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

Jiaojiao Liu,Xiangjun Shi

doi:10.5194/acp-21-10609-2021

Abstract

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±0.18 W m−2, which is approximately two-thirds of that from artificially turning off homogeneous nucleation (-1.98±0.26 W m−2). However, simulations with fixed seeding ice nuclei particle number concentrations of 20 and 200 L−1 show a weak cooling effect of -0.27±0.26 W m−2 and warming effect of 0.35±0.28 W m−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 ± 0.25 W m−2. Overall, the potential cooling effect of cirrus thinning is considerable, and the flexible seeding method is essential.

Highlights

Global warming has been proven by observations and has demonstrated many adverse effects on the environment and economy (Alexander et al, 2006; Feely et al, 2009; Lenton et al, 2019; Milne et al, 2009; Myhre et al, 2013)
This study shows that the potential cooling effect of cirrus thinning cannot be sufficiently exploited due to the fixed seeding method
This study finds that the cooling effect over low-latitude regions is less susceptible to cirrus seeding for other reasons

Summary

Introduction

Global warming has been proven by observations and has demonstrated many adverse effects on the environment and economy (Alexander et al, 2006; Feely et al, 2009; Lenton et al, 2019; Milne et al, 2009; Myhre et al, 2013). Cirrus thinning geoengineering, which allows more longwave radiation to escape to space, leading to a cooling effect on the planet, has been investigated as a new SRM approach and has been proposed in Geoengineering Model Intercomparison Project Phase 6 (GeoMIP6; Kravitz et al, 2015). In GeoMIP6, cirrus thinning is simulated by artificially increasing the sedimentation velocity of ice crystals (ICs). Simulations with this idealized approach indicate that cirrus thinning can produce the desired globally averaged cooling effect (∼ −2.0 W m−2; e.g., Gasparini et al, 2017; Jackson et al, 2016; Muri et al, 2014). Considering the physical feasibility, simulating cirrus thinning by seeding with ice nuclei particles (INPs) is a better approach that can prevent homogeneous nucleation from occurring, thereby decreasing

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