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Geoengineering is the term given to proposals to try to control the climate through technologies, some of them new and bizarre, and unlikely to happen or to work if they did – giant mirrors in space fall into this category. Some of them are new ways of applying old approaches – such as creating ‘biochar’ (see below) – and some of them new but likely to work – such as biomass with CCS. Some environmentalists say that all ‘technological fixes’ should be rejected – that what is needed is behavioural, political and economic transformation. Whatever your view of this argument, it must now be accepted that the need to control climate change is so great and so urgent that all methods must be used, including technological solutions to ‘engineer the climate’. This does not mean that there is no need, or less need, to reduce emissions fast. It means accepting that, even if we were to reduce emissions massively tomorrow (which we will not), there is still a need to reduce the amount of CO₂ already in the atmosphere from historic emissions. 

The UK’s Royal Society, the leading scientific body, published a comprehensive report on this in September 2009 (see The Royal Society: Geoengineering the climate: science, governance and uncertainty). This divides geoengineering into two categories: carbon dioxide removal (CDR) techniques and solar radiation management (SRM) techniques. The former:

“… address the root cause of climate change, rising CO₂ concentrations, [so] they have relatively low uncertainties and risks. However, these techniques work slowly to reduce global temperatures.”

The latter:

“… act quickly, and so may represent the only way to lower global temperatures quickly in the event of a climate crisis. However, they only reduce some, but not all, effects of climate change, while possibly creating other problems . They also do not affect CO₂ levels and therefore fail to address the wider effects of rising CO₂, including ocean acidification.”

Therefore, the Royal Society favours CDR techniques. However, as they say, “… should temperatures rise to such a level where more rapid action needs to be taken”, SRM techniques should also be used.

CO₂ removal methods reviewed in the report include: 

• Land use management to protect or enhance land carbon sinks. 
• The use of biomass for carbon sequestration as well as a carbon neutral energy source. 
• Enhancement of natural weathering processes to remove CO₂ from the atmosphere. 
• Direct engineered capture of CO₂ from ambient air.
• The enhancement of oceanic uptake of CO₂, for example, by fertilisation of the oceans with naturally scarce nutrients, or by increasing upwelling processes.

This article summarises the findings of the Royal Society report and notes the views of other organisations where these are significantly different.

Land carbon sinks

The Royal Society notes that:

“Terrestrial ecosystems remove about 3 GtC yr from the atmosphere through net growth, absorbing about 30% of CO₂ emissions from fossil fuel burning and net deforestation, while the world’s forest ecosystems store more than twice the carbon in the atmosphere.”

It therefore states that:

“… reducing emissions from deforestation and forest degradation is a vital component but afforestation or replanting can play a significant role too, especially in the case of degraded agricultural land.”

The report goes on to note that:

“… at local to regional scales, increased adoption of land use management that incorporates multiple ecosystem services, including carbon storage, could deliver significant benefits. In one spatially explicit watershed scale study in Oregon, USA, carbon storage could be doubled through changed land use policies that were beneficial over a wide range of economic and ecosystem services. Realistic policy changes in this area could potentially increase carbon sequestration by 5 million tons in an area of around 30,000 km².”

Biomass with CCS and biochar

Biomass can be burnt and the carbon captured and stored, just as it can be from the burning of fossil fuels. The Royal Society states that:

“… this technique builds directly on existing technology for bioenergy/biofuels and for CCS, and inherits the advantages and disadvantages of both of these technologies. There is no doubt that it is technically feasible, and there are already some small real-world examples.”

A paper from the University of Calgary concludes that biomass with CCS:

“… represents an efficient strategy for biomass-based carbon mitigation and a mechanism for offsetting emissions sources elsewhere in the economy, fundamentally changing the role of biomass in achieving deep emissions reductions…Emissions offsets generated by biomass energy systems with CO2 sequestration are likely to be more cost effective than many direct mitigation options outside the electric sector.”

(See Biomas Energy with Geological Sequestration of CO₂: Two for the Price of One?.)

Biomass can also be turned into charcoal, known as biochar, and then buried. The Royal Society describes the technique thus: “Biochar (charcoal) is created when organic matter decomposes, usually through heating, in a low- or zero oxygen environment. Known as pyrolysis, the decomposition process produces both biochar and biofuels (syngas and bio-oil). As the carbon atoms in charcoal are bound together much more strongly than in plant matter, biochar is resistant to decomposition by micro-organisms and locks in the carbon for much longer time periods. The range of potential raw materials (‘feedstocks’) for creating biochar is wide, including, for example, wood, leaves, food wastes, straw, and manure, and it is also claimed that addition of biochar to soils can improve agricultural productivity. Biochar is therefore sometimes proposed as an answer to a number of different problems, since it draws down and locks up atmospheric carbon, it can improve crop yields, and it creates biofuels, a renewable energy source. How effectively it achieves each of these goals, at what costs, and with what wider impacts, will determine the influence biochar can have as a geoengineering technology.”

A good example of the proposition that biochar is the answer to many problems is the website biochar.org, which says that:

“Pyrolysis of waste biomass can generate fuels and biochar recalcitrant against decomposition. If biochar is returned to agricultural land it can increase the soil’s carbon content permanently and would establish a carbon sink for atmospheric CO2. In this case the use of crop residues as a potential energy source may improve soil quality and reduce greenhouse gas emissions in a complementary not competing way. Biochar is proposed as a soil amendment in environments with low carbon sequestration capacity and previously depleted soils (especially in the Tropics). From previous studies it is known that soil biochar amendments increase and maintain soil fertility and the human-made Terra Preta soils in the Amazon prove that infertile soils can be transformed into fertile soils and long term carbon enrichment is feasible even in environments with low carbon sequestration capacity.”

However, the Royal Society report states that the “long-term effects on soils not yet known”. Overall, it concludes that:

“Biomass for sequestration could be a significant small-scale contributor to a geoengineering approach to enhancing the global terrestrial carbon sink, and it could, under the right circumstances, also be a benign agricultural practice. However, unless the sustainable sequestration rate exceeds around 1 GtC/yr, it is unlikely that it could make a large contribution. As is the case with biofuels, there is also the significant risk that inappropriately applied incentives to encourage biochar might increase the cost and reduce the availability of food crops, if growing biomass feedstocks becomes more profitable than growing food.”

Enhancement of natural weathering processes                  

The Royal Society describes this technique thus:

“Carbon dioxide is naturally removed from the atmosphere over many thousands of years by processes involving the weathering (dissolution) of carbonate and silicate rocks. Silicate minerals form the most common rocks on Earth, and they react with CO to form carbonates (thereby consuming CO2). Carbon dioxide could be removed from the atmosphere by accelerating the natural weathering process; reacting silicate rocks with CO2 and forming solid carbonate and silicate minerals. This reaction consumes one CO2 molecule for each silicate molecule weathering and stores the carbon as a solid mineral. A variant on this process would be to weather silicate rocks, but instead of forming solid minerals, to release the dissolved materials into the oceans.”

It recognises that there is very large potential for carbon storage in soils and oceans, and that using the oceans in this way would help reduce ocean acidification. However, they also note that neither technique would be cheap or quick, and that there would be local environmental impacts:

“… to be quantitatively important, most of these proposals require large mining and transportation activities. These activities would likely damage the environment locally (and ‘local’ here would mean over large areas, comparable to or greater than those of present-day cement production and coal mining). Some options require large amounts of water. Others require additional energy (for electrolysis or lime production), which would need to come from carbon-free sources. In the case of solid mineral production, there are also issues of disposal (or use) of large amounts of solid material.”

Capture of CO2 from ambient air

The Royal Society describes this technique in the following way:

“Air capture is an industrial process that captures CO₂ from ambient air producing a pure CO₂ stream for use or disposal. There is no doubt that air capture technologies could be developed. The technical feasibility of this is demonstrated, for example, by commercial systems that remove CO₂ from air for use in subsequent industrial processes. Several methods for air capture have been demonstrated at laboratory scale, although as yet no large-scale prototypes have been tested, and it remains to be seen whether any of these processes can be made sufficiently cost effective. Capturing CO₂ from the air where its concentration is 0.04% might well seem unpromising given that there is still no power plant in which CO₂ is captured from the full exhaust stream. Two factors make air capture more dif?cult than capturing CO₂ from exhaust streams; firstly, the thermodynamic barrier due to the lower concentration of CO₂ in air; and secondly, the energy and materials cost of moving air through an absorbing structure.”

However, it accepts that:

“… air capture enables the application of industrial economies of scale to deal with small and hard-to-control sources of CO2 emissions (especially transport-related sources) for which CCS cannot be used. In such contexts it may prove to have a suf?ciently low cost to play an important role in managing emissions, especially if ‘stranded’ energy sources can be utilised…Air capture methods could be useful and important even if the costs are substantially higher than other means of cutting emissions in formulating a long-term climate policy.”

Enhancement of oceanic uptake

The Royal Society describes this technique as follows:

“Carbon dioxide is fixed from surface waters by photosynthesisers – mostly, microscopic plants (algae). Some of the carbon they take up sinks below the surface waters in the form of organic matter composed of the remains of planktonic algal blooms, faecal material and other detritus from the food web. As this material settles into the deep ocean by gravity, it is used as food by bacteria and other organisms. They progressively consume it, and as they respire they reverse the reaction that ?xed the carbon, converting it back into CO₂, that is re-released into the water. The combined effect of photosynthesis in the surface followed by respiration deeper in the water column is to remove CO₂ from the surface and re-release it at depth. This ‘biological pump’ exerts an important control on the CO₂ concentration of surface water, which in turn strongly influences the concentration in the atmosphere…The ability of the biological pump to draw carbon down into deeper waters is limited by the supply of nutrients available that allow net algal growth in the surface layer. Methods have been proposed to add otherwise limiting nutrients to the surface waters, and so promote algal growth, and enhance the biological pump. This would remove CO₂ faster from the surface layer of the ocean, and thereby, it is assumed (sometimes incorrectly) from the atmosphere.”

However, the Royal Society notes that:

“All ocean fertilisation proposals involve intentionally changing the marine ecosystem, but because of its complexity the possible consequences are uncertain. In particular, the complex trophic structures typical of ocean food webs make the ecological impacts and their consequences for nutrient cycling and flow hard to predict.”

Therefore, it concludes that this technique is unproven and with a high risk of unintended consequences.

Solar Radiation Management

What should be done if emissions are not reduced far enough or fast enough to control climate change? The most promising SRM techniques, the Royal Society report found, are:

• “Stratospheric aerosols. These were found to be feasible, and previous volcanic eruptions have effectively provided short-term preliminary case studies of the potential effectiveness of this method. The cost was assessed as likely to be relatively low and the timescale of action short. However, there are some serious questions over adverse effects, particularly depletion of stratospheric ozone.
• “Space-based methods these were considered to be a potential SRM technique for long-term use, if the major problems of implementation and maintenance could be solved. At present the techniques remain prohibitively expensive, complex and would be slow to implement.
• “Cloud albedo approaches (e.g. cloud ships) the effects would be localised and the impacts on regional weather patterns and ocean currents are of considerable concern but are not well understood. The feasibility and effectiveness of the technique is uncertain. A great deal more research would be needed before this technique could be seriously considered.”

Other SRM methods, including painting roofs white, planting reflective crops and installing reflectors in the desert, were found to be “ineffective, expensive and, in some cases, likely to have serious impacts on local and regional weather patterns”.

Conclusion

Geoengineering should be supported, as some CRD techniques will certainly be needed, and some SRM almost certainly will be too. We live, in the words of Nobel-prize winning chemist, Paul Crutzen, in a new geological era – the anthropocene – because, since the industrial revolution, the human influence on climate has been so great that we are already engineering the climate, albeit not deliberately. Geoengineering is not an alternative to rapid and substantial emissions reductions. We need both.