For more than six days Earth has been our friend in the lunar skies. That fragile piece of blue with its ancient rafts of life will continue to be man's home as he journeys ever farther in the solar system. Apollo 17, December 14, 1972

Thursday, March 27, 2008

6. The Carbon Cycle - revolutionary industry

The picture so far....
describes the biological aspects of the carbon cycle. To recap, on the right hand side of the cycle, respiration in living cells releases the energy locked up in the carbon based molecules in a regulated and measured way. One byproduct is generally CO2, in turn produced in modest, measured quantities. There is a corresponding powerful chemical process which uses carbon and generates heat and carbon dioxide. In nature it generally takes place in an uncontrolled and fairly destructive fashion in the form of forest and heathland fires. If their scale is large enough they do have a short term impact on the atmosphere both from the smoke they cause and also the CO2 they generate. The counterbalancing forces do allow balance to return eventually. However, it is Man's continuing use of fire on a massive scale which is having a lasting and worrying effect on the biosphere. This takes two main forms: deliberate deforestation and the burning of fossil fuels. Both are important but it is the latter which is of greatest relevance to this stage in our investigation of the delicate biosphere which relies on a well-balanced carbon cycle.

The Industrial Economy
The quite extraordinary arising of the industrial economy over the last two centuries in the West and its spread to the East represents the biggest threat to the balance. In very general terms it would be true to say that the nineteenth century played host to the coal-fired industrial revolution and the twentieth supplemented coal with oil and natural gas. The inconceivably large amounts of CO2this produces, in comparison to the small regulated quantities needed to sustain biological life is pushing the carbon cycle out of balance - hence the level of CO2 in the atmosphere is not only rising but accelerating - this indicates that natural counterbalancing mechanisms are not working. The challenge for the 21st century is to bring control to the increasing CO2 levels and deal with the consequences of this increase in CO2 from our past and current actions and lifestyles.

Returning to the diagram, the additional arrow on the right hand side represents these new man-made chemical combustion processes which use fossil fuels to generate the goods, services and energy we demand. It also can include the energy required to fire the conversion of limestone to quicklime to make cement for the construction industry and its by product, carbon dioxide. The key point about the diagram is that there is no complementary arrow on the left hand side.

All the biosphere has to counteract these increased emissions are two factors of limited effectiveness.

  1. The first of these is increased activity on the left hand side of the cycle. It has been shown in the USA (Duke Forest Experiment) that plants can grow more quickly in the presence of higher concentrations of CO2 and thereby 'fix' more carbon. However this does not remain fixed for all that long and is likely to end up being decomposed or digested by other organisms in the biosphere. For example a conifer tree in a remote forest may grow for fifty years locking much of the carbon it fixes. However it will die and fall to the ground where it will be digested as food by insects, fungi and microorganisms which will expel the carbon as CO2 and methane.
  2. The second is natural sequestration enabled by organisms using carbon to produce skeletons containing calcium carbonate. Most typically this will be marine organisms with exoskeletons or shells. For example a mollusc builds a large shell over its lifespan and despite eventually dying or getting eaten the shell will remain chemically intact and will sink to the sea bed with its load of carbon. The carbon therein effectively has been removed from the biosphere - this is sequestration.

The next post looks more closely at the implications of digging up fossil fuels from the subterranean layer.


Anonymous said...

Hi Jeremy

That's very interesting once again. So what you are saying is that forestry is not going to make much difference to the overall level of CO2 in the atmosphere. By contrast, sea creatures that take in CO2 do have an effect, because their skeletons fall to the bottom of the ocean and get taken down into the earth's crust for millennia.

I know that James Lovelock recently suggested using clusters of big tubes that hang in the ocean with non-return valves, slowly pumping cold water to the surface by wave power, full of these micro-organisms. It turned out (he didn't know this) that an American company has actually been trialling these.

It seems from what you say that something of this kind has to be done. I suppose there are carbon-sequestration systems being developed for coal burning power plants, but we can't sequester all the carbon like this.

Jeremy said...

Dear Kevin,

Thank you for being so quick off the mark again.

Taking your points one at a time:

1. The natural (non-human caused) deforestation which occurs in nature - volcanoes, forest fires - can have its effect (on the RHS of the cycle) in the short term, but generally there is a recovery. In contrast, the sustained (but probably unsustainable) use of fire by modern man is not so short term. It could be viewed as an example of 'testing to destruction' - a technique used commonly in labs: the lab for this experiment is the one and only beautiful, delicate biosphere, of which we are a part.
I am drawing a distinction between natural perturbations and a concerted drive by humankind to consume more and more with no real thought for the future of system which supports us day to day. A part of this is, what might be termed, aggressive and unrelenting deforestation which will not be allowed to recover and will have both environmental and ecological impact. This will have large and long term effects, but I have not researched these carefully as yet. If I appear to be dismissing this aspect, then I need to change the wording, because that is not the intention. What I am doing is preparing the ground for the next post which talks about the exploitation of fossil fuels lying in the subterranean layer.

2. Natural sequestration (on the LHS of the cycle) is useful on the 'global carbon balance sheet' and you have quite rightly described how the CaCO3 drops out of the active biosphere and eventually 'goes subterranean'.

3. Yes I've read reports on the ingenious big tubes which draw down surface water populated with organisms and allow fresh water to be similarly populated, with a view to making the oceans more productive and thereby bigger CO2 sinks. Like many of these measures they are limited in effect simply because you need so many and in those quantities you cannot predict their effects. As I'll discuss in a later post the oceans probably have a much larger effect as a sink due to the conveyor effect. Keeping that working is a major priority.

4. I've been reading an embargoed copy of the 'next big' climate change book which advocates physical sequestration. I think there are three problems with this. The first is, as you suggest, that it could only be applicable to static carbon dioxide emitters such as power stations - this would make a significant contribution but by no means is enough. Secondly I haven't checked the thermodynamics yet but my suspicion is that the energy required to do the necessary capture of gaseous CO2, cooling it and pressurising it sufficiently will require a lot of energy itself and it will probably have to take place in perpetuity. The third reason, which has pre-historical precedent, is terrifying. Squirting large amounts of volatile, naturally gaseous CO2 down bore holes out of which relatively inert crude oil emanated, to be stored for eternity seems highly risk prone. The impact of a blast into the atmosphere has been experienced in pre-history and had cataclysmic effects. Is this the legacy we want to bestow for future generations? The irony is that CO2, a natural by product of life itself could be a more threatening legacy than nuclear waste simply because we would be dealing with so much of this volatile stuff. I think I heard the figure correctly on the News tonight - we are emitting 1000 tonnes per second, a large proportion of which would have to be taken out of circulation.

My global framework, which is very much work in progress, uses the global carbon balance sheet to show how large the measures need to be. The inevitable answers seem to lie in several directions which tantalisingly follow on from this initial ground work on the carbon cycle.