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.
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 points10–21. 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 0.5ºC or 1ºC22. Such a program would require a
fleet of several hundred large-payload, high-altitude jets of a type
that does not yet exist23. Developing such a prototype
aircraft along with sufficiently powerful engines would consume roughly
a decade, and then manufacturing the fleet would consume a second
decade22. 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
~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 governments24, 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 ~2050 that finally galvanizes
a funded launch decision. That would enable the commencement of a
gradually ramping deployment program starting in ~2060
and the achievement of the cooling target by ~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 AMOC10,12, the SPG12, and the
ice sheets covering the Greenland13,14,20,25 and the
West Antarctic17,18,26. It could ward off abrupt
permafrost melt11,27–29 and prevent Arctic winter sea
ice loss15,30–32. Though not tipping systems, gradual
permafrost melt11,27–29 and September Arctic sea ice
loss15,30–32 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.
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-SO2/yr in each hemisphere, or
a global aggregate of 13.3 Tg
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 time30, 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 seasonal33,34,
longitudinal35,36, and interannual
variability36,37 are relatively minor. During our
proposed injection periods, the tropopause sits at roughly 10 km
altitude at 60ºN and 9 km altitude at 60ºS36.
Tropopause heights at a deployment latitude of 54º in the Southern
Hemisphere rise by only about 1 km compared to
60ºS36, 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 SO2 vented as a gas. After a
few weeks, SO2 would oxidize to
H2SO4, 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-SO2/yr was determined to be required in the Arctic
in order to reduce temperatures by 2°C15,38. 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.
4. 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
payloads23, 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.