Intra‐annual occurrence and risk of humid heatwave in the Northern Hemisphere

Humid heatwaves negatively affect human health due to the integrating effect of temperature and humidity, and thus the early warning and timely mitigating on climate extremes are essential. Yet, systematic assessment on the intra‐annual onset and end of humid heatwaves, which is associated to the occurrence of first and last humid heatwaves, are missing globally. Using a new station‐based data set of daily maximum wet‐bulb temperature, the start and end dates, cumulative anomaly and extremely humid heat of the first and last humid heatwaves in the Northern Hemisphere were explored. It was found that at 91.54% of stations, humid heatwaves started earlier or ended later in the period of 2001–2020 compared to 1981–2000. High cumulative anomalies of the first or last humid heatwaves were found in the mid‐ and high‐latitude regions. Average difference between all humid heatwaves and the first humid heatwaves in cumulative anomalies increased steadily at stations north of 35°N. At regional scales, South East Asia had become the most prominent area with intensification of intra‐annual onset and end of humid heatwaves and will experience more frequent extreme events by 2100.

Furthermore, our focus goes from physical understanding to exposure impacts. Human exposure to humid heatwaves develops with the significant intensification of extreme humid-heat and population agglomeration. Although urban areas are typical spaces of the heat stress, urban heat is expanding outward to rural areas spatially. However, the difference of long-term changes and attributions between urban and rural human exposure to humid heatwaves is still unclear, especially lacking global comparisons supported by continuous series. We also used the new wet-bulb temperature dataset and integrated scenario data to assess historical and future human exposure to humid heatwaves in the Northern Hemisphere. The differences between urban and rural areas in the contribution of enhanced heatwaves and increasing population were quantified. The results showed that about 96.62 % of the stations had pronounced increases in human exposure among those with significant changes. The domination of enhanced heatwaves to human exposure rate was stronger in urban areas in typical developed countries, while domination of increasing population was higher in rural areas in eastern China, with 87.5 % of rural stations dominated by population growth. Under extremely increasing conditions in SSP5 scenario, average rates of human exposure to humid heatwaves in rural areas would be 11.78 % higher than urban areas.

Our findings demonstrated more intensified characteristics of the intra‐annual onset and end of humid heatwaves and provide a scientific cognition for the local risk of humid heatwaves.