For a number of reasons, geoengineering is all over the news. The basic idea is to counteract the effects of climate change induced by greenhouse gasses. This can be accomplished in two basic ways. One is to use a separate mechanism to reduce the amount of energy the Earth absorbs from the sun. Orbiting mirrors and sulfate injection seek to do this. This approach is not ideal, partly because it would cause unknown side effects and partly because it would not stop the oceans from becoming more acidic. A more appealing route focuses on actively removing greenhouse gasses from the atmosphere.
The first way to do this is to encourage the growth of biomass. This is relatively easy, but has limited potential. Biomass is like a giant carbon cushion: it can be thick or thin, but it cannot keep growing forever. Increasing the amount of biomass on Earth could draw down the amount of CO2 in the atmosphere a bit, but only if we also manage to cut our greenhouse gas emissions to practically zero.
The second way – mentioned before – is to draw greenhouse gasses from the air and bury them, using carbon capture and storage technology (CCS). This could be done in two basic ways: (a) draw carbon dioxide (CO2) directly from the air and bury it or (b) grow biomass, burn it, collect the CO2, and bury that. The major limitations here are cost and technology. It remains unclear whether CCS can be made safe, effective, and affordable. It is also unclear whether it could be ramped up to a big enough scale to stop catastrophic climate change, in the absence of strong mitigation action.
The third option is to enhance the weathering of rocks. In the long term, this is where atmospheric CO2 actually ends up going. Some people are talking about speeding up the process, using various suitable types of rock and various mechanisms for increasing its rate of reaction with atmospheric CO2. Once again, the uncertainties concern scale and cost.
The three options that actually remove CO2 from the atmosphere are much more appealing than options that try to interrupt incoming sunlight. Each acts directly on the cause of anthropogenic warming, rather than trying to counter it by proxy. This is a bit like removing poison from a person’s body, as opposed to administering a supposed antidote with unknown effectiveness and side effects.
It remains unknown whether there will ever be a point where geoengineering is less costly per tonne of CO2 than various mitigation approaches. Right now, there are certainly greater opportunities in areas like energy efficiency and building design. That being said, research into CO2-removing technologies strikes me as having merit. They may eventually prove economically comparable to more expensive mitigation options; they may allow us to counteract activities that inevitably produce emissions, such as air travel; and they could give us some last-ditch options, if we find ourselves experiencing abrupt, catastrophic, or runaway climate change as a result of past emissions.