Can Cirrus Cloud Thinning Cool Climate Without Severe Climate Side Effects? CEC14 Berlin Lawrence Jackson, Julia Crook & Piers Forster

Marine cloud brightening Geoengineering challenges Stratospheric SO2 Marine cloud brightening Jones et al. (2011) To return global mean temperature to some earlier state, precipitation is reduced by too much Side effects e.g. Stratospheric O3 depletion, whiter skies Partial solution e.g. ocean acidification Regional variations Only temporary

Cirrus clouds http://www.metoffice.gov.uk/learning/clouds/high-clouds Mean high cloud cover from CloudSat/CALIPSO 2B-GEOPROF R04 & 2B-GEOPROF-LIDAR P1.R04 data sets for 2006–2011 (Mace et al., 2009) http://www.climate4you.com/

What do we know about cirrus cloud thinning? Microphysics Homogeneous nucleation Requires high super-saturation levels Dominates ice crystal production T < -40oC Heterogeneous nucleation Out competes homogeneous nucleation for water vapour Introduce efficient heterogeneous ice nuclei Temperatures < T -40oC Larger ice crystals with greater fall speed “Negative Twomey effect” (Kӓrchar & Lohmann, 2003) Geoengineering Bismuth(III) Iodide (BiI3) Environment is very clean at high altitudes so homogeneous nucleation dominates in cirrus clouds producing small ice crystals. By adding aerosol we can increase heterogeneous nucleation which results in larger ice crystals with greater fall speed. They form quicker but fall out quicker. (Mitchell & Finnegan, 2009) Has potential to offset anthropogenic warming Requires optimum particle concentrations Climate model simulations (Storelvmo et al., 2013)

This study focuses on climate impacts We adopt a simplified implementation in a GCM by modifying ice fall speed Does cirrus cloud thinning: Cool climate? Change precipitation over land? Cause regional variations in precipitation? Could it complement stratospheric SO2 and MCB?

HadGEM2 CC run on HECToR & ARCHER Extension to 60 vertical levels to give improved resolution of stratospheric dynamics 208 x 139 km at equator and 120 x 139 km at 55 degrees Ocean levels – unevenly spaced – about 10m at the surface and in 10m steps down to the layer centred on 125m Atmosphere Ocean Vertical levels 60 40 Horizontal resolution (longitude x latitude) 1.875o by 1.25o 1o by ⅓o - 1o Global grid cells 192 x 145 360 x 216 Martin et al. (2011) The HadGEM2 family of Met Office Unified Model climate Configurations, Geosci. Model Dev, 4, 723-757

Simulation design HadGEM2 forced with: Historical data 1860-2005 RCP4.5 2006-2099 Geoengineering 2020-2069 Control simulation is RCP4.5 Climate baseline is 1986-2005 Assess impacts of geoengineering on 2040-2059 climate means

by cirrus cloud seeding Climate engineering by cirrus cloud seeding Some results …

Outgoing shortwave radiation (Wm-2) Results – Changes in TOA radiation fluxes & cloud cover (2x fall speed) Cloud cover (%) Vertical profile of cloud cover (%) Outgoing longwave radiation (Wm-2) Outgoing shortwave radiation (Wm-2) Hatching indicates changes not significant at 95%

Results - Cirrus cloud thinning TOA radiative forcing

Results – temperature & precipitation Projected warming IPCC 2013 – 2081-2100 relative to 1986-2005 – RCP4.5 +1.1 to +2.6oC – RCP8.5 +2.6 to 4.5oC Our RCP4.5 simulation warms by ~2oC over this time

Results – change in temperature (1.5 K) MCB (-1 K) CIRRUS x2 (-0.6 K) CIRRUS x4 (-1 K)

Results – change in precipitation rate (kg m-2 day-1) (land 3.2%) MCB (land -0.2%) CIRRUS x2 (land 1.8%) CIRRUS x4 (land 1.4%)

Results – effects of climate geoengineering Temperature Precipitation CIRRUS x4 MCB

Conclusions A cooler climate without side effects? But … Regional variations in impact – especially precipitation Is it scalable? In practice how would we know if it works? F (Wm-2) delta T (K) delta P Land (%) CIRRUS x2 -1 -0.6 1.8 CIRRUS x4 -2 1.4 MCB -0.2

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