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 and the Carbon Cycle.

Introduction

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.

The Carbon Cycle

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.

Photosynthetic sequestration

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 already 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.

Chemical sequestration

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.

Physical sequestration

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 $55/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.

Conclusion

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.

References

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.

Discuss anything related to this page at this forum.

The opinions on this site are personal and do not necessarily reflect those of any third party. All information is given in good faith but no responsibility is taken for such information; any person or organisation using such information should ascertain that it is suitable for his/her/their conditions of use.

No reproduction of the contents of part or the whole of this site may be made in any form without the written permission of the owner. An exception is made that a print-out may be made for one individual's private use without seeking permission; it is forbidden to make multiple copies or to photocopy a print-out. Links may be made to this site.

Contact us

Copyright © CypEnv 2004/2007