Since the quantities required to simulate a volcanic eruption are so large that they cannot be injected into the stratosphere all at once and probably this would not be desirable anyway (one junior Pinatubo-like pulse that takes place over a couple of weeks may not have undesirable impacts, but doing this year after year might), the only practical way to achieve the desired stratospheric loading is to have it spread out over time. Likewise, the forcing due to GHGs will increase incrementally over the next 50 years (the time period considered by my analysis) and thus, we only need to offset the additional forcing for each year, one day at a time.
In other words, we don’t have to offset all the forcing in place since 1700 or all the anticipated forcing expected by 2100. It would be nice to be able to do this, but I think this is impractical. If we can simply hold the forcing constant from 2000 to 2050, that will allow the replacement technologies to be developed and become routinely used. This is similar to the logic we used in the plastic cover project.
The table below summarizes the estimates of how much sulfur in jet fuel would allow an offset of the additional forcing from 2000 to 2050.
Tg S emitted into stratosphere and
converted into sulfate aerosol for
given fuel %
YR S, Tg Required 0.04% 0.3% 0.6% 0.9%
2000 0.052 0.027 ----- ----- -----
2001 0.104 0.027 ----- ----- -----
2002 0.156 0.028 ----- ----- -----
2006 0.364 0.036 ----- ----- -----
2010 0.572 0.043 0.324 0.648 0.972
2015 0.832 0.053 0.396 0.792 1.188
2020 1.092 0.062 0.468 0.936 1.404
2030 1.612 0.082 0.612 1.224 1.836
2035 1.872 0.091 0.684 1.368 2.052
2040 2.132 0.101 0.756 1.512 2.268
2050 2.652 0.120 0.900 1.800 2.700
One Tg is one million metric tons. A metric ton is 2200 pounds. S, Tg required is the sulfur required in jet fuel needed to offset an annual additional forcing of 0.04 W/m2.
Conclusion: 0.6-0.9% sulfur in fuel is adequate to offset all additional GHG forcing from 2000 to 2050. If levels above 0.3% are determined to be unacceptable due to performance or other issues, then around one third of the forcing can be reduced this way. If 100% conversion to sulfate aerosol occurs, 0.3-0.5% sulfur-in-fuel is adequate.
Other issues that must be addressed: impact on engine performance and operating life, impact on stratospheric ozone, tropospheric air quality, uneven cooling of the atmosphere, impact on contrail formation and ocean acidification.
The latitude of the releases may be important also, based on studies of sulfate aerosol emissions from volcanic eruptions (36, 37). Releases close to the equator seem to be more equally distributed over time, while those closer to the poles tend to stay north of 30° in the N. Hemisphere and could be assumed to do the same in the S. Hemisphere, although data supporting this are not available. Releases from a volcano in Alaska appeared to have an adverse effect on the Indian monsoon, leading to drier weather, while releases from tropical volcanoes did not. Most commercial airplane flights occur in the N. Hemisphere.
