Researchers find that a program to fight climate change by injecting aerosols into the atmosphere from high-altitude aircraft would be possible, but unreasonably costly and unlikely to remain secret.

With climate change on everyone’s agenda, scientists are embracing fresh approaches based on strict deadlines for setting our planet back on a positive course, with geoengineering at the heart of these efforts. They involve schemes ranging from erecting massive walls on the seafloor to engineering projects aimed at slowing down glacial melting.

Now, a team of scientists has released a study evaluating the progress of various solar geoengineeringprojects. Their work was motivated by the reality that programs designed to inject aerosols into the atmosphere could be environmentally risky and very costly.

Weighing the Odds of Disclosure

The concept, known as stratospheric aerosol injection (SAI), involves the delivery of sulfates into the lower stratosphere. In addition to cost and feasibility, the team also looked at whether or not projects like this should remain confidential, and if they are released to the public, which channels for sharing information should be used.

Dr. Gernot Wagner, from Harvard University’s John A. Paulson School of Engineering and Applied Sciences, and co-author on the paper, discussedhow delicate the subject of solar geoengineering is:

“While we don’t make any judgment about the desirability of SAI, we do show that a hypothetical deployment program starting 15 years from now, while both highly uncertain and ambitious, would be technically possible strictly from an engineering perspective.

It would also be remarkably inexpensive, at an average of around $2 to 2.5 billion per year over the first 15 years.”The Numbers Behind the Plan In terms of the scope of the vision laid out by the team for SAIL (the Loftier version of SAI)

–> Development costs would include $2 billion for the airframe, and an additional $350 million for any necessary engine modifications. 

–> An average of about 4,000 flights in the first year, gradually increasing to 60,000 by the end of a 15-year period.

–> A doubling of the (1) thrust and (2) wing area and (3) number of engines (from four to two) would be needed to make aircrafts ready for the job. 

–> An anticipated fleet of eight to start, which will increase to 100 by the end of a 15-year period.

By all indications, the research shows that SAI is a viable solution, but only if sound planning and a radical restructuring of current models is carried out. And in a refreshing way, the team also views the developments in SAI as positive and cautions against forming geopolitical conspiracy theories about inappropriate use in the future:

“Given the potential benefits of halving average projected increases in radiative forcing from a particular date onward, these numbers invoke the ‘incredible economics’ of solar geoengineering. Dozens of countries could fund such a program, and the required technology is not particularly exotic,” Dr. Wagner shared

Details about the study appear in a paper, titled “Stratospheric aerosol injection tactics and costs in the first 15 years of deployment”, which was published November 22nd in the Environmental Research Letters journal. 




Stratospheric aerosol injection tactics and costs in the first 15 years of deployment

Wake Smith1 and Gernot Wagner2

Published 23 November 2018 • © 2018 The Author(s). Published by IOP Publishing Ltd
Environmental Research LettersVolume 13Number 12


We review the capabilities and costs of various lofting methods intended to deliver sulfates into the lower stratosphere. We lay out a future solar geoengineering deployment scenario of halving the increase in anthropogenic radiative forcing beginning 15 years hence, by deploying material to altitudes as high as ~20 km. After surveying an exhaustive list of potential deployment techniques, we settle upon an aircraft-based delivery system. Unlike the one prior comprehensive study on the topic (McClellan et al 2012 Environ. Res. Lett. 7 034019), we conclude that no existing aircraft design—even with extensive modifications—can reasonably fulfill this mission. However, we also conclude that developing a new, purpose-built high-altitude tanker with substantial payload capabilities would neither be technologically difficult nor prohibitively expensive. We calculate early-year costs of ~$1500 ton−1 of material deployed, resulting in average costs of ~$2.25 billion yr−1 over the first 15 years of deployment. We further calculate the number of flights at ~4000 in year one, linearly increasing by ~4000 yr−1. We conclude by arguing that, while cheap, such an aircraft-based program would unlikely be a secret, given the need for thousands of flights annually by airliner-sized aircraft operating from an international array of bases.

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1. Introduction

Solar geoengineering is commonly seen to be subject to what some call its ‘incredible economics’ (Barrett 2008) and, more specifically, its ‘free driver’ effect: its direct costs are so cheap compared to its potential climate impacts so as to reverse many of the properties of the so-called ‘free rider’ problem governing carbon mitigation decisions and climate policy more broadly (Wagner and Weitzman 20122015, Weitzman 2015). The governance problem becomes one of cooperation to restrain rather than increase action. Here we probe these economic assertions and review the capabilities and costs of various lofting methods intended to deploy sulfates into the lower stratosphere, the leading proposed method of solar geoengineering (Keith 2000, Crutzen 2006, National Research Council 2015). Stratospheric Aerosol Injection (SAI) would require lofting hundreds of thousands to millions of tons of material each year to altitudes up to ~20 km. Here we seek answers to three questions: if SAI deployment were to commence within the foreseeable future with the tools and technologies at our disposal, how would such deployment be physically achieved, how much would it cost, and could it be done in secret?

National Academies of Sciences (NAS), Engineering and Medicine (1992) provides an early review of SAI deployment options, deriving detailed pricing for naval rifles and two different balloon systems (appendix Q.11). McClellan et al (2012) attempt to provide the first comprehensive answer to this question, publishing results from an earlier Aurora Flight Science Corporation analysis (McClellan et al 2010). Like McClellan et al (20102012), and later reviewed by Moriyama et al (2017), we explore an array of different SAI lofting technologies and given our more specific mission criteria, we conclude that aircraft are the only reasonable option. Unlike them, we conclude that modified existing business jets are incapable of flying above ~16 km, a conclusion confirmed directly by the manufacturers of the jets in question. This directly contradicts both McClellan et al (20102012) and IPCC (2018). The latter demonstrates the large influence McClellan et al‘s analysis has had on the broader conversation. IPCC (2018) states that ‘there is high agreement that aircrafts after some modifications could inject millions of tons of SO2 in the lower stratosphere (~20 km)’ (chapter 4). IPCC cites three studies in support of that statement, including McClellan et al (2012). However, both of the other two studies, in turn, base their conclusions, in large part, on McClellan et al‘s earlier analysis. Irvine et al (2016) also cites the other (Davidson et al 2012), which, in turn, cites McClellan et al (2010). Robock et al (2009) provides one further independent analysis, reviewing capabilities of military fighters and tankers. We agree with Robock et al (2009) that military fighters are capable of reaching ~20 km, but they are incapable of sustained flight at that altitude (see table 2 below).

We further conclude that no other existing aircraft have the combination of altitude and payload capabilities required for the mission, leading us instead to the design of a new plane.

We propose such a plane and call it SAI Lofter (SAIL), describing its basic specifications and providing detailed cost estimates for its design, manufacture, and operation under a hypothesized solar geoengineering scenario of halving the increase in radiative forcing from a date 15 years hence. We do not seek to foretell future technological breakthroughs, nor do we guess at costs in 50 or 100 years when next-generation deployment technologies would likely become available. Further, we do not consider solar geoengineering methodologies other than SAI or materials other than sulfate aerosols (Keith 2000, Keith et al 2016). We instead hope to illuminate discussions of direct SAI deployment costs based on existing technologies, thereby facilitating further benefit-cost comparisons and grounding ‘free driver’ discussions in concrete numbers supported by science-based SAI deployment scenarios and sound aerospace engineering.

2. Stratospheric aerosol deployment scenario

Following a research hypothesis proposed by Keith and Irvine (2016), we consider a limited SAI deployment scenario (Sugiyama et al 2018) intended to cut in half the rate of temperature change from the first year of the program onward. While such a scenario is less ambitious (and less environmentally risky) than those aimed at keeping temperatures constant from a certain date forward, it is more ambitious than SAI merely holding the rate of temperature change constant (MacMartin et al 2014).

We further assume anthropogenically driven radiative forcing of ~2.70 W m−2 by 2030, with an assumed decadal increase of ~0.5 W m−2 that is roughly consistent with the Representative Concentration Pathway (RCP) 6.0 scenario (Moss et al 2010, IPCC 2013). Assuming the desire to cut this rate of increase in half implies the need for SAI to reduce radiative forcing by ~0.25 W m−2 by the end of the first decade of deployment. The implied change in global average surface temperatures from SAI deployment is −0.2 K per decade, with an assumed global average temperature sensitivity of 0.8 K per W m−2.

We focus on SAI using sulfates, not because they are optimal—they may not be (Keith et al 2016)—but because the long record of prior analyses on both efficacy and risks of sulfate deployment (National Research Council 2015) renders them the best understood and therefore least uncertain material with which to commence in this hypothetical scenario of partial deployment. In the base case, we assume a top-of-atmosphere (TOA) sulfate forcing sensitivity of −0.25 W m−2 per Tg S yr−1, a value toward the lower end of recent estimates. Pierce et al (2010) estimates −0.34 W m−2 and Dai et al (2018) derives a range from below −0.50 to over −2 W m−2 for injections between 30 °N and 30 °S. Other estimates for different injection scenarios, roughly converted to TOA values, range from −0.15 W m−2 (Kuebbeler et al 2012) to −0.33 W m−2 (Niemeier and Timmreck 2015), while Pitari et al (2014) shows results from the Geoengineering Model Intercomparison Project (GeoMIP), here roughly converted to TOA, for one point of injection at the equator ranging from −0.47 to −0.98 W m−2.

Table 1 summarizes the base-case SAI deployment scenario for the first 15 years of a program commencing in 15 years. The year 2033 is entirely hypothetical. It is not the most likely start date, nor are we suggesting it is an optimal one, but any deployment much sooner seems highly unlikely based on scientific and political considerations. Later deployment may mean the approaches explored here can be revised in light of new scientific and technological developments.

Table 1.  Hypothesized base-case SAI scenario in the first 15 years of deployment commencing in 15 years. Tons of S carried are half of tons SO2 dispersed.

Year Unabated forcing (W m−2) Target forcing (W m−2) SO2 dispersed (Mt)a Temperature reduced (K)b
2033 2.850 2.825 0.2 −0.02
2034 2.900 2.850 0.4 −0.04
2035 2.950 2.875 0.6 −0.06
2036 3.000 2.900 0.8 −0.08
2037 3.050 2.925 1.0 −0.10
2038 3.100 2.950 1.2 −0.12
2039 3.150 2.975 1.4 −0.14
2040 3.200 3.000 1.6 −0.16
2041 3.250 3.025 1.8 −0.18
2042 3.300 3.050 2.0 −0.20
2043 3.350 3.075 2.2 −0.22
2044 3.400 3.100 2.4 −0.24
2045 3.450 3.125 2.6 −0.26
2046 3.500 3.150 2.8 −0.28
2047 3.550 3.175 3.0 −0.30

Assumes −0.25 W m−2 per Tg S. Assumes 0.8 K per W m−2 average temperature sensitivity (see text).

The assumed linear ramp-up, and assumed lofting of sulfate for the purpose of dispersing SO2 (Smith et al 2018), implies the need to loft ~0.1 Mt of S in year one, increasing at a rate of ~0.1 Mt yr−1linearly thereafter. Note this is significantly less material than McClellan et al (2012) assumed mass fluxes of either 1 or 5 Mt yr−1 of S, presenting a more limited and phased deployment scenario (Sugiyama et al 2018).

Another important consideration is the location for SAI. Following Tilmes et al (2018a), we assume base-case injection sites at latitudes of 15° and 30° North and South of the equator. This is no verdict as to these four latitudes being optimal or definitive. It is a statement that, if forced to choose today, these four latitudes appear like a good starting point for discussions (Kravitz et al 2017, MacMartin et al 2017, Richter et al 2017, Tilmes et al 2017, Dai et al 2018). Note that while SAI latitudes matter, longitudes appear not to, as injections at any one longitude mix rapidly to all others. Latitudes, meanwhile, influence the height of injections. At 15 °N and S, injections may be required as high as ~20 km (Pierce et al 2010). Some argue that even higher injection altitudes would provide greater radiative benefit (Tilmes et al 2018b). For the purpose of defining the deployment scenario, we define the service ceiling necessary for the lofting platform at ~20 km.

3. Review of possible lofting technologies

We have undertaken a review of all lofting technologies that seem plausible as methods to hoist 0.1 Mt S to an altitude of up to ~20 km in 2033. Our main research involved engaging directly with commercial aerospace vendors to elicit what current and near-term technology platforms can achieve at what cost. We have met or corresponded directly with: Airbus, Boeing, Bombardier, Gulfstream, Lockheed Martin, Northrup Grumman; GE Engines, Rolls Royce Engines; Atlas Air, Near Space Corporation, Scaled Composites, The Spaceship Company, Virgin Orbit, and NASA, the latter in respect of its high-altitude research aircraft fleet.

Table 2 summaries our findings across lofting technologies. We eliminate technologies we deem insufficiently mature to be used for deployment 15 years hence and those incapable of reaching the required altitude. Existing commercial and military transport aircraft cannot achieve the required altitudes, even with extensive modifications. Modified business jets, noted prominently in McClellan et al (20102012) study, are incapable of reaching altitudes above ~16 km. High payload, high altitude aerostats have been hypothesized but not yet successfully tested, and in all events, are operationally fragile, unable to operate in adverse weather conditions. Tethered hoses are even less technologically mature and to-date untested. Military fighters such as the F-15 have reached altitudes of ~18 km in the context of record-setting ballistic climbs in ideal conditions, but they are incapable of either sustained flight or regular operations at such altitudes.

Table 2.  Cost and capabilities comparison of lofting technologies.

Platform Cost (‘000 $/t) SAIL multiple Source
Mission capable
SAILa 1.4 1×
McClellan New High Altitude Aircraft 1.5b ~1× McClellan et al (20102012)
Delft SAGAc 4.0 ~3× Delft Reportc
McClellan Modernized Gun 19 ~14× McClellan et al (20102012)
Balloons ~40 ~28× Near Spaced
NASA WB57 43 ~30× NASAd
NASA ER2 50 ~35× NASAd
NASA Global Hawk 70 ~50× NASAd
SpaceX Falcon Heavy Rocket 71e ~50× Chang (2018)
Gun Mark 7 16′ 137 ~100× McClellan et al (20102012)
Vector Rocket 1180e ~850× Chang (2018)
Virgin Orbit Rocket 2000e ~1400× Virgin Orbitd
Mission incapable
Existing Commercial Aircraft Not capable of reaching ~20 kmf
Modified Commercial Aircraft Not capable of reaching ~20 kmg
Existing Military Transportersh Not capable of reaching ~20 kmg
Military Fighters Not capable of sustained flight at ~20 kmg
Tethered Hose Not sufficiently mature technologyg
Aerostats/Airships Not sufficiently mature technologyg

aSee section 4 for cost derivations. bAssumes a program deploying ~1 Mt yr−1cTU Delft student report developing SAGA, the Stratospheric Aerosol Geoengineering Aircraft (Design Synthesis Exercise Group 22016). dPersonal communications with individuals at respective entities. eReduced by 95% to account for 20 km target altitude relative to 200 km for Earth orbit; Chang (2018)’s estimates for Vector Rocket confirmed by Vector Launch. fMcClellan et al (20102012) and authors’ analysis (see text). gAuthors’ analysis (see text), including, for military fighters, personal communication with Boeing, Lockheed Martin, and Northrup Grumman. hIncluding existing military tankers.

Among technologies capable of achieving the mission, costs are often prohibitive. NASA’s existing high-altitude aircraft that can reach appropriate altitudes do so with ~1 t payloads, making them very costly. Rockets are intended to reach altitudes 15–25× higher than are required to reach the lower stratosphere, rendering them both ill-suited and extremely costly. Even if the unit-costs of the massive SpaceX Falcon Heavy were reduced by 95% to account for the ratio of its normal target altitude to the ~20 km assumed here, it is still roughly 50× costlier than SAIL. Balloons and large naval-style guns are capable and plausible alternatives, but their per-ton costs are at least 10× as high as those we estimate for SAIL.

Table 2 also shows McClellan et al (20102012) new high-altitude aircraft, which posits a cost-per-ton similar to that of SAIL. While we derive a similar unit-cost, SAIL’s numbers apply to the initial years of deployment, while McClellan et al consider annual masses of both 1 and 5 Mt, the latter of which implies a larger and more mature program that may have achieved substantial economies of scale. For reference, our estimate is that a second-generation platform lofting the same 5 Mt yr−1might have unit costs at least 20% lower than the $1400 calculated here for a first-generation SAIL technology.

4. SAI lofter (SAIL)

Given the apparent inadequacy of existing technologies, especially of previously assumed-to-be-adequate modifications to existing aircraft (McClellan et al 20102012), we propose a novel aircraft with disproportionally large wings relative to its narrow fuselage. We also describe the aircraft fleet requirements, and we calculate development and deployment costs from conception through year 15 of the hypothetical program.

4.1. Design

The aircraft is designed to meet the assumed requirements outlined in section 3 above. In particular, it is capable of level flight at an altitude of ~20 km while carrying a 25 ton payload—large enough to lower operational costs significantly relative to existing high-altitude aircraft, yet small enough to make the mission possible. We have developed the design with direct input from several of the aerospace and engine companies consulted. It assumes a novel aircraft design but utilizes modified pre-existing low-bypass engines, which, though disfavored in commercial service due to their reduced fuel efficiency, will perform better at high altitudes.

Broadly, SAIL is equivalent in weight to a large narrow body passenger aircraft such as the A321, or in Boeing terms, sized between the 737–800 and the 757–200. In order to sustain level flight in the thin air encountered at altitudes approaching ~20 kms, SAIL requires roughly double the wing area of an equivalently sized airliner, and double the thrust, with four engines instead of two. (While maximum thrust requirements of most aircraft are defined by takeoff, SAIL’s engines are configured to perform at high altitudes.) At the same time, its fuselage would seem stubby and narrow, sized to accommodate a heavy but dense mass of molten sulfur rather than the large volume of space and air required for passenger comfort. SAIL would therefore have considerably wider wingspan than length. Its compact fuselage, however, would sit behind a conventional manned cockpit. While it is easy to imagine SAIL migrating to unmanned cockpits over time, under current certification rules, it would be substantially faster and therefore cheaper to certify the aircraft with onboard pilots.

More specifically, the preliminary design for SAIL calls for a length of ~46 m, a wingspan of ~55 m, and a wing area of ~250 m2, with an aspect ratio of ~12:1. The maximum structural payload would be ~25 t, with maximum takeoff weight (MTOW) of ~100 t, operating empty weight (OEW) of ~50 t, and maximum fuel load of ~32 t. The aircraft would have 4 wing-mounted low-bypass engines, modified for high-altitude operations with an aggregate take-off thrust of ~25–30 t and a thrust-to-weight ratio of ~30%. (GE Engines considers its F118 engine adequate, noting that it powers the NASA Global Hawk aircraft to similar altitudes; its Passport 20 engine may similarly be capable. Rolls Royce suggests its BR710 or BR725 engines.) The design will require a smaller fifth centerline auxiliary power unit for bleed air and onboard combustion of the molten sulfur payload.

This highlights another advantage of aircraft as a lofting platform, since they can take advantage of the onboard combustion system from S to SO2 explored by Smith et al (2018). Lofting S would cut in half the payload required compared with lofting SO2. Moreover, S is a less dangerous substance than SO2 to handle on the ground or contend with in the event of an accident. Other possible lofting methods such a balloons and guns could not accommodate this in situ conversion with existing technologies and would, therefore, need to loft SO2 with twice the mass of SAIL’s payload.

SAIL is designed for a service ceiling of ~20 km, with a maximum altitude of up to ~19.8 km in a typical mission. Each mission would last ~5 h, with ~2 h of ascent and descent time each, plus ~1 h on station. The ~2 h for ascent and descent time situates SAIL reasonably between the performance rates of the Global Hawk and U2/ER2. That assumes a ~25 t payload and a conversion of S to SO2 at ~0.5 t S per minute. Operational flights are flown out and back to the same base, with a range of ~4500 km for each plane at maximum payload. While Tilmes et al (2018b) have noted that injections at altitudes 5 km higher would add perhaps 50% to the radiative benefit derived from deployed aerosols, SAIL and similar aircraft deploying conventional engine technology to haul large payloads are unable to substantially exceed ~20 km.

The design assumes 2 pilots plus 1 payload operator, and accommodates 1 supernumerary, possibly a scientific observer. Crucially, there are no passengers, which simplifies regulatory certification for the newly designed plane. SAIL would only have one mission and at most a handful of operators. Ferry and positioning flights aside, SAIL can be expected to fly only in a few remote air corridors, likely enabling it to operate as an experimental aircraft in a restricted category without full commercial certification. This in turn would substantially reduce developmental costs.

4.2. Fleet

We calculate that in year 1 of the deployment program (assumed to be 2033), the SAIL fleet would require 8 new aircraft including one flight-ready spare plane at each of the two initial bases. This assumes that one spare does not substantially influence our cost estimates. Table 3 summarizes SAIL fleet and activity in the first 15 years of deployment.

Table 3.  Total fleet and flight activity by hypothesized deployment year.

Year New aircrafta Total aircrafta Total payload (Mt S)b Flights/year Bases Monthly flight hours/aircraftc Flights/base/day
2033 8 8 0.1 4007 2 278 5
2034 6 14 0.2 8015 2 278 11
2035 8 22 0.3 12 022 4 278 8
2036 6 28 0.4 16 029 4 278 11
2037 6 34 0.5 20 036 4 278 14
2038 6 40 0.6 24 044 4 278 16
2039 7 47 0.7 28 051 4 272 19
2040 6 53 0.8 32 058 4 273 22
2041 6 59 0.9 36 065 4 273 25
2042 6 65 1.0 40 073 4 274 27
2043 6 71 1.1 44 080 4 274 30
2044 6 77 1.2 48 087 4 274 33
2045 6 83 1.3 52 095 4 275 36
2046 6 89 1.4 56 102 4 275 38
2047 6 95 1.5 60 109 4 275 41

aIncludes one spare aircraft per base. bS burned in situ to disperse 2× SO2 (see table 1). cExcludes spare aircraft.

Such a scenario also assumes that by year 16, the ‘first-generation’ SAIL technology is supplanted by a second-generation lofting solution for which much higher development sums would be expended to achieve substantially lower subsequent operating costs. No new SAILs would be manufactured thereafter, though the existing SAIL fleet would serve out its remaining economically useful lifespan. We therefore consider development costs of this first-generation SAIL technology, commencing 7 years before year 1 of the program, but do not include any additional development costs to further refine or supplant the technology.

4.3. Development Costs

We estimate total development costs of $~2 billion for the airframe, and a further $350 million for modifying existing low-bypass engines. These numbers are toward the lower end of McClellan et al(20102012) range of $2.1 to $5.6 billion and significantly below the TU Delft students’ estimates of $14 billion for its purpose-built Stratospheric Aerosol Geoengineering Aircraft, or SAGA (Design Synthesis Exercise Group 2 2016). The former base their estimates largely on RAND Corporation’s Development and Procurement Costs of Aircraft (DAPCA) model first developed in the 1960s and 1970s (Boren 1976, Raymer 1999). The latter use McClellan et al (20102012), and thus DAPCA indirectly, as one data point, but also consider a more granular build-up of development costs by category, and finally compare those numbers to the developmental budget for the A380. We arrive at our numbers by developing the preliminary aircraft design described in section 4.1 and then budgeting the elements of that design in a series of personal conversations with relevant commercial vendors. Among the important findings derived from that approach was that while both McClellan et al and TU Delft devoted roughly half their developmental budget to the development of new engines, we found several pre-existing engines that can power SAIL, though with substantial modifications to account for the high-altitude operations.

We de-emphasize commercial aircraft development programs as relevant data points, since it is very different and significantly costlier to design a flexible aircraft for a range of commercial operations than to design a small batch of specialty aircraft like SAIL that is intended for a novel but very specific mission. SAIL must demonstrate that it can fulfill its mission, but its testing and certification process does not need to explore the entire flight envelope to determine the range of operations for which a variety of operators might purpose the aircraft. Moreover, SAIL does not need to compete against other aircraft based on operating costs. In these senses it is more like a military design exercise—what matters is that the aircraft can achieve the specified mission, but the optimization of operating costs is a substantially lesser consideration. Much of the design, certification, and testing costs for commercial manufacturers like Boeing, Airbus, and Bombardier lies in optimizing the aircraft for operational cost by reducing drag, fuel consumption, and maintenance cost, while increasing operational reliability. These same considerations would be applicable to a second-generation SAI lofting solution, when (and if) the desirability of this intervention has been proven and the lofted masses need to be substantially greater. This may be a more advanced and potentially unmanned aircraft, or a non-aircraft lofting technology. The first-generation solution on the other hand would favor ‘quick and cheap’ experimental aircraft for an experimental mission.

Moreover, the small production run of SAIL is unlikely to attract the world’s biggest airframe developers and is more likely the province of experimental aircraft designers. Two such companies have reviewed detailed SAIL specifications and contributed to the conclusion that development costs for SAIL would be less than the reported $300 million budget for Stratolaunch (Foust 2011), the massive catamaran aircraft currently being built with funding from the late Paul Allen. Given that the 650t MTOW of Stratolaunch is more than 6 times that of SAIL, a $250 million budget for the demonstrator aircraft seems generous. Testing costs for a restricted category certification, meanwhile, would run two to three times that, placing the total airframe budget at $0.75 to $1 billion. We arrive at our $2 billion airframe development figure by taking the high end of the range, and arbitrarily doubling it to account for the well-established history of cost overruns in aircraft developmental programs.

To this airframe budget we have added $350 million for engine modifications and testing, which personal communications with Rolls Royce indicates would be sufficient to purpose one of its existing engines to this program.

Both Scaled Composites and The Spaceship Company estimate five years to be the best-case development timeframe, and would suggest allocating 7 years from the commencement of a fully funded program through certification in the relevant jurisdictions and entry into service of the first production aircraft.

All this assumes a deliberate but standard development program rather than a crash effort intended to deploy SAI as soon as possible in response to a (perceived) crisis. Such a military-style deployment effort could cut several years off the assume 7-year development phase and bypass the required civil certification process, while substantially increasing costs.

4.4. Operating costs

We build a SAIL operating budget using modeling conventions and cost factors common to the air freight industry, including aircraft financing assumptions. Table 4 details SAIL operating cost assumptions based upon the relevant cost drivers. We assume $2.00 per gallon for fuel, which comprises one of the largest elements of operating cost, while the cost of sulfur comprises a mere 3% of the budget. We assume a cost of $80/t for molten S (US Geological Survey 2018, pp 160–161) with an assumed additional $20/t for transport.

Table 4.  SAIL operating cost assumptions.

Category Inputa Units Notes
Aircraft capital cost $1.035 m /aircraft/month Total aircraft cost of $130 m; 0.8% monthly lease factor
Crew $1.4k /block hrb 9 crew/aircraft; narrowbody pay scale; crew utilization of 275 block hrs/month; includes (remote) travel and accommodation, and accounts for payload operator
Airframe maintenance $~43k /aircraft/month 3, 8, 10 and 12 years heavy maintenance checks
Engine maintenance $~2.9k /aircraft cycle Average of industry CFM56 or Rolls Royce RR211 full restoration cost/cycle plus $280/cycle for life-limited parts; 4 engines; full restoration costs = $3.5 m
Landing gear maintenance $~4.2k /aircraft/month Typical narrowbody industry cost; 10-year overhaul interval
APUcmaintenance $20 /block hrb Typical industry cost
Line maintenance $800 /block hrb Average industry cost for narrowbody plus a premium for unique parts; includes both cyclic & hourly drivers, approximated by block hr
Specialized equipment $250 /block hrb Maintenance of specialized aerosol storage, combustion, and dispersal equipment
Insurance $~54k /aircraft/month 0.5% of acquisition cost (typically 0.25%, added 0.25% for non-typical operation)
Fuel usage 1.85k gallons/block hrb Average for Boing 737 and 757, doubled given 4 engines
Fuel price $2 /gallon Projected long-term cost/gallon
Navigation charges $400 /block hrb Typical industry cost
Landing fees $~1.8k /cycle $0.009 kg−1 of MTOW (typical industry cost) × 2 for specialized facilities
Ground handling $2k /cycle Tow, pushback, etc; premium atop typical narrowbody costs to reflect low utilization airports
Cargo handling $0.05 /kg Typical industry loading cost, applied to departures only (no offload on landing)
Overhead $250k /aircraft/month Dispatch, crew scheduling, flight planning, flight and tech ops administration, other general and administrative costs
Initial spares purchase $2 m /aircraft Start-up cost; premium over typical narrowbody cost given unusual airframe; balance sheet item only, not income statement
Spares carrying cost % 20% /yr Typical industry cost; both depreciation and replacement costs
Inventory carrying cost $400k /aircraft/yr Annual income statement impact for spares
Initial crew training $800k /aircraft Typical industry cost; upfront cost only per aircraft; recurring training cost in crew cost numbers
Payload $0.10 /kg $80/t for molten S (US Geological Survey 2018, pp 160–161), plus $20/t for transport

aAll $ figures in 2018 US $. bBlock hour assumed to be 95% of flight hour. cAPU = Auxiliary Power Unit.

We assume the average marginal cost (i.e. excluding amortization of development costs) for each additional aircraft to be $100 million, roughly equal to the actual purchase price (as opposed to list price) of B767-300 and A330 freighters, both of which have about twice the OEW of SAIL. This assumes that SAIL aircraft will be priced at a substantial premium relative to OEW peers such as the A321 because of the much lower projected production volume.

Given that the low annual aircraft production rates will not facilitate optimization of the production line, we assume conservatively that the build time for one SAIL aircraft is two years. That implies that prior to the commencement of operation, the program will have funded not only the $800 million required for the initial complement of 8 aircraft, but an additional $300 million in progress payments towards the additional six aircraft required in year 2.

In addition to pre-start capital costs, we assume the need for an aggregate of $~40 million to fund an administrative entity that will manage the development program for the aircraft over its seven-year gestation cycle as well as to plan for the commencement of operations. During the two years immediately preceding deployment, a yet larger sum will be required for start-up costs such as hiring and training staff, setting up bases, procuring inventory, and certifying the airline(s) that will actually operate the flights. We estimate the capital required for this purpose to be 50% of the first year operating budget, excluding aircraft capital costs—a sum equal to roughly $100 million.

Table 5 summarizes total SAIL capital requirements during the assumed seven-year development phase and the first 15 years of operation. Total pre-deployment capital requirements are ~$3.6 billion. All costs (pre-start and operational) through Year 5 are ~$10 billion. Total costs through Year 15 are ~$36 billion.

Table 5.  Hypothesized SAIL capital requirements and annual operating costs ($m) relative to deployment year, including cost per deployed t SO2 ($k).

Yeara Admin costs Aircraft NREb Aircraft build Ops start-up costs Non-fleet ops costs Total ops costs Cost per tSO2($k)
−7 2.6 335.7
−6 3.3 335.7
−5 4.1 335.7
−4 5.8 335.7
−3 7.1 335.7
−2 8.1 335.7 400 35
−1 9.4 335.7 700 71
1 700 212 311 1.56
2 700 406 580 1.45
3 600 613 886 1.48
4 600 807 1155 1.44
5 650 1003 1426 1.43
6 650 1199 1696 1.41
7 600 1401 1985 1.42
8 600 1596 2254 1.41
9 600 1792 2525 1.40
10 600 1988 2795 1.40
11 600 2184 3066 1.39
12 600 2380 3337 1.39
13 600 2576 3607 1.39
14 300 2772 3878 1.38
15 2968 4148 1.38
Total 40 2350 9500 106 23 899 33 649 1.4c

aDeployment year relative to hypothesized first year of deployment, assumed here to be 2033. bNRE = Non-Recurring Engineering. cWeighted average = $1402/tSO2, unweighted average = $1422/tSO

Total ops costs in table 5 presents the resulting annual operating costs, including capital costs for fleet procurement as well as the amortization of total development costs. All told, year 1 operating costs are ~$310 m, increasing annually in rough proportion to the growing deployment masses. Unit costs per deployed t SO2 decrease slightly in each year due to accumulating but limited economies of scale. Both simple and weighted average operating costs are ~$1400/t SO2 deployed, in 2018 US $. That places total costs well below any other alternative currently available technology and roughly equivalent to McClellan et al (20102012) $1500/t unit cost estimate for 1 Mt deployed via its proposed new aircraft program. For the reasons outlined above, we have significantly more confidence in our estimate. While $1400/t may convey a false sense of precision, we are confident to conclude that average operating costs are <~$1500/t of SO2 deployed throughout the first 15 years of a deployment that is aimed at offsetting half of the increase in radiative forcing, beginning 15 years hence.

4.5. Sensitivity analysis

Table 6 shows sensitivities of the key financial metrics to changes in various assumed inputs. In general, costs per deployed ton prove to be highly consistent across scenarios. Deployed tons vary substantially, leading to large variations in total annual operating costs. Pre-start costs depend primarily on the aircraft development budget.

Table 6.  Sensitivity of average cost per deployed t SO2($), total cost for 15 years (billion $), and total development costs (billion $) to various input assumptions.

Cost/tSO2($k) Total 15 years costs ($b) Total pre-start costs ($b)
Base case (RCP 6.0) 1.40 35.8 3.6
RCP 4.5 1.43 2% 22.7 −36% 3.2 −11%
RCP 8.5 1.39 −1% 55.5 55% 4.2 17%
Forcing target of 0% change 1.38 −1% 68.5 91% 4.6 28%
Sulfate sensitivity * 150% 1.42 1% 24.9 −30% 3.3 −9%
Sulfate sensitivity * 50% 1.38 −1% 68.5 91% 4.6 28%
Aircraft NRE * 200% 1.50 7% 38.3 7% 5.9 65%
Aircraft manufacturing * 150% 1.57 12% 40.7 14% 4.1 15%
Aircraft manufacturing * 50% 1.24 −12% 30.8 −14% 3.0 −15%
Fuel cost * 200% 1.77 26% 44.7 25% 3.6 1%
Fuel cost * 50% 1.22 −13% 31.3 −12% 3.6 −1%
All other ops cost * 150% 1.80 28% 45.3 27% 3.6 1%
All other ops cost * 50% 1.00 −28% 26.2 −27% 3.6 −1%
Sulfur cost * 200% 1.45 4% 37.0 3% 3.6 0%
Sulfur cost * 50% 1.38 −2% 35.2 −2% 3.6 0%

5. Conclusion

Solar geoengineering is often described as ‘fast, cheap, and imperfect’ (Keith et al 2010, Mahajan et al2018). The deployment scenario laid out here assumes the first, though it clarifies that ‘fast’ in this context refers to the immediacy of the impact that would derive from deployment, not necessarily the ramp up to deployment, which, short of a military-style crash deployment scenario outside this present study, would require several years. This paper further confirms ‘cheap,’ but says nothing about ‘imperfect.’ We here make no judgment about the desirability of SAI. We simply show that a hypothetical deployment program commencing 15 years hence, while both highly uncertain and ambitious, would indeed be technically possible from an engineering perspective. It would also be remarkably inexpensive.

Total pre-start costs to launch a hypothetical SAI effort 15 years from now are ~$3.5 billion in 2018 US $. A program that would deploy 0.2 Mt of SO2 in year 1 and ramp up linearly thereafter at 0.2 Mt SO2/yr would require average annual operating costs of ~$2.25 billion/yr over 15 years. While these figures include all development and direct operating costs, they do not include any indirect costs such as for monitoring and measuring the impacts of SAI deployment, leading Reynolds et al (2016) to call SAI’s low costs a solar geoengineering ‘trope’ that has ‘overstayed its welcome’. Estimating such numbers is highly speculative. Keith et al (2017), among others, simply takes the entire US Global Change Research Program budget of $3 billion/yr as a rough proxy (Our Changing Planet 2016), more than doubling our average annual deployment estimates.

Whether the annual number is $2.25 or $5.25 billion to cut average projected increases in radiative forcing in half from a particular date onward, these numbers confirm prior low estimates that invoke the ‘incredible economics’ of solar geoengineering (Barrett 2008) and descriptions of its ‘free driver’ properties (Wagner and Weitzman 20122015, Weitzman 2015). Dozens of countries would have both the expertise and the money to launch such a program. Around 50 countries have military budgets greater than $3 billion, with 30 greater than $6 billion (Stockholm International Peace Research Institute 2017).

6. Further discussion

SAI’s low cost and its resulting ‘free driver’ properties often invoke considerable discomfort (Burns et al 2016, Lawrence and Crutzen 2017). While largely unstated, one possible concern that may underlie this discomfort is what one might call a deployment program’s presumed ‘secret driver’ properties: the prospect that deploying SAI even at scale could remain undetected (Dalby 2014, Hamilton 2014, Stilgoe 2015).

Focusing on a mature 5 Mt SO2 yr−1 deployment program, Lo et al (2016) surveys methods by which to detect the deployed aerosol particles. Instead, we here focus on the detectability of the deployment program. We would argue that dozens of large aircraft flying many thousands of flights annually from multiple bases in several countries would make such a program easily detectable. This may apply even in year one. A hypothesized deployment of 0.2 Mt SO2 yr−1 involves an initial fleet of 8 purpose-built aircraft flying ~4000 flights, far too much flight activity to remain undetected.

Moreover, while the longitude of injection matters little, recent studies show the vastly improved efficacy of SAI at diverse latitudes. Our hypothetical scenario assumes bases at 30 °N, 15 °N, 15 °S, and 30 °S. No country comes close to spanning such territory. In the Americas, representative base locations would include Houston at 29.8 °N and the northernmost tip of Uruguay at 30.2 °S. African bases would span nearly the entire continent, with Cairo, Egypt, at 30.0 °N and Durban, South Africa, at 29.9 °S. Any such operation would require coordination among several countries in both northern and southern hemispheres, further defeating the prospect of it remaining a secret.

All of the above however assumes a rational actor seeking to implement a scientifically sensible SAI program in a reasonably cost-efficient fashion. Might a less cautious or transparent actor still deploy SAI from a single equatorial island in the middle of the Pacific and evade detection? Such an actor would either deploy directly overhead, or fly to the previously proposed latitudes and deploy there. Either scenario creates a serious tradeoff between operational efficiency and costs on the one hand, and purported ‘secrecy’ on the other. Less efficiency for direct SAI deployment above the equator (Dai et al 2018) implies substantially more deployed payload for the same climate impact. More payload requires more or larger aircraft and more flights, making the program more easily detectable. Meanwhile, launching operations from one base but injecting at or near 15 and 30.0 °N and °S requires flight legs of as much as 2000 miles north and south over international waters, if not foreign airspace. This, in turn, will lengthen flights and roughly double both the number of aircraft required and the overall cost of the deployment regime, making the likelihood of a secret program more remote.

Moving from means to motive, while we hope to demonstrate here that no global SAI program could reasonably expect to maintain secrecy, we also consider who if anyone might wish to implement such a covert program. For one, SAI is global in effect, if not implementation (Keith 2000). SAI, thus, is not a precision weapon. Moreover, judged purely by commercial motives, it is difficult to see how one might utilize SAI solely for local gain without triggering substantial global spillovers, both negative and positive. While there might be a long list of contractors who would eagerly bid to vend hardware, supplies, and services to an SAI endeavor, and there might even be a role for patents along that supply chain (Reynolds et al 20172018), we believe strongly that commercial profits must not be a motivating factor in any decisions about whether, when, where, and how to implement SAI. Any entity that intends to engineer the climate of the entire globe must act—and be seen to act—purely out of humanitarian and environmental considerations unclouded by aspirations of direct financial gain.


We thank individuals at Airbus, Atlas Air, Boeing, Bombardier, GE Engines, Gulfstream, Lockheed Martin, NASA, Near Space Corporation, Northrup Grumman, Rolls Royce Engines, Scaled Composites, The Spaceship Company, and Virgin Orbit for inputs into our analysis. We thank Elizabeth Burns, Zhen Dai, David Keith, Doug MacMartin, Jesse Reynolds, and Trude Storelvmo for helpful comments and discussions.


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