Estimating the potential cooling effect of cirrus thinning achieved via the seeding approach

Abstract

Cirrus thinning is a newly emerging geoengineering approach to mitigate global warming. To sufficiently exploit the potential cooling effect of cirrus thinning with the seeding approach, a flexible seeding method is used to calculate the optimal seeding number concentration, which is just enough to prevent homogeneous ice nucleation from occurring. A simulation using the Community Atmosphere Model version 5 (CAM5) with the flexible seeding method shows a global cooling effect of -1.36±0.18Wm-2, which is approximately two-thirds of that from artificially turning off homogeneous nucleation (-1.98±0.26Wm-2). However, simulations with fixed seeding ice nuclei particle number concentrations of 20 and 200L-1 show a weak cooling effect of -0.27±0.26Wm-2 and warming effect of 0.35±0.28Wm-2, respectively. Further analysis shows that cirrus seeding leads to a significant warming effect of liquid and mixed-phase clouds, which counteracts the cooling effect of cirrus clouds. This counteraction is more prominent at low latitudes and leads to a pronounced net warming effect over some low-latitude regions. The sensitivity experiment shows that cirrus seeding carried out at latitudes with solar noon zenith angles greater than 12 could yield a stronger global cooling effect of -2.00±0.25Wm-2. Overall, the potential cooling effect of cirrus thinning is considerable, and the flexible seeding method is essential.