…, should we start experimenting with them, are they the silver bullet or just one viable option among many?

Unfortunately, I won’t – and can’t – answer these questions, since I am asking them myself after getting exposed to this topic at two AGU sessions last year. However, I definitely think it is an interesting topic worth exploring, so I will try to give a very short overview in this post.

This topic is not only covered by some fringe sessions at AGU – or EGU for that matter – also the IPCC has mentioned climate engineering – or geoengineering – in their AR5 report, classifying it into two groups: SRM and CDR. SRM is short for Solar Radiation Management, which is the, by now, quite well known idea of injecting aerosols into the atmosphere, to block out solar radiation, thus cooling the planet. CDR on the other hand is Carbon Dioxide Removal, or the mentioned negative emissions. While the IPCC summary is quite short on the subject, chapter 6.5 offers an interesting overview of CDR and possible schemes which I try to summarize very briefly in the following:

  • Enhanced Carbon Sequestration by Land Ecosystems
    Produce biomass, thus absorbing CO2 and store it long term, e.g. as soil biomass or wood

  • Enhanced Carbon Sequestration in the Ocean
    Produce biomass, possibly increasing growth by (iron) fertilizer, thus absorbing CO2 and store it long term when the biomass dies and sinks to the ocean bottom

  • Accelerated Weathering
    Increase the CO2 binding weathering of carbonates and silicate rocks, by mining, crushing and spreading suitable minerals

  • Carbon Dioxide Removal by Direct Industrial Capture of Atmospheric Carbon Dioxide
    Capture CO2 from the air by technical means and “store” the pure CO2 somewhere, e.g. by sequestering it in suitable, deep rock formations, which is also known as Carbon Capture and Storage (CCS)

Now these schemes range from the easy to understand and to do, at least on a small scale (e.g., plant tree, wait a few years, use tree instead of concrete to build a house that lasts for centuries, thus storing CO2 as a house), to easy to understand and hard to do (somehow remove CO2 from air, put it away somewhere in something like a safe bottle) to the hard to understand and hard to do (mine huge amounts of certain rocks and kickstart a process that naturally takes thousands to millions of years and turns those rocks into some other rocks that now contain CO2) and that is just looking at the possible process itself, without the associated risks and costs.

Where does that leave us? Apart from possibly planting some more trees, all of the mentioned schemes require a lot more research to be done. We must figure out the technical means for the various steps and – probably more important – the risks involved. And for the general issue of climate engineering, we have to split the hope from the hype, to paraphrase the tile of this interesting AGU talk.