As yet, nobody knows. But I am now resuming my hunt for technology that could help keep global warming below the crucial 2 degree - and maybe even 1.5 degree - threshhold.
Weathering (the chemical breakdown of rocks) is a natural process that ultimately leads to carbon dioxide being stored on the ocean floor. It is a very slow phase of the natural carbon cycle but the mechanism could be enhanced as a method for artifical carbon dioxide removal. On the planet Bahshald, where I have just landed, the feasibility and effectiveness of this is being researched.
In a paper published in Nature Climate Change this month, Taylor et. al., (2015) found that distributing pulverised (particulate) silicate rocks across the tropics would increase the rate of terrestrial weathering. They used CMIP5 general circulation models running RCP4.5 and RCP8.5 (two different emission scenarios). They then considered various application rates for three different rocks: dunite, harzbugite and basalt (the former two are commerically mined and major terrestrial reserves exist for the latter).
Taylor et. al.'s simulations suggest enhanced weathering could sequester hundreds of petagrams of carbon dioxide by 2100. They project that in a RCP8.5 scenario, atmospheric CO2 would be significantly lowered through enhanced weathering, and in a RCP4.5 scenario, emissions to date may even be reversed (Fig. 1). Compared weight-for-weight, dunite and harzbugite consumed twice as much CO2 as basalt. All three rocks approach a maximum CO2 consumption as more rock is applied.
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| Figure 1. Atmospheric CO2 concentration lowered by enhanced weathering. Top row (a, c) assumes RCP4.5 scenario, bottom row (b, d) assumes RCP8.5 scenario. Left column (a, b) assumes lower rock application rate of 1 kg per square metre per year. Right column (c, d) assumes higher rock application rate of 5 kg per square metre per year. Blue line shows emission scenario with no enhanced weathering. Black (pink) line shows emission scenario plus enhanced basalt (harzburgite) weathering. Dunite was excluded as it is not present in as large reserves. Source: Taylor et. al. (2015) |
Taylor et. al. (2015) also show that enhanced weathering could have added benefits by reducing oceanic carbon-dioxide concentrations, thus reducing ocean acidification - a huge added benefit which is not possible through solar radiation management. Under scenario RCP4.5 and high harzburgite application, current ocean acidificaiton may even be reversed.
Is it too good to be true?
Well, the authors have not considered any additional emissions that would result from mining, processing, transporting or applying such vast quantities of rock in their simulations - which could reduce carbon sequestration capacity by 8 - 33% (Taylor et. al., 2015). The authors also use idealised scenarios in their models, and do not consider practical or logistical limitations to deploying such a technology. However, their simulations show that enhanced weathering of pulmerized basalt or harzburgite could significantly lower atmospheric CO2, and help reduce ocean acidification.
Furthermore, the identified hotspots for enhanced weathering are primarily tropical forests, excluding Asian cropland (Taylor et. al., 2015), which would doubtless mean considerable objections from conservational or ecological groups. However, basalt can also promote crop growth in acidic tropical soils by making the soil more alkaline (Anda et. al., 2009). Although as Taylor et. al. suggest, areas undergoing REDD+ or afforestation schemes may be well suited to deployment of the technology, as they will already have infrastructure set up.
This technology has promising potential, and warrants more research into ways the rock weathering could be made even more efficient to account for the fact that it is unlikely to be applied accross all the areas that were used in this study and to make up for associated mining and transportation emissions. Further studies could also consider the potential for enhanced weathering in more barren and less ecologically-contentious regions.
Certainly an interesting concept, but I understand why there is huge controversy surrounding geoengineering techniques as there are potential negative impacts of these 'global experiments' that may not be considered until these methods have been implemented! However, studies such as this one play an important part in understanding both the positives and negatives of these methods.
ReplyDeleteYou're absolutely right - there are still so many unknowns, it would be foolish to implement any technique that will have global impacts without more research first. Even so, there is a lot of concern that temperature rises can't stay within 2degrees (see: http://link.springer.com/article/10.1007%2Fs10584-012-0680-5 for instance) without negative emissions, so I do believe there is justification for continued research into safe ways to remove carbon dioxide from the atmosphere (though not necessarily implementation!).
DeleteHi Kitty, a very interesting blog-post! I agree enhanced weathering has great potential as a CDR process and in many cases may be perceived one of the best solutions (in my opinion) that geoengineering can implement on Earth, as it provides many positive influences and the negative impacts are minimal!
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