Geoengineering

Geoengineering is one of the most discussed and contentious topics within climate change mitigation. The methods can range from extremely elaborate, like placing large mirrors into orbit, to more feasible techniques, such as whitening buildings.

What is geoengineering?

Geoengineering refers to a “deliberate and large-scale intervention in the Earth’s climatic system with the aim of reducing global warming” (Stilgoe, 2015). The approaches tend to be able to split into one of two broad categories: carbon dioxide removal or solar engineering (Caldeira et al., 2013).

Solar engineering (SE)

In summary, solar engineering is decreasing the amount of solar radiation absorbed by the Earth in order to counter the additional radiative forcing generated by increased GHGs concentrations in the atmosphere – to offset the forcing from a doubling of CO₂, approximately 1.7% of the incoming sunlight (Caldeira et al.,2013). These techniques (Figure 1) tend to be large-scale and would be expensive to implement, and include ideas such as sending mirrors into orbit or injecting aerosols into the stratosphere.

Figure 1. Summary of different SE approaches.
Source: Caldeira et al., 2013

It is generally accepted that such methods could be successful in reflecting sunlight, but the resulting effects are unknown, and in some locations the techniques could worsen the problem of climate change (Caldeira et al., 2013).

Carbon dioxide removal (CDR)

Focussed on alleviating the actual cause of anthropogenic climate change by removing CO₂ from the atmosphere. In comparison to solar engineering, these techniques (Figure 2) are generally of a smaller scale and can be applied more locally. They include approaches such as reforestation, enhancing weathering, or fertilisation of the oceans (Caldeiraet al., 2013).

Figure 2. Summary of CDR approaches
Source: Caldeira et al., 2013

Whilst being typically cheaper and less controversial to implement, CDR methods tend to be slower-acting and so do not present a solution on the short timescales that are necessary to reduce the impacts of climate change.

Comparison of methods

As mentioned above, the offerings from the two streams of approach are largely opposite. SE methods are expensive, controversial, would require international backing to implement, but would be able to reduce warming within years of introduction. CDR would take longer to take effect and so do not offer an immediate solution to global warming, but they are more inconspicuous, usually cheaper (on smaller scales) and are, in most cases, more localised, allowing for independent introduction by individual countries. There are exceptions to this, of course, with ocean fertilisation being a prime example. As seen in Figure 2, this offers significant carbon removal (up to 200 Pg C by 2100), but the most could be almost as controversial as SE techniques; for example, ocean fertilisation could have dramatic effects on coral reefs and would be expensive (Caldeira et al.,2013).

In my opinion, there seems one fundamental difference between SE and CDR approaches, and it’s connected to the issue that is essential to successfully reducing emissions and mitigating climate change. Whilst solar techniques are generally the more novel of the two, they represent the problem that has underlain the failures of action against climate change so far. They are effectively shifting the responsibility of dealing with our emissions to another source: rather than remove the issue that we have caused, they instead focus on altering the Earth system in an even more dramatic way.

I understand, however, that CDR techniques are not obviously not perfect. To implement them on the scale required would financially infeasible, and the response would likely not be rapid enough to solve fast-acting climate change. Limitations of warming to 2^oC by the Paris agreement include geoengineering within them already, and so it must be assumed that they will eventually be introduced in some capacity. To me, the best solution would be a combination of the less controversial techniques from both sets. Biomass energy with CO₂ sequestration involves the capture of CO₂ from power plants running on biomass, allowing of permanent removal of CO₂ from the atmosphere. 3% of the global land area used for this purpose would lead to a reduction of 1 Pg C each year (Caldeira et al., 2013). From the SE approaches, enhancing solar albedo by installing white roofs globally has been modelled to reduce daily high and low temperatures in urban areas by 0.6^oC and 0.3^oC (Caldeira et al.,2013). By introducing a large quantity of smaller initiatives, we may be able to produce some significant temperature and CO₂ reductions. I believe the large-scale SE techniques offer a safety net that should only be used as a last resort, in the failure of other methods.