Geo-engineering usually refers to deliberate large-scale attempts to modify the climate, usually to mitigate global warming.  Within the environmental movement it is often regarding with suspicion, as a cheap technical fix to avoid fundamental decarbonisation of the world economy.

There are many examples, perhaps no need for details here. There are broadly two classes: technologies that affect incoming radiation, and technologies that alter the composition of the atmosphere, usually by removal of carbon dioxide  and 'sequestration' in some stable form separated from the atmosphere. "Carbon Capture and Storage" might be considered a special case of geo-engineering.

After the almost-universal agreement to 'avoid dangerous climate change' in 1992, it was possible that simple decarbonisation processes might have stabilised the level of GHGs in the atmosphere. Subsequent progress was so slow however, that in the last decade it has become clear that the level threatens to overshoot widely-accepted thresholds, triggering feedback effects. It is now beginning to seem that the climate cannot be stablised without some counter-warming processes, by analogy not just brakes, but a reverse gear.

Once this arithmetical logic is accepted however, a sense of urgency to reduce emissions might be blunted by the prospect of low-cost technical solutions. Further, many of the proposed geoengineering technologies threaten all manner of unintended consequences that are impossible predict in detail. Accordingly, many environmntal groups continue to oppose any forms of geo-engineering. 

In my view this is a mistake. It seems inevitable that we will need a 'reverse gear' and it is important to take part in the important debates about what form this might take. Probably most forms of radiation-management should be put on one side, at least for the time being, for two reasons. One is that the unintended effects are hard to predict; the second is that they do not ameliorate the problem of ocean acidification.

Turning to CO2 removal technologies, it can be readily appreciated that they address both warming and acidification, and in principle could return the CO2 concentration to safer levels, even back to the pre-industrial value of  270ppm. The question then is what kind of CO2 reduction technologies should be supported. Once again there are questions about unexpected side-effects, and the form and fate of the sequestered carbon . A particularly controversial example is ocean fertilisation to encourage the growth of algae that will absorb CO2, then sink to the bottom forming a permanent store. Initial trials have been disappointing, but in my view there should be a well-funded international effort to establish the possibilities. If in some sense it can be shown to work, we might need it and we need to know how to do it properly.

Most other sequestration proposals can be classified according to the method of CO2 capture; and the means of storage.   Wally Broecker and collaborators have promoted, and to some extent demonstrated, the idea of mechanico-chemical 'air capture', followed by sequestration in suitable underground locations. This 'works' but is expensive both in terms of cash and energy. If the energy is derived from fossil fuels, there is little advantage, but if energy is gradually decarbonised until it is essentially  zero-emitting, then there is no theoretical limitation.

In my view again, both these processes need to be ramped up and vigorously developed. The notion of underground storage has become a standard part of the discourse, and conventional energy planners assume there will be some contribution from so-called 'clean coal', where CO2 from the combustion of coal for electricity is captured and stored in various geological formations such us depleted gas fields. It is worth noting that capturing CO2 from an enriched post-combustion source at around 16%  is much more efficient that from air at 0.04%. The whole process is known as Carbon Capture and Storage or CCS.

'Clean coal' however does not capture all the CO2 and still has a much higher carbon intensity than nuclear or most renewables. My own calculations show that it cannot contribute more than 1% of energy supply without exceeding sustainable emission limits. However, if it is co-fired with biomass there is some ratio of coal to biomass that is effectively carbon-neutral. The reason is that biomass withdraws CO2 from the air via photosynthesis, and if this can be captured after combustion and stored, we have essentially a negative-intensity source of energy.  Suitable combination of negative and positive will give zero, so this approach is worth considering further.

Of course pure biomass combustion with CCS would give a genuine net-negative energy source, and this has been described by the Tyndall Centre as 'benign geo-engineering'.  This prospect is so promising that it should be worth supporting all research and trials on CCS even if for the time being they apply to (usually imported) coal. Of course we would have to grow the biomass somewhere, but the ZCB studies show this to possible in principle within the UK, although we should not rule out the possibility of imports, provided we can balance the energy books by exports of (say) wind electricity.

Even more 'benign' processes could be considered, still based on photosynthesis.  Trees absorb CO2 and store it in their trunks and roots during their growth phase, which might last many decades. So reforestation almost anywhere can be a net-negative process.  Alternatively, crops can be grown deliberately to maximise photosynthetic carbon capture, then harvested and used or stored in various permanent forms.  An important idea is to process biomass that generates energy or liquid fuels, leaving a residue of elemental carbon or 'biochar'. This can be incorporated into soils, enhancing crop growth. Soils probably have enough capacity to absorb all the atmosphere's excess CO2.

Carbon stored in frest over 100 years

Carbon stored in frest over 100 years

The conclusion of all these considerations is that net-negative processes are vital and will be increasingly so. It is important to accelerate research in all these fields, so that we know how to do things properly when we have no other options.  In some sense we are already geo-engineering the atmosphere, again with unknown consequences. A logical approach is to very rapidly run down fossil fuel consumotion, replace with a rapid ramp up of renewables, and use benign geoengineering to restore the background  concentration of CO2.