A new magnitude-based approach to detect year-round warm spells and their recent intensification

Warm extremes are intensifying in a warming world; however, most heat-related metrics focus on summer heatwaves. Therefore, linear trends often underestimate anomalously warm conditions occurring throughout the year. This limitation hampers our ability to robustly quantify changes in the frequency and severity of warm spells. Here, we assess global and regional changes in year-round warm spells using the Warm Spell Magnitude Index daily (WSMId), a dimensionless and additive metric that integrates warm-spell intensity, duration, and frequency consistently across seasons.

We calculate WSMId with the aid of ERA5 daily maximum temperature data for the period 1980–2024, aggregating warm-spell magnitudes at annual and seasonal time scales to assess long-term changes in the cumulative severity of warm spells. Trend analysis reveals widespread and statistically significant nonlinear intensification of warm spells since 1980, with more than 70 % of global land areas exhibiting a multi-fold increase in annually aggregated warm-spell magnitude.

Spatial patterns indicate especially large relative increases in warm-spell severity in tropical regions, while pronounced intensification is also evident across the mid-latitudes. Comparison with the Warm Spell Duration Index, an index that captures only event duration, confirms the robustness of the detected trends and underscores the enhanced sensitivity of WSMId, which aggregates changes in warm-spell duration and intensity.

Overall, these findings demonstrate that warm spells are not only intensifying but are increasingly compounding across seasons, emphasizing the value of magnitude-based frameworks for assessing escalating thermal stress under climate change.