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20 November 2009
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Note: This essay was originally published
as a web monograph and is in a slightly different style to the other
essays on this site. It has not been written specifically for the
Cypriot context but is nevertheless relevant.
Introduction
The Carbon Cycle
Photosynthetic sequestration
Chemical sequestration
Physical sequestration
Conclusion
References
Sequestration, in this context, is defined
as a means to reducing the emissions of greenhouse gases by capturing
them and rendering them harmless by some means or another.
One of the measures proposed to reduce the
"global warming" effect is to sequester excess carbon dioxide (CO2)
from the atmosphere. Such propositions come mainly from eco-political NGOs [Johnston
et al., 1999] or as part of a political manifesto [Bush,
2001], rather than scientific circles. It should be noted that carbon
dioxide is not the only "greenhouse gas" (GHG) responsible for anthropogenic
(man-made) climate change, although it is the most important contender. The
bulk of the carbon dioxide in the atmosphere is of natural origin and is
essential to sustain life. However, since the Industrial Revolution, man has
caused ever-increasing amounts of carbon dioxide to be emitted because of
the combustion of fossil fuels, the manufacture of cement and concrete, the
mining of carbonaceous rocks and the effect of acid rains on newly exposed
carbonaceous rock. According to Schimel et al., 1996,
the average carbon dioxide concentration in the atmosphere was 280 ppm in
1880, whereas Barry et al., 1998,
state it was 358 ppm in 1995, an increase of nearly 28 percent, increasing
at an average annual rate of 0.4% from 1980 to 1990. Furthermore, these
figures are confirmed by Machta, 1977, the
observations at Mauna Loa Observatory, Hawaii and others, with an
undisputable concordance. The Intergovernmental Panel on Climate Change
(IPCC) have co-related this increase, combined with that of other man-made
GHGs and various natural phenomena, to the increase of observed globally
averaged temperatures over the past century or so.
The above IPCC graphs [Watson
et al., 2001] show the observed average global temperature (in
red) over nearly 150 years, indicating a
temperature rise of 0.8°C, essentially over 100 years. In grey, is the
theoretical average global temperature, as calculated using the latest
climatic modelling algorithms. These take into account many cyclical and
acyclical natural phenomena (e.g., solar radiation, the earth's orbit,
sunspot cycles, El Niño etc.) in graphs (a) and (c) as well
as radiative forcing due to GHGs in (b) and (c). It can be seen that the
correlation is good in (c) and that the major temperature rise is due to the
anthropogenic forcing produced by the GHGs.
The work of the IPCC does not yet constitute an
absolute scientific proof that the increase of man-made GHGs is responsible
for the observed climate changes, although the circumstantial evidence is
overwhelming.
Sequestration consists of removing some of the
carbon dioxide from the atmosphere in such a way as to reduce the loading
that man has added. Three ways have been proposed to achieve this:
reforestation, chemical capture and physical disposal.
There is a finite quantity of carbon on this
planet, making up only about 0.025 per cent of the earth's crust. A fraction
of this total is constantly being cycled through natural life processes. The
rest is held in carbonaceous rocks, as fossil fuels and in deep ocean waters
and do not normally enter the biological carbon cycle. However, if man
releases them into the atmosphere, particularly in the forms of carbon
dioxide and methane, the natural balance within the carbon cycle is upset.
This diagram [after IPCC, 1990,
after Sundquist et al.] illustrates the carbon
cycle. It shows the quantities of carbon (in black) in each part of the
cycle expressed in gigatonnes (1 Gt = 1012 kg), with annual
changes in italics. The figures in red
show the annual gross flux.
It can be seen that the annual
gross flux out of the atmosphere is 194 Gt, 102 representing photosynthesis
to land plants and 92 Gt being absorbed by the oceans and with a net loss
from there to marine biota (algae) of 4 Gt. The important points are the two
figures on the left, 5 Gt resulting from the combustion of fossil fuels and
2 Gt coming from the land/biota interface as a result of deforestation. This
7 Gt is the gross result of human activity, causing a net annual carbon
loading increase in the atmosphere of 3 Gt. The 4 Gt difference is due to
the two sinks, essentially land vegetation and the ocean, increasing their
uptakes.
There has been some emphasis that
the notion of planting trees will make a significant difference to the
carbon loading in the atmosphere. To be able to absorb the excess annual
loading of 3 Gt would mean that about 15 Gt of extra trees would need to be
grown each year and this would do nothing for the carbon previously added as a
result of human activity. Some of this would be returned in the short term
as a result of rotting leaves. What exactly does this mean? The General
Sherman tree in the Sequoia National Park in California is estimated to
weigh a little over 6 kt; it would therefore require an extra annual growth
equivalent to 2,500,000 such trees, just to sequester the excess carbon
dioxide we are adding to the atmosphere each year, and we would have to
repeat this feat every year. This is unimaginable. Of course, sequoia trees
are not ideal for this, and smaller fast-growing species, such as willows,
pines, hazel etc. would be more suitable. These would require vast
quantities of water, which is a precious commodity in many places, and
nutrients, some of which are derived from fossil fuels sources.
Let us imagine that, by some
means, we are able to plant millions of new trees, obviously quick-growing,
in sufficient quantity to make a significant photosynthetic absorption. What
will happen? Two scenarios are possible: either man culls the new trees when
they have reached the end of their main growth period (say, after 20 or 30
years) or nature takes its course. Such trees are unlikely to be a suitable
source of timber. The main usefulness would be as fuel for renewable energy
generation. So, they are burnt. All the sequestered carbon is therefore
returned to the atmosphere and we are back where we started. The same
applies if they are used for cheap paper production (newsprint): sooner or
later, the carbon will be released back to the atmosphere, no matter how
many times it is recycled. If we let nature take its course, the trees will
die and rot or be burnt in forest fires. Either way, the sequestered carbon
will be returned to the atmosphere.
Such sequestration, at the best,
can be only a very temporary palliative and can never represent a permanent
solution to the excess carbon dioxide in the atmosphere.
This involves capturing the carbon
dioxide from the air by means of a chemical reaction. The most feasible
reaction would be to use calcium oxide ( quicklime) or calcium hydroxide (
slaked lime). The reactions would be
CaO + CO2 > CaCO3
Ca(OH)2 + CO2 > CaCO3 + H2O
In both cases, the carbon dioxide
is captured to form calcium carbonate, which is the main constituent of
limestone, chalk, marble and some other minerals. The idea of the proponents
of this method is to dump the calcium carbonate down disused coal mines or
to fill in open cast mining holes. The only problems are that lime is formed
by subjecting limestone to heat in a kiln, thereby releasing as much carbon
dioxide as it can subsequently sequester and that to react 3 Gt of carbon
dioxide will produce 25 Gt of calcium carbonate, a volume of over 10 billion
m3. To visualise this, imagine a column of solid calcium carbonate with a
base of 1 km x 1 km. It would stretch upwards nearly to the stratosphere,
10,000 metres high! And that, every year.
Another chemical that may
prove marginally useful is monoethanolamine (MEA), if only because it
can selectively absorb acid gases, including the carbon dioxide and then
release them in a concentrated form, while the MEA can be largely
recycled in a closed circuit. This may be useful for absorbing and
separating the GHGs from, say, flue gases, leaving the nitrogen
untouched.
This is a more recent concept of
pumping carbon dioxide into deep submarine aquifers. At 800 to 1,000 metres
depth, the pressure is such that the uptake of carbon dioxide by dissolution
in water (including sea water) can be considerable, making a kind of "super
soda water". The cost of separating flue gases, pumping them to very high
pressures down a deep submarine borehole to a reservoir which will become
saturated, is very high. Once saturated, an aquifer would become useless for
further sequestration. Some boreholes may be able to absorb as much as a
megatonne over a few years, but this is too small to make any real
significant improvement to the global man-made emissions.
There is one unknown in this
technique: would the gas remain in solution for long periods? Some would
almost certainly slowly percolate back to the surface to be re-emitted, but
the time and quantity scales have yet to be determined because the
mechanisms to cause the gas to be released are not yet fully understood.
Rises in geothermal temperatures and tectonic shocks are two factors that
may be implicated in the gas coming out of solution.
On the plus side, this technique
may be used in near-depleted oilfields to force residual hydrocarbons
towards neighbouring boreholes. This would help amortise the high costs,
especially if oil prices rise further (about $120/bbl at the time of
writing).
This technique, which is still
experimental, could be used as an addition to large fossil fuel-burning
power stations but would be useless for transport and domestic heating
applications. The other two techniques of sequestration could be used to
actually remove carbon dioxide from the atmosphere, if only they were
practicable!
A minor variation has been
proposed to simply pump carbon dioxide down disused coal mines. This seems
even less likely to provide a solution to the problem.
The economics of sequestering
in this manner must be alarming. As far as is known, no full scale
experiment has yet been successful. It is possible that the cost would
mean a very significant increase in energy prices.
There is no possibility of being
able to sequester sufficient carbon dioxide from the atmosphere that would
make any significant impact on the amount that man is adding annually, let
alone capturing the amounts that have accumulated over the past century or
so. Nor does it seem practical for large scale schemes to prevent carbon
dioxide from being emitted. As an approximation, it is probable that any
possible action that could be undertaken would fall short of the needs by
many orders of magnitude. It would require a total of about 200 Gt of carbon
to be sequestered to restore the atmosphere to 1850 levels of carbon
loading.
However, many have said that
any contribution, no matter how small, is worthwhile.
Barry, R., Chorley, R., 1998, Atmosphere, Weather & Climate, 7th Ed.,
ISBN 0-415-160019-7, Routledge, London
Bush, George W., 2001: Extract from speech : "We all believe technology
offers great promise to significantly reduce [greenhouse gas] emissions --
especially carbon capture, storage and sequestration technologies.",
President George W. Bush, June 11, 2001
Johnston, P.,
Santillo, D., Stringer, R., Parmentier, R., Hare, W., Krueger, M.,
1999: Ocean Disposal/Sequestration of Carbon Dioxide from Fossil Fuel
Production and Use: An Overview of Rationale, Techniques and Implications,
Greenpeace International
Machta, L., 1972:
The role of the oceans and biosphere in the carbon dioxide cycle, in Dyrssen,
D., Jagner, D., The changing Chemistry of the Oceans, Nobel Symposium 20,
Wiley, New York, 121-45
Schimel, D.
and 26 others, 1996: Radiative forcing of climate changes; in Houghton, J.T.
et al. Climate Change 1995, The Science of Climate Change, Cambridge
University Press, Cambridge, 65-131.
Sundquist
and others, 1990: in Houghton, J.T., Jenkins, G.J., Ephraums, J.J.: Climate
Change: The IPCC Scientific Assessment, Cambridge University Press,
Cambridge
Watson,
R.T. and other editors, 2001: Climate Change 2001: Synthesis Report,
Intergovernmental Panel on Climate Change, Geneva.
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