Carbon Dioxide Removal
Carbon dioxide removal projects address the root cause of warming by removing greenhouse gases from the atmosphere. These kinds of geoengineering schemes have the added benefit that by lowering the CO2 concentration in the atmosphere, they also prevent (and can reverse) ocean acidification. CO2 removal projects are generally slower, more expensive, and less developed than some of the insolation reduction schemes. There are a variety of ways that this could be done, including:
Carbon Capture and Sequestration
You may have heard of this in the context of “clean coal”, which refers to technologies that would allow us to burn coal without emitting CO2 into the atmosphere. Carbon capture and sequestration from power plants that burn fossil fuels involves the chemical removal (sometimes called “scrubbing”) of CO2 from the emissions of power plants, and then the injection of the concentrated CO2 into deep aquifers. In the best case, the sequestered CO2 interacts with minerals in the aquifer to lock the CO2 into a mineral form such as CaCO3 (calcite) which would prevent its release back into the atmosphere. A number of pilot projects of this type have already begun, and it does seem to be technically feasible, but it is not cheap. This would more than double the cost of fossil fuel-generated electricity, making this an expensive proposition, but that in itself would encourage developing clean, renewable energy sources like wind and solar that are also cheaper. Note that this would reduce emissions of CO2, but it would not lower the CO2 concentration in the atmosphere — just prevent it from further increases.
Enhanced Rock Weathering
Other means of carbon capture have been proposed, including the promotion of natural chemical weathering reactions of some rocks in which atmospheric CO2 is consumed. These natural rock weathering reactions could be accelerated by crushing up the rock, which would increase the surface area of the minerals. This would have minimal environmental side effects, but it would also be quite slow and is limited by the availability of the right kinds of rocks. As such, this is not considered as a viable solution to our climate problems.
The surface waters in the southern oceans are depleted in iron, which is an important micronutrient for photosynthesizing plankton, so the plankton in this part of the oceans are under-performing. Adding powdered iron promotes an increase in plankton growth, thus drawing more CO2 from the surface oceans, which in turn enables the oceans to absorb more CO2 from the atmosphere. A few small-scale experiments have been conducted and they appear to work in the short-term, but scaling this up would be challenging and the iron would have to be continuously applied, just as fertilizer is continuously applied to crops. This would be a very expensive solution, and as such, is not considered as a realistic option.