Carbon Capture: no real solution

I was interested to note that carbon dioxide gas concentration rises are not evenly spread in the atmosphere around the world at any particular point in time, due to the location of power plants, factories and cars (mostly for the moment in the global north), the time taken to mix the extra injected CO2  gas globally (about a year), and the differing annual forest absorption/decay cycles and sea absorption/release processes around the world.

http://carboncycle2.lbl.gov/resources/experts-corner/annual-cycles-of-atmospheric-co2-concentration.html

You can watch it happening, and the result getting worse year by year, in this excellent animation: www.youtube.com/watch?v=vA7tfz3k_9A  

So what can be done about it?  Carbon Capture and Storage (CCS) is offered as a way to allow us to continue to burn fossil fuels, and, given the strength of the fossil fuel lobby, it certainly has powerful backers, who argue the case for rapid expansion, for example in the UK: www.ccsassociation.org/press-centre/reports-and-publications/

However, despite pilot projects and programmes around the world, progress has been slow, and so far it is undeveloped on any significant scale, with its cost likely to be high.  www.technologyreview.com/news/428355/will-carbon-capture-be-ready-on-time/

There are also uncertainties at to its long term viability-  e.g. would the stored CO2 stay in place  in open aquifers indefinately?  In any case it would only be a partial solution to emissions-   it would not capture any where near 100% of what a power plant produces. And it would certainly not reduce CO2 levels in the atmosphere. 

One idea for that is to reabsorb CO2 from the air. For example, Air capture of atmospheric CO2, coupled with chemical storage, has been proposed as a somewhat desperate geo-engineering attempt to deal with climate change.  The idea is to absorb CO2 by sucking air through vast towers of sodium hydroxide, with the resultant bicarbonate mulch then being stored, or recycled, to release the CO2 for storage in some other form. Superficially it sounds attractive - you can capture CO2 anywhere, not just directly from power plant exhausts. Fill deserts with so-called ‘green trees’. But quite apart the need to store or process the huge volumes of residues, and keep fresh supplies of NaOH coming, the proportion of CO2 in the air is around 0.039%, so to capture a ton of CO2 you would have to process over 2500 tons of air. The overall energy cost would be high. And, more subtly, it has been argued that absorbing CO2 from the air might lead to disruption of natural CO2 absorption processes, by plants and the sea, so you may be no better off. 

That argument seems odd at first glance. Surely getting CO2 levels back down a bit is good, however you do it. Unfortunately that ignores the fact that the seas have absorbed about 50% of our CO2 emissions, and, if we manage to reduce CO2 levels in the atmosphere by air capture, then it may be that some of this huge sea reservoir would be slowly outgassed to replace at least some it. Certainly one study, reported in Environmental Research Letters, suggested that mass removal of air CO2 ‘leads to an 
increase in the ocean-to-air CO2 flux, largely replacing the air CO2 
removed’  i.e. in time, the seas would outgas trapped CO2 pushing atmospheric levels back up. http://iopscience.iop.org/1748-9326/5/2/024011

This process would take time and the rate of outgassing might be low, so initially air capture would still yield a net reduction in atmospheric CO2. There’s evidently a complex balance between absorption and release processes, depending on, amongst other things (including temperature), the small partial pressure difference.  Also, over time, some of the absorbed CO2 is sequestered as geological carbon in seabed/rock formations and some say we could inject more to get it stored this way. http://web.mit.edu/energylab/www/pubs/overview.PDF

For the moment though, most of what’s there is still in surface layers, interacting with the atmosphere. There is a lot of it. Even so, it is possible that, if air extraction was to go ahead on a very large scale, eventually most of the extra sea-absorbed CO2 would presumably be outgassed, so that then, if air capture was continued, the atmospheric CO2 levels could be reduced much more. But it would take a very long time.

The aforementioned ERL paper notes that there is another approach which might be more effective- direct ocean CO2 extraction i.e. from the sea itself.   It argued that ‘excess ocean CO2 removal is required for any effective air CO2
 capture scheme because removal of air CO2 alone will simply reduce air 
CO2 concentration relative to that in the ocean.’ Indeed it boldly claims that, with sea capture, you then won’t need air capture: ‘schemes that consume/remove and sequester excess ocean 
CO2’ can ‘effectively address both excess
 ocean and air CO2, sidestepping the need for direct air CO2 capture.’

Removing CO2 from sea water has its problems (e.g. it needs energy), and it would still take a very long time and a huge effort to make much difference, but the concentration is about 140 times higher than in air, and some clever ideas for sea extraction have emerged: www.pnas.org/content/early/2013/05/30/1222358110.abstract Though if synfuels are produce using the CO2, as some suggest (to give an economic incentive), when they are burnt the overall process is no longer CO2 negative. For an interesting overview (even it does suggest using nuclear energy to run the system)  see: http://bravenewclimate.com/2013/01/16/zero-emission-synfuel-from-seawater/  

There are other options, with perhaps less eco-worries, e.g. biochar production using biomass- trapping CO2 more permanently as charcoal and using this to help to enhance soil fertility and carbon retention : http://carbon-negative.us/docs/CharcoalVision.pdf  and www.biochar.ac.uk/ But see: http://climate-connections.org/2013/07/24/the-problem-with-biochar/  and this absorption option  doesn’t avoid the sea outgassing problem.

What about BECCS, biomass energy carbon capture and storage?  Depending on the source, BECCs should be CO2 negative, and although that doesn’t escape the sea outgassing problem, the energy output would replace fossil burning and new CO2 additions to the atmosphere. So some say that, since geological/aquifer CO2 storage space will be limited, BECCS projects should be given priority over fossil CCS. Though there are still worries about whether CO2 captured from whatever source can be safely stored underground for ever, and about the ecological and land use impacts of the large scale use of biomass.

Tragically, it seems then that most of these artificial /bio carbon capture options have problems, and, even if expanded massively, BECCS maybe aside, would not be able to make a large difference except over a very long period   Worse still, the albeit slow dissolved CO2 blow back may also mean that, sadly, the CO2 absorption from reafforestation, a much more attractive proposition on many levels, would also be undercut and may not be too much (climate) use long term, unless done on a very large scale. Certainly the overall scale of CO2 re- absorption needed, by whatever means, to make much difference is vast. As the ERL paper noted, ‘to maintain atmospheric CO2 concentrations at pre-industrial levels for centuries, ultimately an amount of CO2 approaching the total cumulative amount of anthropogenic CO2 emissions would need to be removed from the atmosphere’. And, it seems, the sea. And it should also be said, the land- excess CO2 has also ended up being trapped in land sinks. Basically it’s too big a job: http://iopscience.iop.org/1748-9326/5/2/024011/pdf/1748-9326_5_2_024011.pdf

Moreover some of these carbon capture options have environmental side effects. Certainly, when it comes to some of the larger scale geo-engineering projects, then we enter a realm where there could be major impacts: seeding the sea with ferric compounds to increase bio-productivity, blocking sunlight with aerosol particles, orbital reflecting mirrors and so on. A recent study concluded that, not only could there be local or global side effects, some of them possibly irreversible, the overall effectiveness was low: even if continuously deployed on a massive scale, the climate engineering methods it evaluated could ‘only sequester an amount of atmospheric CO2 that is small compared with cumulative anthropogenic emissions’ and were ‘unable to prevent the mean surface temperature from increasing to well above 2C by the year 2100’. www.nature.com/ncomms/2014/140225/ncomms4304/full/ncomms4304.html

So we can’t repair the earth much, except maybe very long term, with some risks, and the higher the temperature the harder that will be. Too much CO2 has be released, and trapped partly in the seas and land, to let us get the planet back even near to how it once was. But we can stop making it worse by not burning fossil fuels. That seems the only major option. Unless Gaia comes to the rescue and allows the sea to absorb a lot more CO2 without getting too acidic!  Or some other natural feedback loop intervenes.

Thanks to Jo Abbess from Claverton Energy Group for some of the links.