Anthropogenic aerosol-induced changes in radiative forcing and temperature over the Mediterranean

Since the 1980s, the increase in greenhouse gases and the decrease in anthropogenic aerosols (AA) over Europe and the Mediterranean (MED), driven by air pollution control policies, along with anthropogenic land-use changes, have been linked to the enhanced regional warming trend in the MED, particularly during boreal summer (JJA). However, the effect of AA changes on the temperature trend over this region requires further investigation. This study examines temperature changes attributed to AA over the MED during the historical period (1850-2014) with respect to changes in effective radiative forcing (ERF) based on experiments from 11 CMIP6 Earth System Models. The transient shortwave ERF due to AA is assessed using the “histSST” and “histSST-piAer” experiments, which share the same historical forcings driven by prescribed sea surface temperatures and sea ice from the corresponding coupled models except that “histSST-piAer” uses pre-industrial aerosol precursor emissions from the year 1850. Shortwave ERFs due to aerosol-radiation interactions and aerosol-cloud interactions, as quantified by the approximate partial radiative perturbation (APRP) method,  present a negative peak in 1965-1984 relative to 1850 over the MED region (multi-model means of -2.17±0.82 W m^-2 and -3.08±1.85 W m^-2, respectively), exhibiting trends towards less negative values in recent past (1995-2014) relative to the 1965-1984 period (changing by 0.94±0.37 W m^-2 and 0.51±1.02 W m^-2, respectively). Furthermore, we quantify the response in near-surface air temperature (monthly mean, and daily maximum and minimum) caused by AA on an annual and seasonal basis using the “historical” and “hist-piAer” experiments, which are driven by historical forcings except that “hist-piAer” uses fixed pre-industrial AA and aerosol precursor emissions. Consistent with the negative ERF changes, we find a surface cooling of -1.23±0.56 K in the 1965-1984 period relative to 1850 on an annual basis. During 1995-2014 there is an annual mean increase of 0.25±0.36 K relative to 1965-1984 pointing towards an amplification of warming due to AA reduction. The regional surface warming in 1995-214 relative to 1965-1984 is more prominent in JJA (0.26±0.34 K) than in boreal winter (0.18±0.57 K) as models show lower agreement on the sign of change during wintertime. Similar results are derived for maximum and minimum temperatures regarding the magnitude and the trend of changes over the MED region.

This work is implemented within the research project REINFORCE (impRovEments in the simulation of aerosol clImate liNkages in earth system models: From glObal to Regional sCalEs) in the framework of HFRI call “Basic Research Financing (Horizontal Support of all Sciences)” under the National Recovery and Resilience Plan “Greece 2.0” funded by the European Union – NextGenerationEU (HFRI; project no. 15155).