Global warming is a widely concerned topic, and the surface temperature has shown an accelerated warming trend during the past several decades. From the perspective of a longer time scale, the 20th century (1900–2000) could be the warmest period in the last millennium, and the global or hemispheric averaged temperature over this period is higher than that over other centuries, particularly compared with that over the Little Ice Age (LIA; 1450–1850). However, we recently found that, in the reconstruction, the 20th-century temperature over some northern mid-latitude regions could be significantly lower than that during the LIA, which contradicts our previous perceptions regarding global warming. Modeling results from the Last Millennium Ensemble Project also reproduce a similar phenomenon, that is, the 20th-century cooling over some northern mid-latitude regions (CNMR). The simulated CNMR can be found in all four seasons. Further analysis indicates that the cooling effects from ozone-aerosol and land use forcings, overcoming greenhouse gas and solar forcing induced warming, play dominant roles in causing the CNMR. The ozone-aerosol forcing reduces the surface net shortwave flux through both direct aerosol–radiation interaction and atmosphere–cloud feedback, while the land use forcing causes negative net shortwave flux anomalies through modulation of surface albedo. Overall, the ozone-aerosol and land use forcings shape the CNMR phenomenon by inducing anomalous surface net shortwave flux, with the ozone-aerosol forcing playing a dominant role. This study highlights the important influences of ozone-aerosol and land use cooling effects on local climate.

全球变暖是一个广泛关注的话题，地表温度在过去几十年中呈现出加速变暖的趋势。从更长的时间尺度来看，20世纪（1900-2000年）可能是上千年最暖的时期，该时期全球或半球平均气温高于其他世纪，特别是与小冰河时代（LIA；1450-1850）。然而，我们最近发现，在重建中，北部中纬度一些地区的 20 世纪气温可能明显低于 LIA 期间的气温，这与我们之前对全球变暖的看法相矛盾。Last Millennium Ensemble Project 的建模结果也再现了类似的现象，即，20 世纪北部中纬度地区的降温 (CNMR)。模拟的 CNMR 在所有四个季节都可以找到。进一步分析表明，臭氧气溶胶和土地利用强迫的冷却效应，克服温室气体和太阳强迫引起的变暖，在引起 CNMR 中起主导作用。臭氧-气溶胶强迫通过直接的气溶胶-辐射相互作用和大气-云反馈降低了地表净短波通量，而土地利用强迫通过调节地表反照率导致负净短波通量异常。总体而言，臭氧气溶胶和土地利用强迫通过诱导异常地表净短波通量来塑造 CNMR 现象，其中臭氧气溶胶强迫起主导作用。