Ozone-aerosol and land use reversed temperature increase over some northern mid-latitude regions between the 20th century and the Little Ice Age based on the CESM-LME

Abstract

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.