Cirrus cloud seeding: a climate engineering mechanism with reduced side effects?

Abstract

Climate engineering, the intentional alteration of Earth's climate, is a multifaceted and controversial topic. Numerous climate engineering mechanisms (CEMs) have been proposed, and the efficacies and potential undesired consequences of some of them have been studied in the safe environments of numerical models. Here, we present a global modelling study of a so far understudied CEM, namely the seeding of cirrus clouds to reduce their lifetimes in the upper troposphere, and hence their greenhouse effect. Different from most CEMs, the intention of cirrus seeding is not to reduce the amount of solar radiation reaching Earth's surface. This particular CEM rather targets the greenhouse effect, by reducing the trapping of infrared radiation by high clouds. This avoids some of the caveats that have been identified for solar radiation management, for example, the delayed recovery of stratospheric ozone or drastic changes to Earth's hydrological cycle. We find that seeding of mid- and high-latitude cirrus clouds has the potential to cool the planet by about 1.4 K, and that this cooling is accompanied by only a modest reduction in rainfall. Intriguingly, seeding of the 15% of the globe with the highest solar noon zenith angles at any given time yields the same global mean cooling as a seeding strategy that involves 45% of the globe. In either case, the cooling is strongest at high latitudes, and could therefore serve to prevent Arctic sea ice loss. With the caveat that there are still significant uncertainties associated with ice nucleation in cirrus clouds and its representation in climate models, cirrus seeding appears to represent a powerful CEM with reduced side effects.

An intensified hydrological cycle in the simulation of geoengineering by cirrus cloud thinning using ice crystal fall speed changes

“…In a simulation using a coupled atmosphere‐ocean general circulation model (GCM), CCT cooled Earth's climate by 1.4°C [ Storelvmo et al ., ]. Climate model simulations of the climate response to CCT, in which CCT was approximated by an increase in ice particle fall speed (fall speed was doubled by Muri et al [] and quadrupled by Crook et al []), yielded reductions in CRF and cooling of global annual mean temperature (−0.94°C and −1.0°C, respectively).…”

“…Bala et al [], Tilmes et al [], and Niemeier et al [] all found that the reduction in net SW radiation flux at Earth's surface from SRM was associated with reduced surface evaporation and weakening of the hydrological cycle. In simulations of CCT, however, global mean precipitation changes were small [ Muri et al ., ] or involved relatively small reductions [ Storelvmo et al ., ]. In a comparison of CCT and SRM methods using the same climate model and identical simulation design, Crook et al [] found that CCT increased global precipitation in contrast to decreases found for all of the SRM methods tested.…”

Sulfate geoengineering: a review of the factors controlling the needed injection of sulfur dioxide

“…Recent studies by Storelvmo et al (2013Storelvmo et al ( , 2014 have quantified the direct radiative effects produced by seeding uppertropospheric cirrus ice clouds with large IN. Although this is not directly related to our specific discussion on SG side effects, it can be considered as indirect evidence of the importance of correctly understanding the balance between the complex microphysical processes regulating the formation and growth of upper-tropospheric ice particles.…”

Cirrus Clouds and Their Response to Anthropogenic Activities

“…The spatially non-homogeneous forcing induces circulation changes with the risk of side effects in remote unseeded regions [65,66]. Another study found that the cooling resulting from seeding of mid-and high-latitude cirrus is accompanied by only a modest reduction in rainfall [67]. Combining Bthinning^of cirrus with projected CO 2 increases in multidecadal climate simulations led to an enhancement of the hydrological cycle [68].…”