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.
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.