Cirrus cloud susceptibility to ice nuclei perturbations in the upper troposphere

Due to their net warming effect, cirrus clouds play a crucial role in the climate system. A recently proposed climate engineering mechanism (CEM) intends to reduce high cloud cover by seeding cirrus clouds with efficient ice nuclei (IN) and therefore cool climate. Here, the susceptibility of cirrus clouds to ice nuclei perturbations in the upper troposphere is investigated in the extended Community Atmospheric Model version 5 (CAM5). Due to large uncertainties associated with the dominant ice nucleation mechanism in cirrus clouds, different control cases were simulated, varying the relative importance of homogeneous and heterogeneous ice nucleation. Whereas additional IN in the pure heterogeneous case leads to a strong warming, the opposite was true for all other control cases. A global mean radiative forcing of -2Wm-2 was obtained in a simulation that introduced particularly efficient IN into the upper troposphere, thereby testing the proposed CEM involving cirrus clouds. Intriguingly, a globally nonuniform injection of IN, with no IN injection in the summer hemisphere and in the tropics, leads to an even stronger cooling effect than globally uniform IN injection. Finally, we present the results of equilibrium climate simulations with and without IN perturbations, and show that climate engineering in the form of IN injection in the upper troposphere at high latitudes may cool the planet by as much as 1.4 K, and avoids many of the caveats associated with the more traditional geoengineering mechansims.