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

2. 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

3. Cirrus 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)

4. 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)

5. 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?

6. 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,

7. Simulation design HadGEM2 forced with: Historical data 1860-2005
RCP
Geoengineering
Control simulation is RCP4.5
Climate baseline is
Assess impacts of geoengineering on climate means

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

9. 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%

10. Results - Cirrus cloud thinning TOA radiative forcing

11. Results – temperature & precipitation
Projected warming IPCC 2013 – relative to – RCP to +2.6oC – RCP to 4.5oC
Our RCP4.5 simulation warms by ~2oC over this time

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

13. 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%)

14. Results – effects of climate geoengineering
Temperature
Precipitation
CIRRUS x4
MCB

15. 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

16. Thank you