Whether or not a focused injection in the lower stratosphere or upper troposphere instead of higher in the stratosphere would tend to stay over the poles and what effect it would have on climate elsewhere is not known. A focused injection would be advantageous if the objective is to slow or stop the melting of the Arctic as recently noted (14, 38). My analysis does not consider injection into the troposphere because of uncertainty as to how long the aerosol would persist. Just how low in the lower stratosphere an injection would have to be made to stay put is also unknown, although the latitude appears to be more important than the altitude.
Additional Assumptions: (a) Increases in fuel usage and forcings are assumed to be linear over the period, with the understanding that increases will likely be more non linear and in some years, there may be a decline relative to the previous year or years as was the case due to 9-11 and the subsequent recession that both impacted air travel.
(b) As sulfur is burned and converted to sulfate aerosol, that aerosol’s reflection of sunlight generally matches the increase in GHG emissions for the same period of time. A one-year average lifetime for the aerosol is assumed. The overlapping of aerosol levels from year to year would need to be determined as the sulfate levels for one year don’t magically disappear on December 31 to be replaced by the higher level for the next year as is assumed in the calculations. This is the basis for Wigley talking about massive injections every few years, since the sulfate aerosol residence time in the stratosphere is 1-2 years.
(c) Based on Crutzen and Wigley’s recent papers, 5.3Tg of sulfur in the stratosphere offsets 4W/m2 or 1.3Tg sulfur/W/m2. It is assumed that an additional 2W/m2 is added to forcing vs. a 2000 baseline by 2050. This requires a maximum injection of 2.6Tg (rounded figure) in 2050 to keep forcing at the 2000 level or an average annual increase in sulfur of 2.6Tg/50 or 0.052Tg.
(d) Jet fuel usage numbers and predictions vary greatly and 2050 estimates range from 270-2300Tg (39, 40). The estimates used here are my own attempt at selecting a realistic level in 2050 of 750Tg and working backwards from that by decreasing it by 12Tg per year (2000, 2001 and 2002 are assumed to be 167, 170 and 174Tg, respectively and the 12Tg per year increase begins with 2002). Boeing and Airbus predict a doubling of the number of planes by 2025, so an increase in the amount of jet fuel burned is to be expected even with improvements in fuel efficiency and the more burned, the more sulfur that is potentially available for stratospheric injection (41).
(e) It is further assumed that 100% of the fuel is either petroleum based or bioethanol based to which sulfur can be added.
(f) The costs of switching to higher sulfur fuel and any impacts on equipment lifetime are borne by the government.
