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arXiv:2401.07829v1 [physics.space-ph] 15 Jan 2024
Cooling the Earth with CO2filled containers in space
Orfeu Bertolami1,2, and Clovis Jacinto de Matos3
1Departamento de F´ısica e Astronomia, Faculdade de Ciˆencias, Universidade do Porto,
Rua do Campo Alegre s/n, 4169-007 Porto, Portugal
2Centro de F´ısica das Universidades do Minho e do Porto, Rua do Campo Alegre s/n,
4169-007 Porto, Portugal
3European Space Agency - ESA Headquarters, 75007 Paris, France
Abstract
We argue that geostationary (GEO) reflective containers filled with CO2could be used
as shading devices to selectively cool areas on Earth’s surface. This proposal would be an
interesting addition to the recently discussed suggestion of dumping CO2to space through
the well of a space lift [1]. We also explore the possibility of producing propellants in GEO
out of greenhouse gases expelled from the space lift. Finally, we discuss the much less
effective idea of filtering the most prominent infrared bands of the incoming solar radiation
using the C O2wrapped in transparent vessels.
E-mail addresses: orfeu.bertolami@fc.up.pt; clovis.de.matos@esa.int
1
1 Introduction
In a recent publication [1] it was proposed that CO2could be transported to space by some
suitable adaptations of the space lift. It was shown that the well of a geostationary orbital lift
or space elevator as it is usually referred to, could be used for dumping greenhouse gases into
space. Naturally, it was assumed that the known requirements to build a stable orbital lift are
satisfied, and it was discussed how to use this infrastructure to dump greenhouse gases away
from Earth’s atmosphere.
Indeed, based on recent advances in material science, more specifically, in the development
of carbon nanotubes and macro-scale single crystal graphene, it has been pointed out that the
space lift could be a reality in a foreseeable future (see Ref. [2] for a recent assessment). In
Ref. [1], the use of the space lift well was considered to dump the excess of anthropogenic
atmospheric CO2into space, but of course, the concept could also be considered for other
greenhouse gases (like for example CH4).
The typical dimensions of the orbital lift are its vertical extension, rG≃35786 km, and
an assumed constant cross-sectional area, A=πr2, where ris the radius of the well. The
anchor of the orbital lift could be a geostationary satellite in an equatorial plane orbit. The
idea of Ref. [1] is to inject CO2into the well of the orbital lift and generate an upward flow
so to dump CO2into space. Natural conditions do not allow for any effective upwards flow as
Earth’s escape velocity is much greater than the typical average velocities of the CO2molecules
in the air. Therefore, conditions for an upward flow must be engineered. The steps to create
an upward flow of CO2molecules are detailed in Ref. [1] and, in its simplest form, involve
the following steps: i) separation of the CO2in the air and its injection into the well, which
has been emptied of its initial content; ii) ionisation of the CO2in the well via soft X-ray
irradiation; iii) acceleration of the charged CO2through an electric field along the vertical axis
of the orbital lift.
Under reasonable assumptions, it was shown that the outward flow of CO2is given by,
Φ = jπr2, where j=ρvf,ρand vfbeing the CO2density and vfthe final velocity at the
upper end of the well. For ρ= 4 ×10−4kg/m3and r= 15 mone gets: Φ1= 4.2ton/s for the
scenario where the first and last sections of the well are under the effect of the electric field and
a middle section with no electric field; Φ2= 3.4ton/s for the scenario where ionised C O2is
accelerated by an electric field in three sections of the well with two intermediate sections with
no electric field [1]. These flows correspond to less than 2% of the anthropogenic C O2yield.
Improvements on the estimated yields are possible and were discussed in Ref. [1]. However,
the point we would like to make in the present work is that the CO2flow could be used to
fill reflective or transparent containers that would reflect or filter part of the incoming solar
radiation at the wavelengths, 4.3µm and 15 µm, making it somewhat ”cooler”. For sure,
in what concerns filtering, the individual CO2molecules may re-emit the absorbed radiation
after a while, however, the absorbed radiation can be dissipated and re-emission avoided if the
concentration of CO2is sufficiently high, the greenhouse effect. We believe that, even though
most of the greenhouse effect is due to the absorption of these infrared wavelengths when solar
radiation is reflected back to space by the ground on Earth, the depletion of these wavelengths
2
in the incoming radiation may have an attenuating effect on the net greenhouse effect.
Before we discuss the details of the present proposal let us point out that the thrust due to
the injection of gas into space that is transmitted on the structure of the space lift can be esti-
mated to be about 6.3×107Nfor the first scenario discussed above. However, this undesirable
effect can be avoided by adopting the simple solution of directing a symmetric ejection of the
CO2along a direction perpendicular to the axis of the well. The desired cancellation can be
achieved by a radially symmetric set of nozzles perpendicular to the axis of the lift at the top
end of the well. In what follows, we shall consider that the delivered CO2gas at the top of the
well is used to fill the reflective or infrared absorbing vessels.
2 Reflective CO2filled containers
The main idea here is to use the CO2ejected by the nozzles at the top of the space lift to
fill vessels wrapped with reflective material so to redirect back to space the incoming solar
radiation. The shade provided by the vessels will depend on their number and configuration.
A sufficiently large set of vessels can cover a considerable area. The vessels can be endowed
with additional features, but their main property is their reflective power, which can be quite
high.
Even though we consider the vessels to be as simple as possible, they can themselves be
endowed with nozzles so to use their CO2content for propulsion. Once the vessels acquire their
final volume, they could be attached to each other, transported away from the space lift and
cover a patch over an area where cooling is most needed (like e.g. at the poles). As for the
vessels themselves they should be wrapped with a reflective material that ensures gas tightness,
is resistant to low/high temperatures and to the interplanetary radiation. The backbone of its
structure should be light, resistant and paramagnetic. The properties for these structures can
be encountered, for instance, in aluminium alloys widely used in the aerospace engineering.
However, as the space lift itself might be built with carbon nanotubes, this material could, in
principle, be used for the backbone of the vessels too. The flux of C O2at the end of the space
lift is sufficient to fill a great number of vessels and thus, in the theory, there is no limit to the
number of vessels to be attached together and to the shading area they can provide.
3 Reflective CO2filled containers in the Inner Lagrange
Point (L1)
The shadow provided by the reflective containers in GEO discussed above is essentially of
the same order of magnitude of the area covered by the ensemble of containers. In order to
shade larger areas it would require either quite large reflecting surfaces in GEO or to move the
mirror-like structure away from Earth, beyond GEO orbital radius [3]. Alternatively, locating
a mirror of about 9.4 million km2at the Lagrange Point (L1) between the Sun and Earth at
a distance of about 1.5×109mfrom the Earth centre, one could reduce the solar radiation
3
over the Earth disk by about 1.8%, which would suffice to relief the global warming situation
[4, 5]. On the other hand, storing C O2and also CH4in GEO, through submitting CH4to
similar manipulations using the space lift as described in Ref. [1] for the CO2, would open new
interesting avenues to fuel future space missions, including a mission to drive reflective CO2
containers to L1 as suggested in Ref. [6].
In GEO, solar energy could be used to power the pyrolysis of methane, to produce hydrogen,
CH4
heat
−−→ C+ 2H2(1)
and to sustain the Sabatier reaction of C O2to produce methane and water.
CO2+ 4H2
heat
−−−−−−−−−−−−→
pressure and catalyst CH4+ 2H2O(2)
The electrolysis of water would then allow producing H2and O2:
2H2Oelectric current
−−−−−−−−→ O2+ 2H2(3)
Hence, the CO2and C H4greenhouse gases dumped to space by the well of the space lift
could allow for yielding and storing important quantities of oxygen, hydrogen and methane
propellants directly at GEO. The resulting infrastructure could fuel all types of space missions
and, in particular, a mission to place CO2filled containers to L1.
In fact, different spacecraft components can, in principle, be brought to assemblage at GEO,
so that spacecraft could be assembled and fueled with propellants produced and stored at GEO.
Thus, the infrastructure provided by the space lift, and its well, can support the entire logistics
of beyond GEO space missions.
The concept proposed here opens the possibility to carry out missions with a challenging
high ∆vcost such as, for instance, Polar orbit ones. This could allow for creating a significant
surface to screen Earth’s polar caps so to reduce their current melting rate.
4 Refractive CO2filled containers
Let us now discuss the main features of the refractive CO2filled containers. As the reflective
containers, the refractive containers are filled with CO2at the top of the space lift. The vessels
are essentially transparent and we consider the simplest possible realization of the idea. The
most relevant property of the vessels is that they absorb radiation in the relevant infrared
wavelengths 4.3µm and 15 µm. The absorption effect can be estimated from the radiative
equation by the intensity attenuation after crossing a medium with density, ρ, and a length, L
[7]:
Iν(L) = I0e−τν,(4)
where the optical depth of the medium at those specific wavelengths, τν, is given in terms of
an integral over a given length:
τν=ZL
0
kνρds, (5)
4
kνbeing the constant that characterises the medium at a given wavelength.
Assuming that the gas density is uniform, then,
τν=kνρL. (6)
The optical depth can also be expressed in terms of microscopic properties through the
number density of the absorbing molecules, n, and the cross section of the absorbing process,
σ:
τν=nσνL. (7)
It is easy to show that if the CO2behaves in the vessels as an ideal gas, then:
σν=kν
µ
NA
,(8)
where µis molecular weight of the CO2and NAAvogadro’s number.
The necessary data to estimate the CO2absorption under realistic conditions can be found,
for instance, in Ref. [8], in terms of the CO2concentration. For typical values, that is 400 ppm,
which under PNT conditions corresponds to ρCO2= 5.16 ×10−4kgm−3, one obtains [8]: k≡
kνρ≃1.0×10−1m−1, implying that kν≃1.93 ×102m2kg−1.
The conditions to avoid that the absorbed energy is re-emitted is that the excess energy
due to the radiation is thermalised. Assuming a reasonable attenuation figure, say 10% of the
incoming radiation on the wavelengths 4.3µm and 15 µm implies from Eq. (4) and (6) that
τν≃10−1and with data obtained from Ref. [8], one obtains that 10−1m−1< k < 6×10−1m−1.
Still assuming the CO2in the vessels behaves as an ideal gas, then for the density figure
corresponding to 400 ppm, one obtains the relations between pressure and temperature:
p
1atm= 2.58 ×10−4T
273 K,(9)
As the CO2freezing point is T= 194.65 K, we consider, for instance, T≃200 Kand hence:
p
1atm= 1.89 ×10−4.(10)
To evaluate the volume of the vessel, we consider a width, L= 1 m, and then the area
facing the sun should be A= 108m2, meaning that for, say, 100 vessels, the area facing the
sun of each vessel should be 106m2, that is, for vessels that have a paving stone shape with a
square face, their length should be 1 km.
Once the set of vessels are positioned, their C O2content will absorb the incoming infrared
radiation and heat up the contained gas, which will increase its pressure. Thus, the vessels
should be built with a material that might allow for an increase in volume of an order of
magnitude or so. Once the pressure reaches a certain level, a built in set of symmetrically
distributed valves would allow for the release of gas into space.
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Notice that the above considerations are equivalent to the discussion of the greenhouse
effect on a layer of atmosphere where one admits for the incoming solar radiation a strong
transmission and weak absorbing coefficient, τs≃0.9 [9].
The infrared absorbing structures that we propose can be an interesting addition to the
arsenal of geoengineering space devices to be built in order to face the ongoing climate change
crisis and are a logical step forward on the use of the space lift to dump greenhouse gases into
space discussed in Ref. [1]. Of course, most of the greenhouse effect is due to the atmospheric
absorption of the solar radiation reflected by Earth’s surface. In rough terms, the main con-
tributors to the reflection are soil, snow, vegetation and water. The last three components do
not reflect any incoming radiation in the 4.3µm and 15 µm wavelengths [10], meaning that
soil reflection is the main component. It is still to be thoroughly studied the effect of depleting
the incoming solar radiation from the main wavelengths that cause the greenhouse effect, but
it is reasonable to think that it might have a beneficial impact.
5 Conclusions and Discussion
It is consensual that anthropogenic climate change due to the accumulation of greenhouse gases
in the atmosphere is a major civilizational challenge. In order to face the disruptive effects on
the patterns of the climate that are already observed, one needs to embark on deep changes
on the tenets of the consumption society powered by cheap fossil fuels and built upon the
mistaken assumptions about Earth’s resources and its capability to admit an indefinite dump
of waste. Naturally, any solution to the problem involves drastic reduction on emissions and
radical socio-economic changes.
Given the structural nature and the depth of the changes that must be implemented, geo-
engineering proposals have been put forward in order to buy us some time. Indeed, severall
proposals have been advanced, some of which controverrsial, such as for instance, ocean fer-
tilisation and alkalinity enhancement, albedo enhancement through passive daytime radiative
cooling [11, 12], the use of sky-facing thermally-emissive surfaces to radiate heat back into space
[13, 14], stratospheric aerosol injection (SAI), the so-called “Budyko blancket” [15, 16, 17, 18],
cloud brightening or a large set of mirrors in the sky to reflect back into space a fraction of the
incoming solar radiation (see Ref. [18] for a review). Other set of ideas involve space reflectors
such as a space mirror [3, 4] or a myriad of reflecting bubbles [6]. Our proposal of reflective
vessels filled with CO2can be seen as a realization of the latter. Of course, geoengineering
proposals involve necessarily some degree of negative effects.
Thus, in principle, any device that reduces and reflects back to space the incoming solar
radiation might be useful to reduce the amount of infrared radiation trapped in the atmosphere.
In the present work we have considered an hypothetical device that involves containers filled
with CO2gas. The vessels can be reflective or transparent so to filter the two main absorbing
wavelength of the C O2. The first possibility is much more promising. The device can be
coupled with the space lift proposed in a previous study [1]. In the case of reflective vessel, an
all spectrum shade is provided. In the case of the refractive vessels, we have shown that an
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infrared ”shadow” can be created by a set of vessels in a geostationary orbit with the net effect
of attenuating the incoming solar radiation in the 4.3µm and 15 µm wavelengths if the vessels
were filled with CO2. The use of other greenhouse gases, such as for instance methane, could
extend the “blanket” effect for other wavelengths in the infrared.
The possibility to produce and store rocket propellants (H2,O2,CH4) in GEO out of
CO2and CH4greenhouse gases pumped from the Earth atmosphere could also support space
missions to transport the reflective/refractive vessels to polar orbits, or to the Inner L1 Lagrange
point. At L1, it becomes possible to shade the entire Earth disk. As a bonus, having a
propellant refuelling station in GEO would unlock many possibilities for new innovative space
exploitation/exploration missions.
Given the urgency to avoid that the continuous climbing of the concentration of greenhouse
gases drive the Earth System (ES) to a Hot House Earth State [22], even the most apparently
unrealistic geoengineering proposals are welcome to partially fix the problem. It should be
pointed out that the most pessimistic predictions about a possible collapse of the great regu-
latory ecosystems [22] are in agreement with theoretical analyses based on a physical model of
the Earth System and the Hot House Earth State as an inevitable outcome given the present
intensity of human activities (see e.g. Refs. [23, 24, 25, 26, 27, 28]).
For sure, the magnitude of the problem asks, first of all, for a deep change on the assumptions
of the neoliberal market economy based on the wrong assumption that a finite planet can
afford an indefinite period of economical growth without an inevitable collapse. It is becoming
increasingly evident that no sustainable future is possible without an encompassing plan for
global economic deceleration and a drastic reduction in the use of fossil fuels.
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