Global warming cannot simply be reversed by turning off the tap of fossil fuel burning. Two centuries’ worth of accumulated anthropogenic carbon dioxide would continue to trap solar energy, even supposing that an immediate shutdown of emissions was feasible; a pure fantasy for any kind of society hooked on coal, oil and gas. It takes too long for natural processes to download CO2 from the atmosphere into oceans, living organic matter or, ultimately, back once more into geological storage. In the carbon cycle, it has been estimated that an individual molecule of the gas returns to one of these ‘sinks’ in about 30 to 95 years. But that is going on all the time for both natural and anthropogenic emissions. Despite the fact that annual human emissions are at present only about 4.5 % of the amount emitted by natural processes, clearly the drawdown processes in the carbon cycle are incapable of balancing them, at present. Currently the anthropogenic excess of CO2 over that in the pre-industrial atmosphere is more than 100 parts per million achieved in only 250 years or so. The record of natural CO2 levels measured in cores through polar ice caps suggests that natural processes would take between 5 to 20 thousand years to achieve a reduction of that amount.
Whatever happens as regards international pledges to reduce emissions, such as those reported by the Paris Agreement, so called ‘net-zero emissions’ leave the planet still a lot warmer than it would be in the ‘natural course of things’. This is why actively attempting to reduce atmospheric carbon dioxide may be the most important thing on the real agenda. The means of carbon sequestration that is most widely touted is pumping emissions from fossil fuel burning into deep geological storage (carbon capture and storage or CCS), but oddly that did not figure in the Paris Agreement, as I mentioned in EPN December 2015. In that post I noted that CCS promised by the actual emitters was not making much progress: a cost of US$50 to 100 per tonne sequestered makes most fossil fuel power stations unprofitable. Last week CCS hit the worlds headlines through reports that an Icelandic initiative to explore a permanent, leak-proof approach had made what appears to be a major breakthrough (Matter, J.M. and 17 others, 2016. Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions. Science, v. 352, p. 1312-1314). EPN January 2009 discussed the method that has now been tested in Iceland. It stems from the common observation that some of the minerals in mafic and ultramafic igneous rocks tend to breakdown in the presence of carbon dioxide dissolved in slightly acid water. The minerals are olivine ([Fe,Mg]2SiO4)] and pyroxene ([Fe,Mg]CaSi2O6), from whose breakdown the elements calcium and magnesium combine with CO2 to form carbonates.
Iceland is not short of basalts, being on the axial ridge of the North Atlantic. Surprisingly for a country that uses geothermal power to generate electricity it is not short of carbon dioxide either, as the hot steam contains large quantities of it. In 2012 the CarbFix experiment began to inject a 2 km deep basalt flow with 220 t of geothermal CO2 ‘spiked’ with 14C to check where the gas had ended up This was in two phases, each about 3 months long. After 18 months the pump that extracted groundwater directly from the lave flow for continuous monitoring of changes in the tracer and pH broke down. The fault was due to a build up of carbonate – a cause for astonishment and rapid evaluation of the data gathered. In just 18 months 95% of the 14C in the injected CO2 had been taken up by carbonation reactions. A similar injection experiment into the Snake River flood basalts in Washington State, USA, is said to have achieved similar results (not yet published). A test would be to drill core from the target flow to see if any carbonates containing the radioactive tracer filled either vesicles of cracks in the rock – some press reports have shown Icelandic basalt cores that contain carbonates, but no evidence that they contain the tracer .
Although this seems a much more beneficial use of well-injection than fracking, the problem is essentially the same as reinjection of carbon dioxide into old oil and gas fields; the high cost. Alternatives might be to spread basaltic or ultramafic gravel over large areas so that it reacts with CO2 dissolved in rainwater or to lay bear fresh rocks of that kind by removal of soil cover.
Tucking away vast amounts of atmospheric carbon dioxide (carbon capture and storage or CCS), or at least that emitted by fossil-fuel power stations, is a widely suggested and well supported approach to slowing down global warming. It has two main downsides: if successful it helps maintain the dominance of fossil fuels and vast amounts of buried greenhouse gas might simply leak out some time. Ideally, the storage part of CCS would involve CO2 being taken up by an inert solid. Carbonates may be stable enough but arranging the chemical reactions to make them seem difficult, the most widely considered being by encouraging weathering of ultramafic rocks to form magnesium carbonates as a by-product: huge areas would have be coated with finely-ground peridotite. A less satisfactory approach would to dissolve the gas in water held at great depths in sedimentary aquifers, but if that water doesn’t move and doesn’t get warmed it might do the trick.
Unsurprisingly, a lot of funds are available to research CCS and ideas are pouring forth, a recent, sober assessment focussing on the solubility option (Steele-MacInnis, M. et al. 2012. Volumetrics of CO2 storage in deep saline formations. Environmental Science and Technology (August 2012 online) DOI: 10.1021/es301598t). The team from Virginia Tech and the US Department of Energy conclude that solution in brines trapped in deep aquifers may help, although solution is an equilibrium between gas and dissolved CO2, so that a gas layer in the aquifer is always likely to be present, even at high pressures. The only way of avoiding that is if the dissolved gas reacted with carbonate in the aquifer so that calcium and hydrogen-carbonate (HCO3–) ions entered solution. That ‘enhanced’ solution is not so easy since, although it mimics the calcite-weathering effect by acid rain that naturally takes CO2 from the atmosphere, calcite dissolves very sluggishly. But solution adds to the density of already dense brine so that it is less likely to leak upwards into more shallow aquifers. Their preferred technology is to liquefy the gas under pressure and pump that to deep aquifers where eventually the supercritical CO2 liquid will dissolve. The problem is this: while experiment and theory suggest the approach will work, nobody knows how long CO2 solution in brine will take. There needs to be a sizeable pilot study…
Not many people would like to visit a waste heap at an asbestos mine. That is not because waste heaps are generally boring but all forms of asbestos are carcinogens when inhaled. Encountering pits in the tailings that emits puffs of warm air would cause health and safety alarm bells to ring. Yet that is exactly what has attracted researchers to the huge asbestos mining complex at Thetford in Quebec, Canada: the air leaving the vents can be extremely depleted in carbon dioxide (Pronost, J. and 10 others 2012. CO3-depleted warm air venting from chrysotile milling waste (Thetford Mines, Canada): Evidence for in-situ carbon capture and storage. Geology, v. 40, p. 275-278). More precisely, the depletion – down to less than 10 parts per million (ppm) compared with normal atmospheric levels of 385 ppm – occurs in winter, when the puffing pits emit warm air far above the frigid air temperatures encountered in winter Quebec. The chrysotile must be reacting with groundwater and CO2, and is therefore a potential means of using near-surface natural materials for carbon capture and storage (CCS). The end product is an innocuous carbonate – Mg5(OH)2(CO3)4·4H2O – and dissolved silica. Quite a find, it might seem, as the reaction is exothermic too: CCS plus geothermal energy plus safe decomposition of a major environmental hazard. In fact any magnesium-rich silicates are likely to undergo the same carbonation reaction, especially if ground-up to increase the net surface area exposed to moist air.
The parent asbestos rock at Thetford is a metamorphic derivative from mantle ultramafic rocks in an ophiolite, and the asbestos insulation business, both for extremely hazardous blue (crocidolite) and less dangerous white (chrysotile) asbestos has been hugely profitable since the 19th century. Consequently, wherever there are altered ophiolites, generally in collision-zone orogenic belts, asbestos has been exposed either naturally or through mining and processing. There are many related cancer ‘hot spots’ in populous mining areas of Canada, India, the Alps and southern Africa, and in dry climates even natural exposures pose considerable risk. Could these blighted areas take on a new role in lessening the chance of global warming? About 30 billion tonnes of CO2 are emitted by burning fossil fuels each year. To keep pace, at the current atmospheric concentration of CO2, some 75 trillion tonnes of air would have to react annually with about 100 billion tonnes of magnesian silicate, making this form of CCS the largest industry on the planet (http://www.newscientist.com/article/mg21428593.800-stripping-co2-from-air-requires-largest-industry-ever.html).
Another factor tempering somewhat forced optimism for CCS as a way of having our fossil fuel cake and eating it is that direct injection of greenhouse gases into deep storage may have an unforeseen down-side. Deep drilling and injection of fluids may trigger earthquakes. The alarm raised by small yet disturbing seismicity accompanying sites for shale-gas development by ‘fracking’ (http://earth-pages.co.uk/2011/11/04/fracking-check-list/ and http://earth-pages.co.uk/2011/10/14/britain-to-be-comprehensively-fracked/) has died down to some extent following detailed analysis of small earthquakes around drilling sites. It turns out that they are triggered not by the drilling itself but the subsurface disposal of the large amounts of fluids that have to be passed through the oil shales to make the tight rock permeable to gas (Kerr, R.A. 2012 Learning how to NOT make earthquakes. Science, v. 23 p. 1436-1437). Safe subsurface disposal requires injection wells penetrating 1 to 3 km below the surface, often below the cover of sedimentary strata and into crystalline basement. Such hard rocks store elastic strain induced by burial and tectonics, and release it when lubricated by fluids, especially if they contain dormant faults. Once impermeable rock can thus be hydrofractured in the same manner as ‘fracked’ gas-prone shales and old, often unsuspected faults reactivate: a catastrophic prospect for injected CO2. In sedimentary sequences, drilling CCS wells into porous rocks capped by impermeable ones – the scenario for ‘safe’ gas storage – could also induce ‘fracking’ of the sealing rocks and thereby causing leakage (see also http://www.newscientist.com/article/dn21633-fracking-could-foil-carbon-capture-plans.html).