The authors examined the contrasting factors behind the distinct increases in hot days (HDs, daytime hot extremes) and warm nights (WNs, nighttime hot extremes) over land in the Northern Hemisphere during the boreal summers (June to August) of 1980–2018. While the occurrence of HDs has increased gradually since 1980, that of WNs increased abruptly during the late 1990s and has changed little since then. This sudden increase observed in the occurrence of WNs was found to be related to the low frequency variability in sea surface temperature, including the Pacific Decadal Oscillation/Interdecadal Pacific Oscillation. Comparisons of observational data with historical simulations by CMIP5 climate models under different forcings suggest that the increasing trend in observed HDs was largely due to anthropogenic forcing, but that the observed regime shift-like increase in WNs could not be reproduced by external forcing alone. Unlike the observations, the CMIP5 models exhibited accelerated increasing trends in both HDs and WNs, indicating distinct factors controlling the observed trends in HDs and WNs.

作者研究了 1980 年至 2018 年北半球夏季（6 月至 8 月）期间北半球陆地上热天（HD，白天极端炎热）和暖夜（WN，夜间极端炎热）明显增加背后的对比因素。虽然自 1980 年以来 HD 的发生率逐渐增加，但 WN 的发生率在 1990 年代后期突然增加，此后几乎没有变化。发现这种在 WN 发生中观察到的突然增加与海面温度的低频变化有关，包括太平洋年代际振荡/年代际太平洋振荡。CMIP5 气候模型在不同强迫下的观测数据与历史模拟的比较表明，观测到的 HDs 的增加趋势主要是由于人为强迫，但是观察到的 WN 中类似政权转移的增加不能仅通过外部强迫来复制。与观察结果不同，CMIP5 模型在 HDs 和 WNs 中表现出加速增长的趋势，表明控制 HDs 和 WNs 观察到的趋势的不同因素。