PowerPoint Presentation

Can Cirrus Cloud Thinning Cool Climate Without Severe Climate Side Effects?

CEC14 BerlinLawrence Jackson, Julia Crook & Piers Forster1

Jones et al. (2011)

Stratospheric SO2

Marine cloud brighteningTo return global mean temperature to some earlier state, precipitation is reduced by too muchRegional variations

Side effects e.g. Stratospheric O3 depletion, whiter skiesPartial solution e.g. ocean acidificationOnly temporaryGeoengineering challenges

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 20062011(Mace et al., 2009)

http://www.climate4you.com/Cirrus clouds

Bismuth(III) Iodide (BiI3)What do we know about cirrus cloud thinning?Homogeneous nucleationRequires high super-saturation levelsDominates ice crystal production T < -40oC

Heterogeneous nucleationOut competes homogeneous nucleation for water vapourMicrophysicsGeoengineeringIntroduce efficient heterogeneous ice nucleiTemperatures < T -40oCLarger ice crystals with greater fall speedNegative Twomey effect (Krchar & Lohmann, 2003)(Mitchell & Finnegan, 2009)Climate model simulationsHas potential to offset anthropogenic warmingRequires optimum particle concentrations(Storelvmo et al., 2013)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.4

This study focuses on climate impactsWe 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

Martin et al. (2011) The HadGEM2 family of Met Office Unified Model climate Configurations, Geosci. Model Dev, 4, 723-757AtmosphereOceanVertical levels6040Horizontal resolution(longitude x latitude)1.875o by 1.25o1o by o - 1o

Global grid cells192 x 145360 x 216Extension 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 125m6

Simulation design HadGEM2 forced with:Historical data 1860-2005RCP4.5 2006-2099

Geoengineering 2020-2069Control simulation is RCP4.5Climate baseline is 1986-2005Assess impacts of geoengineering on 2040-2059 climate means

Some results Climate engineeringby cirrus cloud seeding

Outgoing longwave radiation (Wm-2)Outgoing shortwave radiation (Wm-2)

Results Changes in TOA radiation fluxes & cloud cover (2x fall speed)

Cloud cover (%)

Vertical profile of cloud cover (%)Hatching indicates changes not significant at 95%

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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 4.5oCOur RCP4.5 simulation warms by ~2oC over this time

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Results change in temperature

CIRRUS x2 (-0.6 K)CIRRUS x4 (-1 K)

MCB (-1 K)(1.5 K)

Results change in precipitation rate (kg m-2 day-1)

CIRRUS x2 (land 1.8%)

MCB (land -0.2%)CIRRUS x4 (land 1.4%)(land 3.2%)

Results effects of climate geoengineering

TemperaturePrecipitationCIRRUS x4MCB

A cooler climate without side effects?ConclusionsBut Regional variations in impact especially precipitationIs it scalable?In practice how would we know if it works?F (Wm-2)delta T (K)delta P Land (%)CIRRUS x2-1-0.61.8CIRRUS x4-2-11.4MCB-2-1-0.2

Thank you