1991 - Present
Scientists propose how to geoengineer contrails with biofuels and sulfur-doped jet fuel
1991 - Stratospheric welsbach seeding for reduction of global warming
“The particles may be seeded by dispersal from seeding aircraft; one exemplary technique may be via the jet fuel as suggested by prior work regarding the metallic particles. Once the tiny particles have been dispersed into the atmosphere, the particles may remain in suspension for up to one year.” 
1998 - First direct sulfuric acid detection in the exhaust plume of a jet aircraft in flight
“Direct detection of total sulfuric acid (SA) has been achieved for the first time in the plume of a jet aircraft in flight. The measurements show the same SA signatures for the case when SA was injected directly into the exhaust jet and the case when sulfur was provided to the engine with the fuel.” 
2002 - Influence of fuel sulfur on the composition of aircraft exhaust plumes: The experiments SULFUR 1–7
“A series of experiments (SULFUR 1–7, abbreviated as S1–S7) was performed in the years from 1994 to 1999 in order to determine the particle and contrail formation properties of aircraft exhaust plumes for different fuel sulfur content (FSC) and atmospheric conditions. This paper describes the series of experiments and summarizes the results obtained. In particular, the paper discusses the evolution of our understanding of particle formation and contrails as obtained during the course of these and related experiments.” 
2006 - Albedo enhancement by stratospheric sulfur injections: A contribution to resolve a policy dilemma?
“Although climate cooling by sulfate aerosols also occurs in the troposphere, the great advantage of placing reflective particles in the stratosphere is their long residence time of about 1–2 years, compared to a week in the troposphere. Thus, much less sulfur, only a few percent, would be required in the stratosphere to achieve similar cooling as the tropospheric sulfate aerosol.” 
2008 - Weather and Climate Engineering
Video Presentation (15:53) • Use of commuter aircraft with their jet fuels doped with aerosol generators is another possibility. Also the use of UAVs or blimps for aerosol dispersal could be considered. Potential adverse consequences, however, are likely including impacts on precipitation, local cold temperature extremes (which would also impact fossil fuel demands) and the hydrological cycle. …
3.4 Seeding cirrus clouds or making more contrails
On an annual average clouds cover between 55 to 60% of the earth (Matveev 1984) and much of that cloud cover consists of middle and high clouds. It is thought that globally cirrus clouds contribute to warming of the atmosphere owing to their contribution to downward transfer of LW radiation. In other words they are a greenhouse agent. Human activity is already modifying the cirrus clouds through the production of aircraft contrails. Kuhn (1970) found that contrails depleted solar radiation and increased downward LW radiation but during the daytime their shortwave influence dominates and they contribute to a net surface cooling. Kuhn (1970) calculated that if contrails persist over 24h their net effect would be cooling. Others have concluded that they lead to surface warming (Liou et al. 1991; Schumann 1994) but Sassen (1997) notes that the sign of the climatic impact of contrails is dependent upon particle size. Global estimates of the effects of contrails are they contribute to a net warming (Minnis et al. 2004).
It has even been proposed to seed in clear air in the upper troposphere to produce artificial cirrus which would warm the surface enough to reduce cold-season heating demands (Detwiler and Cho 1982). So the prospects for seeding cirrus to contribute to global surface cooling do not seem to be very good.
The only approach that might be feasible is to perform wide-area seeding with soot or carbonaceous aerosols which would absorb solar radiation and warm cirrus layers enough to perhaps dissipate cirrus clouds (a semi-direct effect). This strategy would be similar to that proposed by Watts (1997) and Crutzen (2006) for implementation in the stratosphere. As noted by Crutzen (2006) only 1.7% of the mass of sulfur is needed to produce a similar magnitude of surface cooling. Application at cirrus levels in the upper troposphere would have the double benefit of absorbing solar radiation thus contributing to surface cooling and dissipating cirrus clouds which would increase outgoing longwave radiation. Of course, the soot that becomes attached to ice crystals will reduce the albedo of cirrus thus countering the longwave warming effect to some degree. In addition, there is evidence that soot particles can act as ice nuclei, thus contributing to greater concentrations of ice crystals by heterogeneous nucleation but possibly reduced crystal production by homogeneous nucleation (DeMott et al. 1994; Kärcher et al. 2007). Thus it would be best to engineer carbonaceous aerosol to be ineffective as IN.
The possible adverse consequences of such a procedure can only be conjectured at this time but are mostly likely to impact the hydrological cycle. Complex chemical, cloud-resolving, and global models are required to evaluate the feasibility of this approach and to estimate possible adverse consequences. The feasibility of this approach in terms of implementation strategies is probably comparable to seeding sulfates in the lower stratosphere. The costs would be similar to Crutzen’s estimates for stratospheric seeding. 
2009 - Modification of cirrus clouds to reduce global warming
“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.”
“dissolved or suspended in their jet fuel and later burned with the fuel to create seeding aerosol, or (2) injected into the hot engine exhaust, which should vaporize the seeding material, allowing it to condense as aerosol in the jet contrail” 
2009 - Benefits, risks, and costs of stratospheric geoengineering
“Options for dispersing gases from planes include the addition of sulfur to the fuel, which would release the aerosol through the exhaust system of the plane, or the attachment of a nozzle to release the sulfur from its own tank within the plane, which would be the better option.” 
2010 - Efficient formation of stratospheric aerosol for climate engineering by emission of condensible vapor from aircraft
“Here we describe an alternate method in which aerosol is formed rapidly in the plume following injection of H2SO4 (sulfuric acid), a condensable vapor, from an aircraft. ... We explore the possibility of forming sulfate aerosols in an aircraft plume by emission of H2SO4 vapor rather than SO2. (Depending on the injection method, SO3 vapor may be emitted rather than H2SO4, but it is quickly converted to H2SO4.” 
2013 - Stratospheric passenger flights are likely an inefficient geoengineering strategy
“Here, we show that using stratospheric passenger flights to inject sulfate aerosols would not cause significant forcing under realistic injection scenarios: even if all present-day intercontinental flights were lifted above the tropopause.” 
2014 - Alternative-Fuel Effects on Contrails & Cruise Emissions (ACCESS-2) Flight Experiment
“Three different fuel types are discussed: a low-sulfur JP-8 fuel, a 50:50 blend of JP-8 and a camelina-based HEFA fuel, and the JP-8 fuel doped with sulfur.” 
2015 - Academy of Finland’s FICCA “COOL” Project
“Another technique examined was the use of commercial passenger aircraft flying at high altitudes to inject sulphate aerosols, emitted by aviation fuel, into the stratosphere. This would mimic a volcanic eruption, during which sulphur compounds are released into the stratosphere. They reflect solar radiation and thereby have a clear cooling effect on the climate. No previous calculations are available on the viability of using commercial flights in this way.
“In terms of efficient geoengineering strategies, this technique proved unviable. It would work best close to the equator, but little air traffic operates there – commercial flight routes are operated further north. In addition, current commercial aircraft are unable to fly high enough in the stratosphere. We would need new planes with large amounts of sulphur added to their fuel,” Laaksonen says. 
Commercial aircraft could be used to deliever sulfate into stratosphere by increasing fuel sulfur content and the flight altitude of inter-continental flights • The sulfur content of the fuel should be increased to about 50 times the current level to have a significant cooling effect • The cooling effect would be confined to the Northern Hemisphere. 
2015 - Impacts of aviation fuel sulfur content on climate and human health
Applying high FSCs [fuel sulfur content] at aviation cruise altitudes combined with ULSJ [ultra-low sulfur jet fuel, aviation biofuel] fuel at lower altitudes result in reduced aviation-induced mortality and increased negative RE compared to the baseline aviation scenario. 
(1) Use biofuels on takeoff. Create less carbon black dust (soot) around airports, kill less people.
(2) Use high-sulfur jet fuel at altitude. Mimic "Pinatubo effect" to do stratospheric sulfur injections for solar radiation management purposes.
Here is how the two-fuel geoengineering solution works
United States Patent 8430360B2 • Inventor: Malte Schwarze and Andreas Westenberger • Assignee: Airbus Operations GmbH
The control unit is adapted for being connected to at least one first supply apparatus for supplying a first fuel to the internal combustion engine and to at least one second supply apparatus for supplying a second fuel to the internal combustion engine. 
United States Patent US8849541B2 • Inventor: Peter Swann • Assignee: ROLLS-ROYCE PLC
A method includes i) identifying a period of operation corresponding to a fuel supply requirement; ii) determining at least one ambient air condition in which the machine will operate during the period; iii) determining a duration of time in which, whilst in the ambient air condition, it is required to achieve a predetermined vapour trail characteristic; iv) determining a resultant fuel composition for use by the machine in the ambient air condition to achieve the characteristic, where the resultant fuel composition includes at least one of the first and second fuel compositions; v) determining the ratio of at least the first and second fuel compositions required for sufficient resultant fuel composition for the duration of time determined in step iii); and vi) producing a first signal indicative of the ratio of at least the first and second fuel compositions required for the duration of time determined in step iii). 
United States Patent US9518965B2 • Inventor: Peter Swann • Assignee: ROLLS-ROYCE PLC
A fuel system (12) comprising a vapor trail detection sensor (20) configured to generate a first signal (28) which indicates the optical depth of a vapor trail (35). A control unit (40) is provided responsive to the first signal (28) and configured to generate a second signal (80) in dependence upon the first signal (28). The second signal (80) defines a percentage of at least one of a first fuel composition and second fuel composition required to produce a resultant fuel composition. At least one regulator (42) is provided configured to receive and be responsive to the second signal (80) and regulate the percentage of first and second fuel composition required to produce the resultant fuel composition. 
Soot and black carbon carries sulfur into stratosphere, destroys ozone layer, affects rainfall and monsoons worldwide.
2010 - Photophoretic levitation of engineered aerosols for geoengineering
Aerosols could be injected into the upper atmosphere to engineer the climate by scattering incident sunlight so as to produce a cooling tendency that may mitigate the risks posed by the accumulation of greenhouse gases. Analysis of climate engineering has focused on sulfate aerosols. Here I examine the possibility that engineered nanoparticles could exploit photophoretic forces, enabling more control over particle distribution and lifetime than is possible with sulfates, perhaps allowing climate engineering to be accomplished with fewer side effects. The use of electrostatic or magnetic materials enables a class of photophoretic forces not found in nature. Photophoretic levitation could loft particles above the stratosphere, reducing their capacity to interfere with ozone chemistry; and, by increasing particle lifetimes, it would reduce the need for continual replenishment of the aerosol. Moreover, particles might be engineered to drift poleward enabling albedo modification to be tailored to counter polar warming while minimizing the impact on equatorial climates. 
2017 - Black carbon particles from aviation exist up to 18 km into the stratosphere
Aeroplanes may be ejecting significant amounts of black carbon (BC) — a pollutant known to aggravate breathing disorders, upset the monsoon and quicken glacier melt — and may be depleting the ozone layer, according to a study by climate researchers from multiple institutions in the country.
Though airborne, BC (black carbon) is known to dissipate and settle down in a few months under the influence of rain and wind and is unlikely to travel upward of 4 km. However, a group of scientists — including from the Indian Institute of Science and ISRO’s Vikram Sarabhai Space Centre — say they now have evidence of such particles existing up to 18 km into the stratosphere and there are about 10,000 of them in every cubic centimetre.
Given the shape and location of these particles, they argue, it could only derive from emissions from aviation fuel and they pose a problem because these black carbon particles can linger long enough to provide a fertile ground for other chemical reactions that can deplete the ozone layer. 
2017 - Possible climatic implications of high-altitude black carbon emissions
Furthermore, we show that such aircraft-emitted BC can be transported to upper tropospheric or lower stratospheric heights (∼17km) aided by the strong monsoonal convection occurring over the region, which is known to overshoot the tropical tropopause, leading to the injection of tropospheric air mass (along with its constituent aerosols) into the stratosphere. 
Commercial aviation has been geoengineering for years. Now we understand how. Sulfuric acid rides on carbon black dust into the stratosphere.
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