minimal work is 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.

WSA found two large significant signals. The first is in the central-eastern Pacific Ocean: hot extremes (above two standard deviations, about 27 °C) became less frequent by more than 10 days per year, while cold extremes (below minus two standard deviations, about 19 °C) became more frequent by the same amount. This resembles a shift toward La Niña conditions (the Pacific phase with cold water) and agrees with earlier findings that during the warming hiatus the Pacific was in the negative phase of the Pacific Decadal Oscillation (slow temperature fluctuations of the Pacific). The second signal is in the Barents and Kara Seas. There, WSA showed a significant distribution change at point B (66°E, 79°N), where the average temperature rose from −10.2 °C to −6.5 °C and the standard deviation decreased.

The cause is sea ice melt. Ice used to tightly cap the maximum air temperature near the freezing point, giving the distribution a sharp peak. As ice diminished, the cap loosened and the peak smoothed out. At point C (131°E, 79°N), where the ice cover shrank less, the distribution change was weaker and did not pass the significance threshold. Sea ice acts as a 'ceiling' for air temperature, and its disappearance changes not only the average but the very structure of variability.

Previously it was unclear how to detect climate change during periods when the average temperature barely moves. Now it is clear that even without a trend, the temperature distribution can change substantially. The method can be applied to validate climate models and to monitor changes in extreme weather, especially in regions with sea ice. Currently, WSA can only tell that the distribution has changed, not in which direction: whether there is more heat or more cold. If directionality can be added, WSA could become a standard tool for real-time monitoring of climate shifts. The interpreter who hears the topic shift will finally be able to say what exactly is being talked about.