ESTIMATING THE COOLING AND BRIGHTENING EFFECT OF CIRRUS THINNING GEOENGINEERING BY HINDERING HOMOGENEOUS ICE NUCLEATION

Abstract:

Solar radiation management (SRM) aims at reducing the incoming solar radiation to cooling the earth has been widely discussed. In recent years, cirrus thinning is proposed to decrease cirrus cloud coverage and lifetime to increase the outgoing longwave radiation (OLR), and then also get the cooling effect. Cirrus thinning implemented by seeding INPs (ice nucleating particles including ice crystals) can hinder homogeneous ice nucleation, which can produce a large number of ice crystals. However, seeding too few INPs might not inhibit entirely homogeneous ice nucleation, and seeding too much INPs might increase cirrus cloud in cloud-free regions. To maximize the cooling effect of cirrus thinning, in this study, we developed a method (optimal seeding method) to calculate the minimum concentration of seeding INPs, which is just enough to prevent homogeneous nucleation from happening. Our simulation results with CAM5 (Community Atmosphere Model version 5) show that the optimal seeding method reduces the global cloud radiative effect for –1.36 W m^-2 (cooling effect), which reach up the two-thirds of the idealized cooling potential (–1.98 W m^-2) derived from the pure heterogeneous simulation (i.e., homogeneous ice nucleation is artificially switched off). While seeding with fixed 20 L^-1 (underseeding simulation) and 200 L^-1 INPs (overseeding simulation) respectively show weaker cooling effect (–0.27 W m^-2) and slight warming effect (0.35 W m^-2). In addition, we also find the surface downwelling shortwave radiation (FSDS) increases in cirrus thinning simulation (+6.68 W m^-2, brightening effect), which is different from other SRM that usually increase the earth's reflectivity. Furthermore, the spatial pattern of the brightening effect is not consistent with the cooling effect. Cirrus thinning shows stronger cooling effect in extra-tropical regions, while in most tropical regions, shows stronger brightening effect (stronger than 10 W m^-2), but weaker cooling effect (weaker than –1 W m^-2). Overall, cirrus thinning by using optimal seeding method shows the obvious cooling effect, and it can be considered that combine cirrus thinning with other SRM to counteract up the dimming effect caused by the other SRM, especially for the tropical regions.