Over the last couple of weeks I’ve been writing about ways to draw CO2 back out of the atmosphere. The first post looked at natural ways to do it, the second at negative emissions technologies, and this week there’s a third category of techniques that I want to look at. Next week I’m going to round up by looking at some of the problems around negative emissions, and try to set the whole debate in context.
I’ve looked at this area of climate change action in order of practicality. Negative emissions techniques based around land management, such as reforestation or wetlands restoration, have multiple benefits and are things we should be doing anyway. New technology to draw carbon out of the atmosphere is more expensive and harder to justify. This third category is less practical again, and we’re getting more into geo-engineering territory. So I mention these three techniques not to endorse them, but because they are being discussed and its worth being aware of them.
With that caveat, here are three more radical ways to draw carbon out of the atmosphere:
- Clouds – there are a number of different cloud-related ideas for engineering the climate, from generating white clouds to make the earth more reflective, to encouraging thinner cirrus clouds. One negative emissions form of this is to seed clouds with an alkali compound. This would enhance CO2 solubility, in theory creating ‘alkaline rain’ that would rinse CO2 out of the air as it falls. Needless to say, it would be very difficult to do this at scale. The role of clouds in climate change is not well understood either, so it’s too early to tell whether this is going to be remotely viable.
- Oceans– the same logic of cloud seeding can be applied to the oceans too – using alkali to increase the sea’s capacity to absorb CO2. This could be done by adding lime to the sea, and this would help to re-balance ocean acidification as well as storing carbon. Again, to make any kind of difference would require limestone quarrying at a truly massive scale, which makes the whole project rather unlikely.
An alternative ocean-based approach would be to add iron or nitrogen to the sea to encourage the growth of zooplankton. These would grow, take in CO2 as they photosynthesize, and then carry that carbon to the ocean floor when they died. This is called ocean fertilization, and it’s one of the recurring ideas around geo-engineering. Since we already have algal blooms and dead zones, the consequences for marine biology are a major complication.
- Rock – I hadn’t heard of ‘enhanced rock weathering’ until Carbon Brief mentioned it recently, but it works on the principle that rainfall very slowly erodes rock. Being mildly acidic, rain wears down rock and rinses carbon into the sea in the form of bicarbonate. This is a naturally occurring process that could be accelerated by grinding up rock and spreading it on the soil. More affordable than spraying chemicals out of a plane or boat, this could potentially improve soil by adding minerals too. However, as the Oxford Geoengineering Programme puts it, “size reduction of rock requires a substantial amount of energy.”
Is there a place for this sort of planetary tinkering? That’s an impassioned debate in climate circles, and we will look a little more at that question next week.