Option 5: Running Commercial Jet Engines with Richer Fuel to Air Ratio
This can be done right now, with the planes only running rich while in the stratosphere. This is based on about 1.3% fuel converted to soot and relies on a 1989 estimate that 1% offsets 6.5GtC forcing (22). The 1.3% takes into consideration growth in forcing since 1989 and applies it to a 2000 baseline. The additional soot will block enough sunlight to completely offset warming due to GHG emissions and hold the forcing at 2000 levels.
Like with the sulfur emissions, more soot would be required each year. Due to uncertainty of the response to additional soot, I have not provided a schedule for the next 50 years. If it is linear and has to match GHG emissions growth between now and 2050, then around 2.6% soot might be enough to offset the warming then. That would still seem doable, since only 2.6% of jet fuel is burned for this purpose. However, a doubling of jet fuel use might allow 1.3% to continue to be used.
The black carbon aerosol released may generate more cirrus clouds and enhance stratospheric warming. That could offset any ozone loss due to sulfate aerosols if they are also used. While reducing ozone loss would be beneficial, the soot might also result in some as yet unknown harm. The net effect on GHG forcing and any possible harmful effects must be determined to gauge the true benefit. It isn’t clear if a single country could carry this out by having its planes generate even higher soot levels, but if so, effective distribution would require use of the airspace of other countries.
Running engines rich could wear them out faster. Attempting to run rich and use higher sulfur fuel at the same time could complicate the engine operation as well as reduce the efficiency of the sulfate aerosols if they combine with the soot. Having some planes run rich and others with higher sulfur fuel may provide the proper mix of these approaches to achieve the desired result.
It could allow the sulfur content to be decreased to below 0.3%. Or, finally, the decision could be made to run rich for as long as possible and then start using the higher sulfur fuel if the soot levels are found to be causing problems. As was assumed for the higher sulfur fuel, the cost of additional fuel and engine wear is borne by the government.
