Abstract. Climate models simulate lower rates of North Atlantic heat transport under greenhouse gas climates than at present due to a reduction in the strength of the Atlantic Meridional Overturning Circulation (AMOC). Solar geoengineering whereby surface temperatures are cooled by reduction of incoming shortwave radiation may be expected to ameliorate this effect. We investigate this using six Earth system models running scenarios from GeoMIP (Geoengineering Model Intercomparison Project) in the cases of (i) reduction in the solar constant, mimicking dimming of the sun; (ii) sulfate aerosol injection into the lower equatorial stratosphere; and (iii) brightening of the ocean regions, mimicking enhancing tropospheric cloud amounts. We find that despite across-model differences, AMOC decreases are attributable to reduced air–ocean temperature differences and reduced September Arctic sea ice extent, with no significant impact from changing surface winds or precipitation − evaporation. Reversing the surface freshening of the North Atlantic overturning regions caused by decreased summer sea ice sea helps to promote AMOC. When comparing the geoengineering types after normalizing them for the differences in top-of-atmosphere radiative forcing, we find that solar dimming is more effective than either marine cloud brightening or stratospheric aerosol injection.

Aerosol particles can strongly affect both air quality and the radiation budget of the atmosphere. Above Beijing, the capital city of China, large amounts of aerosols within the atmospheric column have caused the deterioration of local air quality and have influenced radiative forcings at both the top and the bottom of the atmosphere (BOA and TOA). Observations of aerosol radiative forcing and its efficiency have been made using two sun-photometers in urban Beijing between 2013 and 2015, and have been analyzed alongside two air quality monitoring stations' data by dividing air quality conditions into unpolluted, moderately polluted, and heavily polluted days. Daily average PM 2.5 concentrations varied greatly in urban Beijing (5.5-485.0 µg/m 3 ) and more than one-third of the analyzed period is classified as being polluted according to the national ambient air quality standards of China. The heavily polluted days had the largest bottom of atmosphere (BOA) and top of atmosphere (TOA) radiative forcings, but the smallest radiative forcing efficiencies, while the unpolluted days showed the opposite characteristics. On heavily polluted days, the averaged BOA aerosol radiative forcing occasionally exceeded´150 W/m 2 , which represents a value about three-times greater than that for unpolluted days. BOA aerosol radiative forcing was around two-to-three times as large as TOA aerosol radiative forcing under various air quality conditions, although both were mostly negative, suggesting that aerosols had different magnitudes of cooling effects at both the surface and the top of the atmosphere. Unpolluted days had the largest average values of aerosol radiative forcing efficiencies at BOA (and TOA) levels, which exceeded 190 W/m 2 (´70 W/m 2 ), compared with the lowest average values in heavily polluted days of around´120 W/m 2 (´55 W/m 2 ). These results suggest that the high concentrations of particulate matter pollution in the urban Beijing area had a strong cooling effect at both BOA and TOA levels.

Abstract. Climate models simulate lower rates of North Atlantic heat transport under greenhouse gas climates than at present due to a reduction in the strength of the North Atlantic meridional overturning circulation (AMOC). Solar geoengineering whereby surface temperatures are cooled by reduction of incoming shortwave radiation may be expected to ameliorate this effect. We investigate this using six Earth System Models running scenarios from GeoMIP (Geoengineering model intercomparison project) in the cases of: i) reduction in the solar constant, mimicking dimming of the sun; ii) sulfate aerosol injection into the lower equatorial stratosphere; and iii) brightening of the ocean regions mimicking enhancing tropospheric cloud amounts. We find that despite across model differences, AMOC decreases are attributable to reduced air-ocean temperature differences, and reduced September Arctic sea ice extent, with no significant impact from changing surface winds or precipitation-evaporation. Reversing the surface freshening of the North Atlantic overturning regions caused by decreased summer sea ice sea helps to promote AMOC. Comparing the geoengineering types after normalizing them for the differences in top of atmosphere radiative forcing, we find that solar dimming is more effective than either marine cloud brightening or stratospheric aerosol injection.

Author contributions: H.M.L. and Y.M.Q. equally contributed to this work, performed the experiments and drafted the manuscript. M.D.X. analyzed the data. X.Y.D. and H.Z. participated in the experiments. X.L.T. designed the experiments and revised the manuscript. W.D., C.S.O., and Y.H.X. contributed to the edition of the manuscript. All authors have reviewed the results and approved the final manuscript.

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