This week we have been learning about climate models and their projections, and also considering proposals to geoengineer the climate. Here are answers to a few of the questions that have come up repeatedly:

Why do models diverge from natural factors after 1970, when emissions have been going on before that?

Prior to this the sulphate aerosol produced from burning ‘dirty’ coal and other fossil fuels acted to cool the climate by roughly the same amount as the CO2 emitted from burning the same fossil fuels. However, sulphur dioxide emissions and the resulting sulphate (after oxidation in the atmosphere) caused acid rain (as well as choking smogs) and in the 1970s it became clear that acid rain was seriously damaging forests (as well as buildings), especially in Europe. This led to stronger legislation to ‘clean up’ fossil fuel burning with e.g. sulphur dioxide scrubbers fitted to major power stations. This reduced the cooling effect but left the CO2 warming effect. Some fossil fuel burning e.g. ship fuel was exempt, and hence ships have continued to produce cooling ‘ship tracks’ of aerosols and clouds that counteract the CO2 they emit. This ‘ship track’ effect is very similar to some geoengineering schemes for deliberate sunlight reflection.

Many of your questions have focused on the other type of geoengineering – carbon dioxide removal, and in particular the chemistry of ‘artificial trees’ that absorb atmospheric carbon dioxide using sodium carbonate, e.g.: How does this work, chemically, and do we have enough sodium carbonate to compensate for the fact that it’ll saturate quickly to make this a viable option?

The key point here is that the ‘sorbent’ for CO2 – here sodium carbonate – is regenerated when the CO2 is desorbed from it (and then transferred to some form of storage). This step costs energy (as does pressurising and pumping into storage the resulting CO2) and that is key to setting the cost of this CDR option. There are other passive sorbents that different groups are working on – but still there must be some energy investment in concentrating CO2 from its very dilute form in the atmosphere to concentrated form – because this amounts to working against the second law of thermodynamics. In one scheme that energy comes from the latent heat of water but that in turn makes the CO2 capture step very demanding of water.

What is the potential that stored carbon, from carbon capture schemes, could release? What impact would this have on the climate?

If liquid CO2 is stored underground at appropriate pressure and temperature then it can be very stable there. If carbon is stored as charcoal in soils (‘biochar’) then this can break down slowly. We certainly wouldn’t want to put CO2 somewhere that it could escape easily or abruptly – not only would this have the potential to asphyxiate animals locally (as happens in natural CO2 seeps) – but it would obviously contribute to climate change.

Professor Tim