David L Mitchell and William Finnegan 2009 Environ. Res. Lett. 4 045102 doi:10.1088/1748-9326/4/4/045102
David L Mitchell and William Finnegan
Show affiliationsPart of Focus on Climate Engineering: Intentional Intervention in the Climate System
Greenhouse gases and cirrus clouds regulate outgoing longwave radiation (OLR) and cirrus cloud coverage is predicted to be sensitive to the ice fall speed which depends on ice crystal size. The higher the cirrus, the greater their impact is on OLR. Thus by changing ice crystal size in the coldest cirrus, OLR and climate might be modified. Fortunately the coldest cirrus have the highest ice supersaturation due to the dominance of homogeneous freezing nucleation. Seeding such cirrus with very efficient heterogeneous ice nuclei should produce larger ice crystals due to vapor competition effects, thus increasing OLR and surface cooling. Preliminary estimates of this global net cloud forcing are more negative than −2.8 W m−2 and could neutralize the radiative forcing due to a CO2 doubling (3.7 W m−2). A potential delivery mechanism for the seeding material is already in place: the airline industry. Since seeding aerosol residence times in the troposphere are relatively short, the climate might return to its normal state within months after stopping the geoengineering experiment. The main known drawback to this approach is that it would not stop ocean acidification. It does not have many of the drawbacks that stratospheric injection of sulfur species has.
92.60.Jq Water in the atmosphere (humidity, clouds, evaporation, precipitation)
92.70.Mn Impacts of global change; global warming
92.60.Nv Cloud physics; stratus and cumulus clouds
92.60.hf Tropospheric composition and chemistry, constituent transport and chemistry
Issue 4 (October-December 2009)
Received 1 April 2009, accepted for publication 12 August 2009
Published 30 October 2009
David L Mitchell and William Finnegan 2009 Environ. Res. Lett. 4 045102
H Damon Matthews and Sarah E Turner 2009 Environ. Res. Lett. 4 045105
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