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\begin{document}

\title{On thin ice: Solar geoengineering to manage tipping element risks in the
cryosphere by 2040}



\author[1]{Wake Smith}%
\author[1]{Madeline Bartels }%
\author[1]{Jasper Boers}%
\author[1]{Christian Rice}%
\affil[1]{Affiliation not available}%


\vspace{-1em}



  
  \date{June 15, 2024}


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\sloppy


\textbf{Abstract}

Tipping elements are features of the climate system which can display
self-reinforcing and non-linear responses if pushed beyond a certain
threshold (the ``tipping point'').~~Models suggest that we may surpass
several of these tipping points in the next few decades, irrespective of
which emissions pathway humanity follows.~~Some tipping elements reside
in the Arctic and Antarctic and could potentially be avoided or arrested
via a solar geoengineering program only at the poles.~~This paper
considers the utility of proactively developing the capacity to respond
to emergent tipping element threats at the poles as a matter of risk
management.~~It then examines both the air and ground infrastructure
that would be required to operationalize such capability by 2040 and
finds that this would require a funded launch decision by a financially
credible actor by roughly 2030.~

\par\null

\textbf{Plain Language Summary~}

Stratospheric aerosol injection is a solar geoengineering method by
which tiny particles cast into the stratosphere reflect a portion of
incoming sunlight away from Earth.~~Recent work has demonstrated that
solar geoengineering may be effective in preventing components of the
climate system known as ``tipping elements'' from collapsing by cooling
the atmosphere and thereby preventing the crossing of dangerous
temperature thresholds.~~In particular, many of the highest risk tipping
elements reside in the cryosphere and their temperature thresholds are
expected to be passed in coming decades regardless of global mitigation
pathways.~~A solar geoengineering program at high latitudes could
effectively manage climate risk by cooling the Arctic and Antarctic,
thereby preventing tipping elements from collapsing.~~This paper
investigates the ground and air infrastructure that would be required to
operate such a solar geoengineering program by the year 2040.~~We first
describe the logistics of such a program, including airport base
location choice, plane procurement and modification strategies, and
projected costs.~~We find that infrastructure development for a polar
solar geoengineering program would need to commence by 2030 in order to
successfully safeguard cryospheric tipping elements, suggesting that
research and decision-making processes for a stratospheric aerosol
injection program should be approached with urgency.~~

\par\null

\textbf{1.}~\textbf{Introduction}

Despite the rapid growth of solar and wind power along with advancements
in battery technology capabilities to store such energy, demand for
fossil fuels remains undiminished\textsuperscript{1}~and greenhouse gas
emissions continue to increase\textsuperscript{2}. Without rapid and
dramatic changes in our emissions trajectory, humanity may surpass a
1.5\selectlanguage{ngerman}°C global average surface temperature anomaly by 2030 and 2°C by
mid-century\textsuperscript{3}. Such a temperature trajectory would not
only be damaging in its own right, but would risk triggering
self-reinforcing transitions in various tipping elements of the climate
system\textsuperscript{4--9}, which in turn could dramatically
accelerate climate damages.

The potential urgency of devising responses to tipping elements is
illustrated in Table 1, which is derived from Armstrong McKay et al.
(2022). The minimum estimates of the global mean surface temperatures
that would trigger many of these tipping elements are in many cases well
below 2ºC, and expected tipping temperature values for key tipping
elements such as abrupt permafrost thaw and subpolar gyre (SPG) collapse
are also below 2ºC\textsuperscript{5}. These tipping temperature
thresholds will be surpassed irrespective of which Representative
Concentration Pathway (RCP) the world ultimately selects. Even diverting
to a radical emissions reduction scenario such as RCP2.6 would not
prevent us from crossing before 2050 the minimum temperature threshold
that might trigger a tip of the Atlantic Meridional Overturning
Circulation (AMOC)\textsuperscript{5}. It is possible that we have
already crossed the thresholds that might lead to an irrecoverable loss
of the West Antarctic or Greenland ice sheets\textsuperscript{5}. Or,
those thresholds may lie just ahead.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.98\columnwidth]{figures/Screenshot-2024-04-16-at-6-41-37-PM/Screenshot-2024-04-16-at-6-41-37-PM}
\caption{{Thresholds and timescales of tipping element targets and non-tipping
targets addressed by polar SAI deployment, sourced from Armstrong McKay
et al. (2022).
{\label{260432}}%
}}
\end{center}
\end{figure}

\emph{}In all events, these circumstances emphasize how imprudent it is
to rely upon emissions cuts as our sole means of defense against tipping
elements. Mitigation remains a plausible defense against triggering
tipping elements whose thresholds lie in the latter part of this century
or the next, but if thresholds lurk in the next several decades and
before the world's net zero emission point, mitigation is likely to be
an ineffective means of avoiding them. We would remain on a collision
course with climate disasters that could unfold in a non-linear fashion
and evade any subsequent efforts to control them. That would portend
runaway climate change.

\par\null

\textbf{2. SAI and tipping elements}

A growing body of literature supports the assertion that a global
stratospheric aerosol injection program could be effective in delaying
or avoiding many tipping points\textsuperscript{10--21}. This would
suggest a distinct rationale for the commencement of global SAI -- not
for the direct purpose of reducing global mean surface temperatures to
avoid heat-related damages, but rather to avoid tipping thresholds until
mitigation or carbon capture could obviate the need for further solar
geoengineering. This is a twist on the common conception that SAI may be
a means to ``buy time'' before other interventions can kick in.

However, this too may prove too late. While global SAI is often
portrayed as a possible ``emergency'' climate intervention, humanity
should anticipate a roughly two-decade interval between a funded launch
decision and the achievement of substantial global cooling on the order
of 0.5\selectlanguage{ngerman}ºC or 1ºC\textsuperscript{22}. Such a program would require a
fleet of several hundred large-payload, high-altitude jets of a type
that does not yet exist\textsuperscript{23}. Developing such a prototype
aircraft along with sufficiently powerful engines would consume roughly
a decade, and then manufacturing the fleet would consume a second
decade\textsuperscript{22}. Limited and ramping deployment could begin
at the beginning of the second decade, but only when the fleet is
complete would the target level of cooling be reached.

Moreover, this does not imply that we can expect global SAI capability
\textasciitilde{}20 years from today. What would commence this countdown
to achieving a cooling target would be a ``funded launch'' -- a firm
contractual commitment by a financially capable actor to spend tens of
billions of dollars to purchase a large fleet of new jets from one of
the world's major airframers. A roughly equivalent sum would also be
required for ground infrastructure -- principally airports and aerosol
management facilities. Such a spending spree can only be underwritten by
major governments\textsuperscript{24}, whose approval or consent would
also be required to authorize such a program. No government of any size
is known to be even considering embarking on such a program, so a funded
launch of a global SAI endeavor is not a near-term prospect.

Given the immensity of the technological, scientific and governance
obstacles that would need to be overcome before undertaking such a
launch decision, it is our subjective view that a funded launch decision
seems all but impossible in the coming decade and improbable in the
subsequent one. To draw an admittedly arbitrary but no longer flatly
implausible line in the sand, let us imagine that it is the crossing of
the 2ºC GMST threshold in \textasciitilde{}2050 that finally galvanizes
a funded launch decision. That would enable the commencement of a
gradually ramping deployment program starting in \textasciitilde{}2060
and the achievement of the cooling target by \textasciitilde{}2070. So
long as no thresholds had been transgressed before 2070, humanity would
thereafter have the ability not only to cool the planet generally, but
to ward off tipping elements thereafter. We would have successfully
crossed the abyss.

But this would leave a task undone. If there are any tipping points
lying in our path before 2070, we would be destined for a head-on
collision. This begs the question as to whether there are any tools that
could reduce our risk of triggering tips before 2070. Fortunately, the
answer appears to be -- perhaps.

An alternative to the global ``peak-shaving'' conception of SAI is a
program that would target just the poles. Aerosols injected into the
stratosphere equatorward of the Arctic and Antarctic circles would be
carried poleward by the Brewer Dobson Circulation, fitting a parasol
over the top and bottom of the earth. Such a sunshade appears likely to
slow or prevent the deterioration of high-latitude tipping elements such
as the AMOC\textsuperscript{10,12}, the SPG\textsuperscript{12}, and the
ice sheets covering the Greenland\textsuperscript{13,14,20,25} and the
West Antarctic\textsuperscript{17,18,26}. It could ward off abrupt
permafrost melt\textsuperscript{11,27--29} and prevent Arctic winter sea
ice loss\textsuperscript{15,30--32}. Though not tipping systems, gradual
permafrost melt\textsuperscript{11,27--29} and September Arctic sea ice
loss\textsuperscript{15,30--32} can also be ameliorated by a polar SAI
program. This would provide humanity with a direct defense against
colliding with some, though not all, of the tipping thresholds that may
be encountered in the middle of this century.

We posit here another line in the sand, though we perceive this one to
be less arbitrary. It would be prudent for humanity to arm itself to
defend directly against tipping high latitude tipping elements by 2040.
We choose this temporal target not because no protection is required
sooner, but rather because we judge this timescale to be as soon as is
reasonably possible. Later is certainly plausible (much more so, in
fact), but sooner is not. Therefore, from here, we proceed from a simple
question, which is ``if humanity were to seek by 2040 the capability to
respond within a year to a high-latitude tipping point emergency, what
would we need to do between now and then?''

We will first describe in greater detail the deployment scenarios that
might be responsive to the sorts of climate emergencies we can envision
at the poles. We will next consider the air infrastructure that would be
required to mount such responses, and subsequently, the required ground
infrastructure.

\par\null

\textbf{3. Polar solar geoengineering deployment scenario}

In describing a deployment scenario that might prevent or delay crossing
tipping thresholds at the poles, we start with the sub-polar focused
program described in Smith et al. (2022), which was characterized as
follows:

a) Temperature anomaly target: Annual average surface temperature
reduction of 2°C for the area between 60ºN and the North Pole

b) North/south symmetry: Any deployment at one pole must be
countervailed by a roughly equivalent deployment at the opposite pole

c) Injection seasonality: March through June in the north, and September
through December in the south

d) Injection locations: At latitudes of roughly 60ºN and as close to
60ºS as logistics allow

e) Injection altitude: 13 km

f) Deployed material: Sulfur dioxide, vented as a gas

g) Deployed masses: 6.7 Tg-SO\textsubscript{2}/yr in each hemisphere, or
a global aggregate of 13.3 Tg

\par\null

Several elements of the above program merit further discussion or
modification here.

a) While an average annual surface temperature reduction of 2°C was
chosen for Smith et al. (2022), there was no suggestion that this level
of cooling was in any way optimal or fit for purpose. Moreover, in the
current paper, we note many different cryospheric hazards to which we
might potentially seek to respond, and the level of cooling that may be
optimal to suppress one hazard is unlikely to be the same as might be
warranted in respect of another. Further modeling beyond our intent here
will be required to calibrate cooling objectives to particular hazards.
Nonetheless, a -2°C annual temperature change in the area north of 60°N
would be a very substantial cooling, particularly since the seasonal
imbalance would concentrate most of the cooling in the summer months

b) As conceived here, the trigger for an SAI response would be
convincing data suggesting that a particular high-latitude target
tipping element was approaching its threshold, but any large-scale SAI
response at one pole must be countervailed by a roughly equivalent one
in the opposite hemisphere. This is because a program in just one
hemisphere would substantially shift the location of the Intertropical
Convergence Zone (ITCZ), which in turn materially impacts rainfall
patterns in the tropics. So as not to disturb agriculture and ecosystems
in the most densely populated parts of the world, any polar SAI program
must be bi-polar and roughly symmetrical. Symmetry could be conceived as
meaning the same deployed mass in each hemisphere, or the same cooling
impact, or such ratio of hemispheric distribution as creates the least
disturbance to the ITCZ. Noting these prospectively different
definitions, we will assume herein that equivalent masses are deployed
in each hemisphere.

c) Given the extraordinary seasonal imbalance of insolation at the
poles, one would intervene only in the sunny months and not in the dark
ones. Building on conclusions reached in Lee et al. (2021), one would
inject only in the spring/early summer months, such that the aerosols
are aloft when the days are long and the insolation is at its maximum.
While aerosols vented above the tropics would have an atmospheric
lifetime on the order of a year or more, aerosols injected at 60ºN/S
would endure for less than half that time\textsuperscript{30}, meaning
the particles injected in the spring would have mostly fallen out of the
air by autumn.

~d) In respect of injection locations, latitude matters greatly but
longitude seems not to substantially impact deployed aerosol efficacy,
given the efficient east/west mixing provided by the earth's rotation.
Since the hazards we seek to manage reside in the high latitudes and
since material vented into the lower stratosphere flows mostly poleward,
high-latitude injection locations are sensible for this purpose. In the
Arctic, a sunshade from 60ºN northward would shield all of Greenland,
the Arctic Ocean, and most of the permafrost regions from the most
intense summer sunlight. Likewise, all of Antarctica lies south of 60ºS.
Bases near 60ºN are plentiful, with each of Anchorage, Oslo, Stockholm,
Helsinki, and St. Petersburg being proximate. On the other hand, the
southernmost widebody-capable airports in the world are at the tip of
Tierra del Fuego, lying only 54 degrees south of the equator. These are
less ideal than their northern counterparts, but will have to do.

e) An injection height of 13 km is chosen based on the height of the
tropopause at deployment latitudes, noting that aerosols must be
injected above the tropopause to retain sufficiently high residence
times in the stratosphere. Compared to our proposed program, the
principal source of difficulty in respect of tropical deployment is that
an altitude of roughly 20 km is required to access the lower
stratosphere in these regions, requiring the development of novel
aircraft. Latitude is the primary determinant of tropopause height, and
fluctuations due to seasonal\textsuperscript{33,34},
longitudinal\textsuperscript{35,36}, and interannual
variability\textsuperscript{36,37} are relatively minor. During our
proposed injection periods, the tropopause sits at roughly 10 km
altitude at 60ºN and 9 km altitude at 60ºS\textsuperscript{36}.
Tropopause heights at a deployment latitude of 54º in the Southern
Hemisphere rise by only about 1 km compared to 60ºS\textsuperscript{36},
suggesting that deployment from 54ºS is an acceptable alternative. To
all of these altitudes, we add a 3 km buffer to account for tropopause
abnormalities and minor seasonal, longitudinal, and interannual
variation, arriving at an injection height of 13 km. However, tropopause
height variability may mean that on certain days in the deployment
window, an abnormally high tropopause may render deployment ineffectual.
We have reduced dispatch reliability by 5\% herein to account for this,
noting that that is simply a plug number intended to take account of
this issue, subject to further refinement.

f) While other forms of sulfur are possible and perhaps preferable in
deployment schemes, and non-sulfur aerosols would avoid interfering with
the recovery of the ozone layer particularly in the Antarctic, we have
for simplicity retained the most common assumption, which is that our
program's aerosol is SO\textsubscript{2} vented as a gas. After a few
weeks, SO\textsubscript{2} would oxidize to
H\textsubscript{2}SO\textsubscript{4}, in which state it would be
effective at deflecting the intense summer sunlight at the poles.

g) Sourcing from Lee et al. (2021) and assuming linear scaling, 6.7
Tg-SO\textsubscript{2}/yr was determined to be required in the Arctic in
order to reduce temperatures by 2°C\textsuperscript{15,38}. An
equivalent deployed mass was assumed in the Antarctic. We reiterate that
the resulting aggregate deployed mass of 13.3 Tg/yr should be perceived
as an order-of-magnitude first guess, subject to substantial further
refinement.

\par\null

\textbf{4.} \textbf{Air infrastructure}

The lower altitude required for high-latitude aerosol injections is a
game-changer in respect of aeronautical infrastructure. Unlike tropical
injections that would require novel high-altitude aircraft with minimal
payloads\textsuperscript{23}, sub-polar injections could be performed
with existing aircraft designs, because the 13 km altitude requirement
(roughly 43,000 feet) is at or near the service ceilings certified for
many modern jetliners. Moreover, contrary to the conclusions reached in
Smith et al. (2022), we conclude here that some existing aircraft would
be well-suited to this mission and that purpose-built novel aircraft
would not be required. This removes one substantial task from the
infrastructural build-out necessary to undertake such an intervention.

Smith et al. (2022) examined the capabilities of existing air-to-air
refueling tankers for polar SAI deployment given the mission
similarities, since in both cases the planes are tasked with lofting
dense loads of fluids into the air. However, the aging aircraft designs
used for air-to-air refueling caused these aircraft to perform poorly at
13 km, suggesting that purpose-built SAI aircraft would be substantially
more economical despite the required upfront investment in a new design.
In this study, we have analyzed the capabilities of the most modern
freighters either in production or on the drawing boards at Boeing and
Airbus and find those to be much more capable than the tanker platforms
and well-suited to a 13 km deployment mission. In particular, our
analysis suggests that a modified variant of the 777F (a ``777 Special
Tanker'') could haul a whopping 110.6 metric tonnes (243,750 lbs) to the
target altitude, which is roughly 3 times the payload of the tankers
Boeing and Airbus are currently delivering to air forces around the
world and over 1/3 more than the payload of the ``SAIL-43k'' bespoke
deployment aircraft conceived in Smith et al. (2022). For the task of
developing an emergency response capability to tipping point phenomena
at the poles, the 777F appears to be an excellent starting point.

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.98\columnwidth]{figures/Screenshot-2024-04-16-at-6-44-51-PM/Screenshot-2024-04-16-at-6-44-51-PM}
\caption{{Altitude and payload capabilities for candidate aircraft for a polar
solar geoengineering program at an altitude of 43,000 feet
(\textasciitilde{}13 km).
{\label{599177}}%
}}
\end{center}
\end{figure}

Even more capable is the final variant of the venerable 747 program, the
747-8F, which could carry a substantially larger payload of 314,500
pounds to the target altitude. The problem with planning the fleet based
on this super-jumbo is that the last 747-8 was delivered in January 2023
and the production line has closed\textsuperscript{39}. Consequently,
obtaining new aircraft from Boeing would require a line restart roughly
a decade from now, and the chances of that are remote. Just 107 747-8Fs
were delivered, along with 48 in passenger
configuration\textsuperscript{40}. The polar SAI mission would require
more than 70 such aircraft, which is to say most of the production
freighter fleet and roughly half the total fleet. The likelihood that so
large a proportion of the delivered fleet could be acquired on the
secondhand market is low, and passenger aircraft converted to the tanker
mission would have marginally reduced payloads relative to the
production freighters. Nonetheless, some substantial proportion of a
polar SAI fleet perhaps could be assembled via used 747-8s. This raises
the prospect of a mixed SAI fleet, consisting of some repurposed 747-8s
and perhaps some similarly repurposed mid-life 777s, operating alongside
a core of factory- delivered 777 Special Tankers. Freight operators are
opportunistic in their fleet acquisitions and therefore customarily have
a mix of production freighters and repurposed second-hand aircraft in
their fleets. An SAI fleet might be similarly composed. Nonetheless, for
the purpose of the discussion hereafter, we will assume an all 777
Special Tanker fleet is newly acquired from Boeing, noting that in fact
some portion of it might be otherwise acquired.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.98\columnwidth]{figures/Screenshot-2024-04-16-at-6-46-17-PM/Screenshot-2024-04-16-at-6-46-17-PM}
\caption{{Activity and fleet requirements for different candidate aircraft.
{\label{794906}}%
}}
\end{center}
\end{figure}

Even in respect of aircraft newly delivered from the factory, a design
modification program would be needed to adapt the 777F to this unique
mission. This could be done in the 777 production facility, but is more
likely to be done in a separate modification line elsewhere. Insulated
double-walled pressurized tanks would need to be installed through the
existing freighter side door. These tanks should provide safe carriage
of the aerosols and maintain it at the desired temperature in flight.
The payload weight concentration near the aircraft's center of gravity
will~require local strengthening of the main deck and surrounding
airframe structure.~ An internal plumbing system would need to be
developed and installed to enable aerosol loading on the ground and
venting at altitude.~ Nozzles to vent the aerosols would need to be
designed and installed. Local modifications to the aircraft electrical
system would be required to provide power to pumps, control valves, and
monitoring systems.~If required by the FAA, the cargo bay may need to be
isolated from the cockpit via a physical pressure bulkhead and positive
pressure in the crewstation. This would ensure that an inadvertent
escape of the highly toxic aerosols on the ground or in flight would not
endanger the flight crew. Modifications to the cockpit instrument panel
would allow the crew to manage and control the payload. All of these
modifications would need to be engineered, tested, and certified before
they could actually be installed in an operational aircraft.

As illustrated in Addendum 1, a reasonable time frame in which to
engineer and certify the modification program that would transform the
777F into the 777 Special Tanker might be three years. Thereafter, a
fleet of more than 90 777s must be manufactured, and then modified to
the Special Tanker configuration. This is far too large a fleet to be
sourced second-hand on the open market, so some or all of these would be
new deliveries from the factory. Boeing currently produces 36 777s per
year\textsuperscript{41}. If the SAI program were to elbow aside the
world's airlines and claim half the production line, it would take
another 5 years to build the fleet. 15 aircraft a year is also the rate
at which Boeing is delivering new KC-46 tankers to the
USAF\textsuperscript{42}, so this is a market-reasonable production
rate. Nonetheless, even with an existing aircraft program as our
starting point, it would require roughly a decade to stand up the
aeronautical infrastructure required to be capable of responding to
tipping points in the cryosphere.

\par\null

\textbf{5.} \textbf{Ground infrastructure}

A decade may prove short in comparison with the span required for the
corresponding ground infrastructure. We envision a minimum of four air
bases as being required for the polar SAI program -- two in the north
and two in the south. From a robustness/resilience/reliability
standpoint, operating from a single base in each hemisphere would be
unreasonably risky. As previously noted, there are several options from
which to choose for bases near 60\selectlanguage{ngerman}ºN, but for the purpose of this
analysis, we focus on the two busiest among them, Anchorage (ANC) and
Stockholm (ARN). Each hosts roughly 100,000 aircraft landings per
year\textsuperscript{43,44}. While the SAI program would be merely
seasonal, during the four month deployment window, daily operations
would roughly double at both airports. This would require a rough
doubling of capacity of nearly everything at the airport -- hotels, fuel
storage, maintenance hangars, kitchens, roadways, automobile parking
garages, medical facilities, warehouses, etc.

The most acute shortage noted in dialogue and correspondence with the
operators of both airports is hard stand aircraft parking spots, so a
substantial apron expansion would be required. This would be relatively
straightforward for ARN, which owns ample adjacent land, but not so for
ANC, which is surrounded by urban development and the Cook Inlet.
Nonetheless, with further landfill into the Inlet, such an apron
expansion with related hydrant fueling capacity and taxiway
infrastructure could occur.~~Other identified needs would include 777
maintenance hangar, storage and servicing facilities for additional
airfield maintenance and ground service equipment, additional de-icing
capacity, crew quarters, and several supplemental storage tanks for fuel
and separately for sulfur.~

However, the most substantial further infrastructure requirement would
be an additional runway, which would almost certainly be required to
accommodate not only an SAI program but projected continued growth in
normal passenger and freight operations.~~A new 10,000 foot north/south
runway and adjacent taxiways at ANC would require a substantial landfill
protrusion into Cook Inlet, where a threatened local population of
Beluga whales are the object of great local
affection\textsuperscript{45}.~~Citizen outcry and lawsuits should
therefore be expected.~~A new runway requirement would boost the total
project budget into the low billions of dollars (see Table 5) and define
a project span to 12 -- 15 years (see Table 6), with the majority of
that pertaining to planning, permitting, and
approvals\textsuperscript{46-50}.~The construction itself could be done
in perhaps five years.

Alternatively, building a new airport on a green field site near but not
protruding into Cook Inlet would appear to add minimally to the budget
and not at all to the span. Choosing a site that is not hemmed in by
city and sea would eliminate the landfill requirement and substantially
reduce the likely environmental and citizen outcry. The operators at ANC
estimate that a brand new airport dedicated to the SAI operation could
be built across the inlet on Point Mackenzie for \$3 billion.

Given their greater control of adjacent property, ARN might confront
fewer land constraints in building out airport capacity. Nonetheless,
their estimate for the span to add a runway is 15 -- 20 years.
Construction itself would require no more than three years, so the vast
majority of that span is consumed by planning and approvals.
Nonetheless, were a new runway needed by 2040, the airport would need
already to be in motion, which it is not.

A novel trans-national financing arrangement may be required to fund any
airport build-out for emergency response SAI capability since the timing
and likelihood of its use would be highly uncertain. One can imagine the
voters of both Alaska and Sweden declining to fund such a project with
immense global but limited local benefit. Nonetheless, with sufficient
global will, there would undoubtedly be a way to overcome the local
politics -- there is ample precedent for doubling capacities at major
airports.

The same must be true for the southern airports, but problems there are
far more acute. Although we might seek a latitudinally symmetrical
southern base at or near 60ºS, there is no proximate land mass to
accommodate one. The southernmost tips of Africa, Tasmania, and the main
islands of New Zealand lie at southern latitudes of 34.5, 43.4, and 47.2
degrees respectively. Stanley, the capital of the Falkland Islands, sits
at 51.4°S. South Georgia is farther south at 54.2°S, but hosts only a
tiny non-permanent population and no airport. No other islands deeper
into the Southern Ocean offer sensible options. The closest landmass to
our target latitude lies across the Southern Ocean in the South Shetland
Islands at the tip of the Antarctic Peninsula, but this is utterly
inhospitable territory lying too far south at 62.1ºS. Hosting the
northernmost airport in Antarctica, it has merely a gravel runway a bit
longer than 4200 feet.

Re-crossing the Southern Ocean, the tip of Cape Horn is at 56.0°S on an
uninhabited island whose notoriously foul weather at the juncture of the
world's two largest oceans has bedeviled sea operations for centuries
and would create challenging conditions for air operations as well. The
most southerly permanently settled place in the world is Puerto
Williams, Chile at 54.9°S on the southern side of the Beagle Channel in
Tierra del Fuego. With a population of merely \textasciitilde{}2500, its
small airport has a single 4700 foot runway that handles roughly 1000
landings per year. In contrast, each airport in the SAI program would
call for more than 250~\emph{per day} on runways twice as long. Moving
northward in search of more viable options, next up are Ushuaia,
Argentina (54.5ºS) on the opposite (northern) side of the Beagle
Channel, and Punta Arenas, Chile (53.0ºS) on the Strait of Magellan.
Each has a runway longer than 9000 feet that could accommodate a 777.
However, the SAI program would increase daily operations by roughly 20X
at Punta Arenas and 30X at Ushuaia.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.98\columnwidth]{figures/Screenshot-2024-04-16-at-6-47-12-PM/Screenshot-2024-04-16-at-6-47-12-PM}
\caption{{Prospective bases for a polar solar geoengineering program.
{\label{914477}}%
}}
\end{center}
\end{figure}

Given such dramatic traffic increases, one could consider simply
building a brand-new airport in one or both countries, but it is not
merely tarmac and hangars that are required, but a local economy to
employ, house, and feed the many thousands of people required to staff
this operation. To enable airport construction, the adjacent seaport
would need to be expanded to bring in all the construction materials,
fuel, sulfur, and other supplies. There is no large economic engine such
as Anchorage or Stockholm in this region on which to piggyback, and
there is no reasonable geographic alternative to Patagonia. One could
perhaps reconceive these southern bases as isolated military
installations, but that wouldn't much change the infrastructure
build-out required. Neither Chile nor Argentina would have any incentive
to commence such a project until some external funding source stood
ready to foot the bill. Nonetheless, given the low ebb at which these
projects would start, it may be that the ground infrastructure build-out
at the tip of Patagonia would define the critical path for global
readiness to arrest the emergence of tipping points at the poles.

A review of public information about newly finished or ongoing airport
construction projects around the world provides context regarding the
granular cost and span information received from ANC and ARN. A list of
substantially all new large commercial airports opened in the last 30
years for which publicly reported cost data is available suggests an
average initial development cost of 5.8 billion in 2023 dollars.
However, as this includes both very large and mid-sized airports, it is
a very crude metric. Dividing these costs by the number of aircraft
movements in a recent year yields a cost-per-100,000-movements of \$2.9
billion. Alternatively, simply dividing development cost by the number
of runways (another crude metric of airport capacity) yields a cost per
runway of \$2.6 billion. As we envision airports with 2 runways, this
would suggest a cost of \$5.2 billion for new SAI airports. This array
of figures confirms that ANC's estimate of \$3 billion for a brand new
airport is on the low end of a very reasonable range. We will therefore
assume that a new airport of the size envisioned here might cost \$3 --
5 billion. They also confirm that ANC's estimate \$2.3 billion to double
the capacity of an airport currently handling roughly 100,000 aircraft
movements per year is also reasonable, leading to a planning figure for
such an expansion of \$2 -- 3 billion.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.98\columnwidth]{figures/Screenshot-2024-04-16-at-6-48-57-PM/Screenshot-2024-04-16-at-6-48-57-PM}
\caption{{Cost metrics for recent and ongoing new airport construction projects.
{\label{150875}}%
}}
\end{center}
\end{figure}

The average span from initial start date to opening for the airports
listed is nearly 19 years. This exceeds the 15 years estimated by ANC
and suggests that 15 -- 20 years for a new airport would be a prudent
assumption. \emph{}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.98\columnwidth]{figures/Screenshot-2024-04-16-at-6-49-31-PM/Screenshot-2024-04-16-at-6-49-31-PM}
\caption{{Time span metrics for recent and ongoing new airport construction
projects.
{\label{258352}}%
}}
\end{center}
\end{figure}

\par\null

\textbf{6.} \textbf{Discussion}

To summarize, the large infrastructural tasks required to put a tipping
element response capability in place by 2040 are to:

\begin{itemize}
\tightlist
\item
  design and certify a modification program for Special Tankers
\item
  manufacture or otherwise acquire a fleet of over 90 777s and run them
  through the modification line
\item
  roughly double the capacity of two large northern airports
\item
  build nearly from scratch two large airports and related support
  capacity in Patagonia
\end{itemize}

As previously noted, a roughly decadal time span should be sufficient to
procure the fleet. That would be more than enough time to build out
capacity at ANC and ARN if no new runways are required, but likely
insufficient time to build no runways. It appears unlikely that the
southern bases would be ready on this schedule, irrespective of whether
they are built upon the existing infrastructure at USH and PUQ or simply
erected on green fields elsewhere in Tierra del Fuego. However, it may
be that a smaller operation could commence from the existing southern
airports while the new ones are completed, implying that the attainment
of the full Antarctic annual deployment would lag its Arctic counterpart
by several years. Even with the benefit of this optimistic assumption,
in order to have our tool ready by January 1, 2040, we would need to
start on our longest span task by roughly January 1, 2030.

As earlier noted, ``start'' in this case would not mean ``commence the
governance discussions'' -- it would mean one or more large governments
actually dispensing funds to contractors on three continents to begin
engineering on aircraft and airports. Within a few years, dirt would
need to be moving at various airports and Boeing or Airbus would need to
start delivering purpose-built aircraft. But if the funds started to
flow in 2030, with some fudge factor for the southern airfields, we
could conceivably have our response capability in place by our target
date. All that seems feasible.

What does not seem feasible is that we would be ready to pull that
trigger by 2030. This article is being written in 2024 and with luck
will be published in the same year. Insofar as we are aware, no
government is yet in motion to prepare a tipping point response
capability at all, let alone by 2040. There is no awareness among policy
makers of such a prospect, and no public support for it. And yet, in
five years and change, some consortium of governments must be ready to
sign a \textasciitilde{}\$20 billion purchase contract with a major
airframer and another roughly \$15 billion in construction contracts
with Argentina, Chile, Sweden and the State of Alaska. Failing that, the
2040 date will recede into the future, and the span during which the
world will be vulnerable to and unprepared for tipping points in the
cryosphere will be elongated.

This flips the standard conception of the governance problem related to
SAI on its head. Governance is customarily conceived in negative terms
-- i.e., the task is to prevent actors from deploying prematurely,
irresponsibly, and/or without widespread consensus and
legitimacy\textsuperscript{46,47}. It is a matter of preventing parties
from doing things we don't want them to do. However, the governance task
here is the opposite -- how to get someone to step up to the plate by
2030 with a roughly \$35 billion checkbook to put in place a response
capability that would protect all of humanity but that few people yet
understand to be required. This protection would be a non-excludable
good, creating incentives for all actors to free-ride. In light of that,
how do we motivate a first mover, and how do we distribute the financial
burden of implementing the program?

Addressing that is beyond our scope here, but we hope that a first step
in deriving an answer is to pose the question.

\par\null

\textbf{7.} \textbf{Conclusion}

In the climate arena as elsewhere, when one finds oneself in a hole, the
first step is to stop digging. Emissions reductions are the
indispensable response to the climate problem, and neither SAI nor any
other palliative can replace them. However, it is increasingly clear
that those may not arrive in time to ward off substantial climate
damages in the coming decades, particularly from prospective tipping
elements with low temperature thresholds. Mitigation alone is no longer
sufficient to secure the future climate that we and our successors on
this planet will desire. A global peak-shaving SAI program could also
prove effective in delaying or preventing tips in both high and
low-latitude regions, but for both infrastructural and sociopolitical
reasons, it seems a distant prospect. The infrastructural hurdle related
to an SAI program targeting just the poles appears to be substantially
smaller, as would be the resulting deployment program. It is unclear on
the other hand whether the remoteness of the deployment program and the
prospective urgency of the tipping element threat may render this
intervention more acceptable to policy makers and the general public
than the global sort, but that is the question that needs to be promptly
presented. Policy makers are only dimly aware that tipping element
threats may be looming in our near future, and we know of no prior
literature suggesting that the intervention discussed here may be a
practicable remedy. A prudent response to these circumstances would be
to dramatically accelerate both research and communications in respect
of a polar solar geoengineering program such that by 2030, policymakers
might be capable of making an informed decision as to whether to
proceed.

On the one hand, this seems like an impossibly tall order given the low
awareness of and regard for SAI that obtains currently. And yet, the
response to the 1985 identification of the ``ozone hole'' demonstrates
that concrete pending threats have a galvanizing effect that slowly
rising temperatures do not. The analogy is not precisely apt, but the
tipping element threats appear to be real if somewhat indeterminate. We
are steaming heedlessly towards shoals that we are warned may lie
directly ahead. As we seem unable to rapidly stop the ship, it would
seem unjustifiably cavalier not to explore other response options that
appear promising.

\par\null

\textbf{Acknowledgements}

The authors extend their gratitude to John Johansen of Ted Stevens
Anchorage International Airport (ANC) and Fredrik Jaresved of Stockholm
Arlanda Airport (ARN) for their support in providing ground
infrastructure logistics and costing information.~

\par\null

\textbf{Data Availability Statement~}

The data sources compiled in Tables 5 and 6 on airport movements, costs,
and construction timelines are listed in Addendums 2 and 3.~

\par\null

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