Energi- Forsynings- og Klimaudvalget 2017-18
EFK Alm.del Bilag 179
Offentligt
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Redistribution of CO
2
between atmospheric layers as a means to mitigate global warming –
two thought experiments
John R Porter
Professor D.Sc.
Faculty of Sciences
University of Copenhagen
Denmark
[email protected]
Summary: Climate change mitigation and negative emission technologies need to consider options
for redistributing CO
2
between different layers of the atmosphere, which may raise the global
albedo as well as mitigate global warming. This short paper is an attempt to stimulate thinking
about how this might be done. I am convinced that radical technical solutions will be required to
limit global warming.
Negative greenhouse emission technologies (NETs) attempt to redistribute CO
2
between
atmospheric, land, ocean and geological reservoirs to reduce the concentration of greenhouse gases
in the atmospheric reservoir. The biophysical and economic limits to a range of such processes were
recently presented
1
, with the conclusion that 3.3 Gt C is the level of NETs needed for a possible 2
o
C
future, in the absence of drastic fossil fuel emissions. Given the current lack of international urgency
to ratify agreements made at the COP21 meeting in December 2015, which could be considered
modest at best and extremely unlikely to hold warming below 2
o
C about pre-industrial – it is likely
that ‘technical fixes’ involving large scale geo-engineering will perhaps be the only way to avoid lethal
threats to the planet from high levels of warming. Finding reservoirs for greenhouse gases may mean
that perhaps we need to look upwards, as well as downwards from the Earth surface, for other
possible reservoirs
2
for CO
2
and consider how it might be possible to remove CO
2
from the global
atmosphere and out into near-space. The atmosphere is structured with the three lowest layers
being the troposphere (up to about 10km above the land surface), followed by the stratosphere (up
to about 50km above the land surface). Above these two is the mesosphere and there is a boundary
layer between this and the lower layers such that the CO
2
concentration (ca. 150 ppmv CO
2
) in the
mesosphere is less than half that in the lower layers.
The key property to enable the biological storage of C is the fact that the terrestrial ecosystem is
composed of strata - for example above- and below-ground reservoirs of C. These strata have
different residence times for C, caused by the degree and types of chemical linkages of C with other
 EFK, Alm.del - 2017-18 - Bilag 179: Henvendelse af 5/3-18 fra John R Porter, University of Montpellier, France, vedrørende klimaforandringer
elements, particularly H. At one extreme there is oxygenated C, as CO
2
, and at the other extreme are
hydrocarbons of one form or another giving chemically reduced C. The terrestrial C strata are a
reflection of the different forms of C. On the other hand the global atmosphere has an almost
complete predominance of C as the CO
2
form and thus the question arises as to what is the basis of
the atmospheric stratification into the troposphere, stratosphere and mesosphere (ignoring the
thermosphere – or near-space layer at about 110 km height above the Earth surface). In the
atmosphere, stratification is caused (according to the people who know about these things) by
vertical differences in O
3
amounts and concentrations but much more importantly by the spectral
properties of CO
2
– which absorbs long-wave radiation but is transparent to short wave-lengths.
Planetary stratification of C depends, in the biosphere, on the chemical form of C, but in the
atmosphere on the physical radiative preferences of gaseous CO
2
; the generic element being that
both planetary chemistry and physics generate ‘boxes or reservoirs’ in which C can be stored.
Technologies and policies, such as climate-smart agriculture and forest preservation, are being used
in order to redistribute C from atmosphere to biological sinks to offset the warming effects of
burning C from historical reservoirs, with chemistry being the driving ‘metier’. But can the physics of
the atmosphere be used to alleviate tropospheric warming by altering the strata-based distribution
of C in the planetary atmosphere? We will examine two ideas.
The mesosphere is characterized by two important features – it harbours so-called noctilucent clouds
formed by ice crystals and, whereas in the lower atmosphere CO
2
warms the planetary surface by
absorbing infrared radiation radiated by the earth’s surface; in the mesosphere CO
2
cools the
atmosphere by radiating heat into space
3
. Noctilucent clouds are formed by ice crystals from water
vapour and seeded by small dust particles with sizes of about 40-100nm and are thought to have a
role as indicators of climate change
4
. In addition, the mesosphere temperature is about 10-40
o
C
colder than the STP sublimation temperature of CO2 (-75
o
C at 100kPa or 1bar(B), decreasing to -
140
o
C at 100Pa or 1mB), at which point CO
2
changes directly from a gas to a solid and, in the
mesosphere, changes to white crystals. Thus at first glance, if more CO
2
could be injected into the
mesosphere via geostationary machine and what might be called ‘atmospheric stomata’ (to borrow
an analogy from leaves) then this would reduce levels in the lower layers and at the same time raise
the global albedo via the presence of reflective crystals in the mesosphere, thus having a solar
radiation management effect on the stratrosphere
4
. The main physics questions would be what is the
effect of moving CO
2
upwards, how much needs to be moved, how long is its residence time in the
mesosphere and what effect would an increase of CO
2
in the mesosphere have on the global albedo.
However and unfortunately, this scenario is not realistic for two decisive reasons; the first is shown
 EFK, Alm.del - 2017-18 - Bilag 179: Henvendelse af 5/3-18 fra John R Porter, University of Montpellier, France, vedrørende klimaforandringer
from the CO
2
phase diagram (Figure 1) that the sublimation temperature decreases to about -140
o
C
for the kind of extremely low pressures that exist in the mesosphere, which is to all intents a vacuum.
Such extremely low temperatures are only found occasionally in the mesosphere and only at high
latitudes. The second issue would be the engineering aspect that would be very difficult, as it is for all
NETs. The main challenge is how to concentrate the CO
2
so that one does not have to move large
volumes of air. There are technologies on the horizon that might help here; catalytically enhanced
sorption-desorption
5
, nano-sieve membrane separation
6
or cyclonic separation technologies
7
, that
are capable of separating CO
2
from air and are currently being developed. However, overcoming
technical and engineering barriers is a feature common to many forms of NETs. The energy aspect of
such technologies is not a concern as solar energy at almost the power density of the solar constant
would be available to drive the required processes.
Initial calculations indicate that about 3.3 Gt C per year would be a reasonable aspirational target for
atmospheric stomata, but in concert with other NETs, a level of 1 Gt C of NETs removed via
atmospheric stomata is another option. What is needed is an effort to make some models of the
processes involved and work on possible designs of machines for sorption-desorption , membrane
separation or cyclonic separation that could be used to efficiently concentrate and transport CO2
from the stratosphere to the mesosphere. It is impossible to disagree with Smith
et al.’s
conclusion
that the preferred route to a livable planet in the future is via a rapid transfer from fossil to non-
fossil based fuels, but there is likely to be a need for new thinking in terms of NETs in the future.
Combination of the removal of CO2 together with an increase in the atmospheric albedo may offer
one such novel route.
If the mesosphere as an enhanced reservoir for CO
2
has to be excluded for the reasons given above,
then is there any mileage in considering the next level down - the stratosphere, which extends from
about 10km to 50km above the Earth’s surface. The pressure-temperature relationship of the
stratosphere is almost as unconducive to sublimation of CO
2
as in the mesosphere but there is
evidence that increasing levels of CO
2
in the stratosphere does lead to cooling of this layer
10
. The
reasons for this are unclear, but has to do with the differential emissivity and absorption responses
of the troposphere and stratosphere in the presence of CO
2
. We know that emissivity of long-wave
radiation from the troposphere to the stratosphere declines as CO
2
increases (as it is doing via fossil
fuel burning); however emissivity of the stratosphere either remains constant or slightly increases
with higher levels of CO
2
(ozone in this layer also plays a minor role in the stratospheric short-wave
radiation emission and energy balance). The net effect is that emissivity increases relative to
 EFK, Alm.del - 2017-18 - Bilag 179: Henvendelse af 5/3-18 fra John R Porter, University of Montpellier, France, vedrørende klimaforandringer
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absorption in the stratosphere with increased CO
2
in the troposphere and thus the stratosphere
paradoxically cools with increased levels of tropospheric CO
2
. The big question is – can this
understanding be used to cool the global atmosphere by physically pumping CO
2
from the
troposphere to the stratosphere? At first sight this would have the effect of reducing warming in the
troposphere whilst increasing cooling in the stratosphere – and thereby contribute to a reduction in
global warming. There are no doubt many who will say that the engineering challenges of doing such
changes of carbon pools in the atmosphere are prohibitive – but are they really more challenging as
physics than the chemical challenges of converting CO
2
into more reduced terrestrial forms and
trying to store them in global ecosystems, including agriculture. We need more research on the
physical and chemical properties of the higher atmosphere and to think ‘big’ about how we are to
avoid the pitfalls and Earth system shocks of an enhanced Anthropocene, which is the direction in
which humans are moving at ever faster speeds.
Figure 1. The Temperature-Pressure Phase Diagram for CO
2
showing how the sublimation
temperature of CO
2
decreases with decreasing pressure.
http://hub.globalccsinstitute.com.
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References
1. Smith P. et al., Biophysical and economic limits to negative CO
2
emissions.
Nature Clim. Change
DOI: 10.1038/NCLIMATE2870 (2015).
2. The Economist. A stairway to heaven? May 31 2007. http://www.economist.com/node/9253976.
3.
www.Mesosphere&Mesopause.htm
4. Thomas, GE; Olivero, J (2001). "Noctilucent clouds as possible indicators of global change in the
mesosphere". Advances in Space Research 28 (7): 939–946.
5. Boucher, O., D. Randall, P. Artaxo, C. Bretherton, G. Feingold, P. Forster, V.-M. Kerminen, Y. Kondo,
H. Liao, U. Lohmann, P. Rasch, S.K. Satheesh, S. Sherwood, B. Stevens and X.Y. Zhang. Clouds and
Aerosols. In:
Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to
the Fifth Assessment Report of the Intergovernmental Panel on Climate Change
[Stocker, T.F., D.
Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley
(eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA (2013).
6. Wang, Q., Luo, J., Zhong, Z., Borgna, A. 2011. CO2 capture by solid adsorbents and their
applications: current status and new trends. Energy and Environmental Science 4, pp 42-55
7. Du, N.Y., Park, H.B., Robertson, G.P.,Dal-Cin, M.M, Visser, T., Scoles, L., Guiver, M.D. 2011. Polymer
nano-sieve membranes for CO2-capture applications. Nature Materials 10, pp 372-375
8. Wang, H.L., Zhang, Y.H., Wang, J.G., Liu, H.L. 2012. Cyclonic Separation Technology: Researches
and Developments. Chinese Journal of Chemical Engineering 20, pp 212-219
9.
http://hub.globalccsinstitute.com/publications/co2-liquid-logistics-shipping-concept-llsc-
overall-supply-chain-optimization/53-co2
10. Connolley W 2011 Why does the stratosphere cool under global warming (GW)? Science Blogs,
1-41.
Short information on the author:
Consultant Professor, Supagro Montpellier, France
Emeritus Professor of Climate and Food Security, University of Copenhagen (UC), Denmark
Emeritus Professor of Agriculture and Climate Change, Natural Resources Institute, University of Greenwich, UK
Honorary Professor, Lincoln University, New Zealand
Honorary Fellow, University of Bangor, UK
Career Summary
Porter is an internationally known agro-ecological scientist with an expertise in ecosystem services in agro-ecosystems,
including agro-ecology, simulation modelling and food system ecology. His main contribution has been multi-
disciplinary and collaborative experimental and modelling work in the response of arable crops, energy crops and
complex agro-ecosystems to their environment with an emphasis on climate change, ecosystem services and food
systems. Porter has published 145 papers in peer-reviewed journals out of a total of about 350 publications. On
average, his peer-reviewed papers have been cited more than 100 times each. He has personally received three
international prizes for his research and teaching and two others jointly with his research group. His career H index is 57
and with 129 papers receiving over 10 citations.
From 2011 to 2014 he led the writing of the critically important chapter for the IPCC 5th Assessment in Working Group
2 on food production systems and food security, including fisheries and livestock. This chapter was one of the most
th
cited from the IPCC 5 Assessment and formed an important scientific bedrock of the COP21 agreement in Paris in
2015.