摘要：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.
