Concluding post.

There are uncertainties, but it appears that we may need to be prepared to use geo-engineering. However, before we give geo-engineering the go ahead, there are several areas that I have come across over the past few months that first need to be addressed:

Public: More effort needs to be made to bring the debate into the public sphere - after all, the choices made in this area are likely to effect everyone in someway.  This means that people need to be informed objectively regarding both climate projections and geo-engineering.  It is right that the public is presented with both sides of the coin, the good and the bad, the potential and the uncertainty.

More research: This needs to be done safely, and incrementally.  In particular, more needs to be known about the regional and localised impacts of geo-engineering (particularly if SRM is on the cards!).  The least this will do is inform adaptation strategies for those regions that are likely to experience the most adverse environmental impacts under a geo-engineering regime.

How geo-engineering would be used:  What specific role will it play in the battle against global warming? A short term helping hand or a long term commitment?  An insurance measure to keep temperature below critical levels or a means to completely balance anthropogenic forcing (if even feasible)? I think that for geo-engineering to be justified, we need to minimise the extent of interventions to only what is absolutely necessary.

Politics: The amendment and creation of new international policies and laws will be necessary to prepare our global political system to meet the demands of such large-scale trans-boundary interventions.  These need to address requirements, restrictions and accountability measures.

CO2 mitigation: this stone should not go unturned! Ultimately every effort should still be made to reduce global carbon emissions.  Geo-engineering should be used to achieve what mitigation alone cannot, it should not be considered an alternative.

The methods: There needs to be careful consideration of each method and their intrinsic qualities.  The types of methods I've been able to cover have spanned a spectrum of effectiveness, feasibility, affordability and risk.  My conclusion is that the greatest potential lies in CDR methods, such as reforestation, and more specifically bio-char production.  In comparison with other methods they provide the best combination of effectiveness, feasibility, affordability and risk.  Unlike SRM methods, they don't commit us to long term geo-engineering and the high risks of stopping. They may be limited in how much CO2 they can draw down from the atmosphere, but I honestly believe this can only be a good thing; if they are only able to ease global warming and not completely solve it then there would still be a need to address the crux of the problem: emissions!  If we are heading towards a climate emergency lets prevent it before we have to implement any high-impact, high-risk strategies.

And finally, even if political agendas render my conclusions invalid, thank you geo-engineering for an interesting introduction into the world of blogging!

Geo-engineering in context

I’m going to get straight to the point.  Doing this blog has not convinced me that geo-engineering is good.  Far from it, I actually feel that if the prospect of global warming and climate change wasn’t an incentive to reduce CO2 emissions, then geo-engineering certainly should be! And here is why"

• ·      SRM: some high impact solutions but ignore the cause of the problem (CO2), poses what could be global-scale environmental and social risks.
• ·    CDR: addresses the part of the problem (CO2, but not emissions), but has limited capacity for CO2 draw down, requires large scale coverage and has the potential for high regional environmental and socio-economic impact.
• ·      None of the effective methods could be implemented (safely) tomorrow; most require more thorough research and design.
• ·      This poses another problem: for many techniques, accurate global climate research cannot be confined to a lab.  Adequate research requires that we actively experiment on our planets climate; to full understand the risks we would need to live them.
• ·      All techniques require significant investment, some exceeding the value of the global CO2 emitting economy.  This investment not only includes the cost of implementation, but there are also the research and development costs to consider.
• ·      Global climate manipulation renders this a topic open for international debate, and we all know that consensus is highly unlikely short of exercising geopolitical power inequalities and manipulation.  Political interest is likely to be particularly high, as there are winners and losers in climate change, geo-engineering (the type and the extent) will change who wins and who loses (as frequently stated by David Keith).
• ·      Knowing that we have a back up plan takes the emphasis off of CO2 mitigation and climate change adaptation. We need more political leverage for reducing CO2 emissions, not less.            

           … the list goes on.

So, the question is: are all these factors enough to justify putting geo-engineering to bed and focusing on the bigger issue?

The answer is yes, BUT (and this is a massive elephant-in-the-room type but) perhaps global warming has already reached a point whereby simply curbing our emissions will not prevent a runaway global climate, a point at which we need geo-engineering.

A recent piece in New Scientist suggests that we are.  Focusing on the Arctic, it details reports that suggest 2015 will see ice retreat that exposes climate to accelerated warming, not only due to a reduced albedo, but due to methane release from the increasingly exposed Siberian Arctic seabed (obviously, as with everything related to climate change, this is highly debated.  Right or wrong, this article raises the crucial point about whether it is within our capacity to truly mitigate runaway climate warming.  Drawing from palaeo-records of the Pleistocene and Cenozoic eras, Hansen et al. (2008) discusses inertia in the earths system.   The paper tends towards the argument that the complete temperature increase that can occur in response to our current level of anthropogenic CO2 forcing has yet to be fully realised.  They suggest that inertia, particularly in delayed ocean warming, compounded by the high atmospheric residency time of CO2, means that even if we stopped emissions, thus stabilising atmospheric CO2 concentrations, it is possible that temperatures would continue to rise to ‘dangerous’ levels.  They conclude that a reduction in atmospheric CO2 of approximately 45ppm is required to stabilise these slow positive feedbacks – is geo-engineering therefore the answer?

On principle, I would warn against geo-engineering, however, on reflection I think it would be naïve to be so dismissive.  As reluctant as I am to say it, within the current context of both climate (and political) inertia, it seems that in the not so distant future geo-engineering may prove to be extremely useful.

Geo-engineering, Sir David Attenborough, politics and ethics

A few days ago, in my attempts to keep up with geo-engineering in the public sphere, I found an article detailing David Attenborough's stance on geo-engineering.  If the legend that is Sir David has something to say on the matter you can be sure I was keen to find out more.  I tracked down the original interview on the Andrew Marr show, you can take a look below (my feeble attempt to embed BBC videos has failed so it's a link I'm afraid):

CLICK HERE!

It is interesting that he focuses not on the intricacies or reliability of the technologies, but on the political implications of geo-engineering.  After yet more climate talks it is clear that we are  incapable of coming to satisfactory agreements regarding C02 mitigation, so the future looks bleak for such discussions of geo-engineering! Humphreys (2011) discusses the political difficulties posed by a global climate intervention: Where? When? How? To what extent? Who decides? who pays? - a recipe for political tension and perhaps even conflict. The ethical considerations of climate are blurred and uncertain, open to manipulation and compromise - a global case of the tragedy of the commons.  Geo-engineering is popularly stated to be the 'lesser of two evils', deemed by many a preferable option to a climate catastrophe. In light of the political complexities, Gardiner (2010) suggests that there is an alternative two evils scenario that should be considered, that of a climate catastrophe or a geopolitical catastrophe. In addition, implementation is likely to confuse CO2 mitigation discussions further, as geo-engineering maybe perceived as an alternative, rather than an additional measure. 

I think we'd all benefit from CO2 mitigation rather than sending the world into further geopolitical turmoil....

Illustration by Stephanie McMillan @ Code green

CDR: Terrestrial biomass ... afforestation, reforestation or biochar

We all know that plants absorb CO2 during photosynthesis.  Consequently, there have been many geo-engineering ideas that propose we embrace this natural phenomenon to aid our battle against global warming. In addition to the fertilization of marine phytoplankton as mentioned in the previous post, there are also several terrestrial biomass methods that work to draw down CO2 in this way, namely afforestation, reforestation and biochar.

Firstly I am going to look at afforestation and reforestation: 

Increasing terrestrial biomass has great potential for absorbing CO2 and sequestering carbon.  Depending on who you ask, anthropogenic deforestation has been causing increases in atmospheric carbon dioxide concentrations for thousands of years (Ruddiman, 2003).   It makes sense, therefore, that reforestation and afforestation would help to mediate the current CO2 problem.

But what to what capacity can these two options really help?

Firstly, reforestation (restocking of forest and woodlands that have been depleted)  has the potential to reduce CO2 concentration by 40-70 ppm by 2100 (Candell and Raupach, 2008) - this figure is by no means conservative! - these figures assume that complete reforestation is possible, it is not.  Should we implement afforestation also, the possibilities are limited owing to competing land-use requirements such as food production, urbanisation and conservation etc (Candell and Raupach, 2008).  Already (up until 2000) anthropogenic fossil fuel combustion has caused an increase of 90ppm, exceeding the potential of reforestation.   It is clear that reforestation falls short of being able to truly mitigate atmospheric CO2 currently, let alone if atmospheric CO2 continues to climb.  Additionally, this method is by no means a quick fix! - I doesn't take a genius to work out that we can't grow entire forests over night!

What are the impacts? - (Zomer et al., 2008)

9% of land surface available for af/re-forestation, ~700 Mha of land, in terms of being biophysically suitable.  However, more than half of this is cropland posing high socioeconomic cost and compromise of food security.  Such a conversion of land-use would also see a displacement of tenant farmers due to low labour requirements of forestry activities, loss of land access rights to indigenous peoples.

Af/re-forestation will also likely have hydrological impacts.  Generally it has been shown that this would cause and increase in 'green vapour' flow and a reduction in runoff.  The extent of this impact varies regionally, however,  afforestation in 27% of suitable areas would likely result in a 80-100% decrease in runoff, whilst 50% of suitable areas are vulnerable to a significant reduction of less than 60% (Trabucco et al., 2008).  Careful consideration  is required as af/reforestation in terms of hydrology, would likely have impacts on water resources, water management, biodiversity, downstream food security.  Water resources are a particularly important consideration in-light of the escalating concerns over water scarcity (Trabucco et al., 2008).

Biochar

A pre-Columbian agricultural tool used in the Amazon basin, biochar or terra preta was applied to increase soil fertility.  Its persistance in this region for thousands of years is indicative of its potential of a truly long term sink of carbon.  Biochar essentially describes a man-made charcoal.  This can be produced through the burning of biomass in low an low or no oxygen environment, this process is known as pyrolysis.  Can be used not only as a means of trapping carbon, but also increases soil fertility as it was originally intended for historically. 

Capacity to reduce CO2?

By 2100, it seems possible that this technology could sequester 400 billion tonnes of carbon, lowering atmospheric concentrations CO2 by 37 ppm, though this number is considered to be at the high end of estimates (Kleiner 2009). 

Impacts? - (Kleiner 2009)

There are concerns that promotion of this CRD method could result in significant land-use changes in order to make room for the necessary plantations.  Similar responses have been the case with encouragement of biofuel production and the destruction of virgin forests for palm-oil plantations.

Concerns with biochar also lie in its true ability to sequester carbon.  In its application to soils to increase fertility, there are concerns that this will in fact increase microbial breakdown in the humus.    This works in opposition to the primary purpose of biochar, as it would accelerate release of CO2 from the soil.  The extent of this is unknown and  more work needs to be done to truly undertand the level of stability of biochar in soil. 

Conclusion

Each of these technologies are unable to solely solve global warming.  The speed at which reforestation can occur for example, and the conflicts of land use in all cases mean that these methods are restricted in timescale and extent.  However, I think that each could have a significant role in mitigating high atmospheric CO2 levels, in combination with other methods.  Consideration of the compromises that would have to be made socioeconomically in balance with the benefits would ned to be carefully considered. 

CDR: Ocean fertilization

Initially this method was perceived to be the solution to our budding CO2 problem with John Martin in 1988 declaring, 'Give me half a tanker of iron, and I'll give you an ice age'.  However, after numerous small-scale studies and growing knowledge regarding aquatic ecosystems this technique has been rendered obsolete with two UN conventions explicitly banning large scale ocean fertilization and a growing opinion that continuing investigation is a waste of resources (Strong et al., 2009). But before I get to the problems with this method, let me first outline what it involves:

How does it work?

This method proposes that by increasing primary production in the ocean surface waters, we can increase the amount of CO2 drawn down from the atmosphere.  Carbon fixing, photosynthetic organisms such as phytoplankton are nutrient limited, thus this increase in primary production can be achieved by supplying additional nutrients such as nitrogen (N), phosphorus (P) and Iron (Fe) to surface waters.

Martin's idea was born from the then contemporary studies exploring changes atmopsheric CO2 concentrations between the last glacial maximum (~200ppm) compared to the last interglacial and the pre-industrial levels of the current interglacial (~280ppm) (Martin, 1990).  The Vostock ice core indicated a negative relationship between atmospheric CO2 levels and Fe, as indicated in the diagram below.  This suggests that during periods of low CO2, iron was more widely distributed as dust from arid areas, palaeo-productivity studies indicate that during these time productivity was also high, supporting the Iron hypothesis that much ocean productivity is iron limited and clearly interacts with atmospheric CO2 concentrations.

The majority of studies and reviews focus mainly on Fe fertilization for two reasons:

• Iron is significantly more effective at increasing CO draw down, being 2 and 5 orders of magnitude more effective that P and N, respectively (Royal Society, 2009).
• N and P are significantly more expensive due to high demand for food production (Royal Society, 2009) ... also if you check out Cordell et al.'s (2009) story of phosphorus you'll see this particular resource is a hot commodity at the moment and arguably finite if we don't change our ways.

How effective would it be? - not very!

As previously mentioned, there have been numerous small-scale experiments but scaling up provides very little additional understanding as to global impact of large-scale implementation (Strong et al., 2009).  Subsequently, modelling has been employed to establish the likely impacts.  

The general consensus is that fertilization would be insufficient in mitigating global warming .  The study by Zaharlev et al. (2008) calculated that even if the Southern ocean were continuously fertilised such that there were no Fe limitation, enhanced uptake of CO2 peaked at less that 1Gt of CO2, less than 11% of annual emissions.  This mean that this technique alone could never really mitigate the high concentration of CO2 that we've put into the atmosphere, not now nor if annual emissions continue to rise!

Environmental risks

Now, I've spent A LOT of time reading about how terrible eutrophication can be (admittedly this has been in relation to terrestrial water bodies, but still - ), the effects can be devastating to aquatic ecosystems on all trophic levels!  The complexity of the ocean trophic structure far exceeds that of any terrestrial water body, making the effects of ocean fertilization fairly uncertain. Thats aid there are two likely effects that I believe to be worth mentioning:

• Ocean oxygen depletion: anoxia is a common symptom of eutrophication in lakes and coastal waters, with severe impacts on benthic communities and thus structuring of the entire food web. Sarmiento and Orr (1991) modelled various scenarios of CO2 increase and iron fertilization of the Southern Ocean.  The model indicated that oxygen depletion would occur with the biggest risk of anoxia in the Indian Ocean.  Palaeo-records have revealed that there have been numerous nutrient related oceanic anoxic events throughout the Earth's history, many of which have been so extreme as to induce marine mass-extinction (Kaiho and Hasegowa, 1994; Mort et al., 2007) ... we don't need to cause another!
• Increased greenhouse gases  (Fuhrman and Capone 1991): 'but I the aim was to reduce greenhouse gases?', I hear you ask.  This is where you begin to appreciate just how complex the Earths system is.  Increased productivity  would likely see an increase in methanogenesis that occurs during digestion of phytoplankton by zooplankton.  Whilst anoxia would also increased the production of methane by archaebacteria that dwell in anoxic waters.  Fe fertilisation would also cause an increase in N cycling, resulting in an increased production of N2O.  

Conclusion

Categorically banned under UN convention, large-scale ocean fertilization is considered to be ineffective.   It is unable to remove a high enough concentration of atmospheric carbon dioxide to be a viable option whilst simultaneously enhancing global warming through the increased production of other greenhouse gases.  

p.s. for the previous post

Another point I wanted to add to the previous post:

One of the greatest problems with many of the large scale SRM methods is the lack of knowledge.  However, to rectify this requires experimentation, and unlike many other scientific pursuits, this one cannot be confined to a laboratory.  The scale and dynamism of the climate system is something we cannot physically replicate in small-scale experiments to test these methods.  Thus we are presented with a catch-22 situation, whereby implementation requires greater understanding and experimentation requires implementation.   Some of the smaller scale techniques, such as cloud brightening and increasing surface albedo,  could be conducted at a more localised scale that the other techniques, however, we still run the risk of localised consequences.  Essentially, to test these methods requires that our planet becomes the laboratory - risky business!

Geo-engineering interlude and other SRM considerations

Over the last couple of weeks I have hopefully given you a bit of insight into key features of the main solar radiation management (SRM) approaches.  I expect that you have been able to gauge the imperfect nature of the methods proposed, that their feasibility (cost, effectiveness and risks) are flawed.  They should not go ahead without question nor further investigation. There are numerous reasons that I ask you to think beyond what I've been able relay to you, as there is so much more to consider beyond the summarised version I have so far portrayed:

• Local/regional scale impacts: Where will be subjected to the most significant climate and weather anomalies due to geo-engineering? Who will be affected? To what extent are these societies and ecosystems able to successfully adapt to these adverse conditions? What are the financial implications? These are all questions that require thorough evaluation before such measures be taken.  Take the South Asian Summer monsoon for example. I’ve referred to several studies forecasting the disruption of this monsoon system under the influence of geo-engineering (such as Bollasina et al, 2011).  The lives of 1.5 billion people in South Asia alone are intrinsically linked to this dynamic system.  The success of their water resources, agriculture, ecosystems, health and economies is shaped by the 70-80% of annual rainfall that falls in just 4 short months.  Compounded by high levels of poverty and a significant reliance on agriculture in this region, the acknowledged sensitivity of the monsoon system ensures that this region is highly vulnerable to geo-engineering.  The capabilities of such large and growing populations to adapt, and the financial implications (the cost of adaptation and the potential losses induced by declines in productivity) need to be fully appreciated.  This applies to all sensitive areas, including consideration to the impacts on the biosphere.

• Unexpected outcomes: No amount of modelling can guarantee the future outcomes of geo-engineering.  There is much progress to be made towards further understanding the non-linear and variable nature of climate and its associated feedbacks.  The knowledge that we lack is reflected within our models, as their performance is dependant on the data and parameters we put into them. We have a long history of ‘accidentally’ altering our natural environments (acidification, global warming, holes in the ozone), with many of the mechanisms driving this change becoming apparent once the negative impacts have been realised.  Hindsight is a wonderful thing, but can we afford to risk unpredictable climate changes when the stakes are so high?  Similarly, with the stakes being so high execution of the more feasible SRM techniques would be need to be meticulously executed.  As with our history of ‘unexpected’ impacts, we also have a long history of just doing things wrong – everyone makes mistakes, but can we afford to? If we consider the possibility of implementing stratospheric aerosols.  If we found ourselves in a situation whereby we were had been successfully geo-engineering our climate for an extended period of time and something went wrong (somebody forgot to do something, a system failed, or we find that the negative impacts are simply to great), that leads us to a new problem - the termination problem (Matthews and Caldeira, 2007).  If geo-engineering were to stop abruptly subsequent years would undergo a rate of climate warming 20 times greater than we are currently experiencing (with 20 times more catastrophic impacts?).  Where A2 is a scenario for continued global warming, GEO showing the effect of geo-engineering and OFF_2025/2050/2075 indicating the abrupt stopping of geo-engineering at for each of those respective years; the diagram demonstrates that the longer we employ geo-engineering, the more significant the termination would be.

Figure from Matthews and Caldeira (2007)

          ... which raises a new point, geo-engineering is a commitment – if we start we can’t 

          simply stop! We would be committing future generations to sustaining an SRM   

          program.

• High levels of CO2 are still high! A final point I’d like to raise is that of global warming not being a lone symptom of anthropogenic CO2 emissions.  Thus, to treat this one symptom with SRM geo-engineering neglects the other real threat of ocean acidification.  Integrated within the Earths biogeochemical cycle, the ocean is a key sink for C02. As we have emitted C02 in to the atmosphere, the ocean has taken up equivalent to half these emissions serving to lower the average pH by 0.1.  Already we are seeing significant impacts on ocean biota, such as coral bleaching and subsequent food web alterations.  Based on past records it has been estimated that if we continue on our current trajectory of CO2 emissions the pH of the ocean will decline by a further 0.4 by 2100 with devastating impacts on ocean biodiversity and associated food-webs including human livelihoods (Royal Society, 2009).  Ice core records indicate that in the past Earth has experienced substantially higher atmospheric C02 than at present.  However, the fastest know increase was 80ppm over 6000 years IPCC (2001). Slow changes accommodate inertia in the ocean systems, such that they can gradually alter and buffer the impacts of changing CO2 levels. After 200 short years of rapidly increased CO2 draw down into the oceans not only are we experiencing dramatic impacts, but the oceans will require tens of thousands of years to fully readjust.  Artificial means of reducing acidification are currently not an option; the only sure solution is to reduce emissions.  If we fail to remedy ocean acidification, additionally the rate of CO2 draw down will decline thus increasing the demand for geo-engineering.  For me this highlights the need to directly address CO2 emissions and not SRM!

And with that, over the next few posts I am going to move on to the remaining category of geo-engineering, CDR. These methods could be considered a form of mitigation as they address the issue of high atmospheric CO2 concentrations.  Will these prove to be viable options or by entertaining these ideas are we simply ignoring the elephant in the room?

SRM: Surface methods

The final approach considered under the category of SRM geo-engineering is that of increasing surface albedo.  This blog won’t be overly extensive as there a re a few fundamental limitations to this concept that negate the need (as well as the ability) to fully critique it.

Increasing the surface albedo is a method that proposes doing exactly what it says on the tin.  Though, since the Earth is not one uniform surface there are several ways in which this could be achieved (Royal society, 2009):

• Whitening roofs, road and pavements in urban settlements

            - cost: 1% of earths land surface $300 billion per year

• Increase more reflective crop varieties

            - cost: unknown

• Installing reflective materials into the desert

            - cost: $ trillions per year and extensive ecological consequensces.

• Reforestation

            - cost: unknown

The overlying issue with these techniques is their inefficiency (relative to cost!) compared to other SRM methods.  What has become apparent as I’ve been blogging it that SRM methods closer to earth are less efficient and more limited in reducing global temperatures whilst having a much more significant regional or local climate signature.  This area of SRM is no different, and would likely have the greatest regional impacts and the smallest overall global climate forcing concordant with its direct implementation on the Earth itself. 

In order to achieve the adjusted albedo necessary to correct global warming, extensive resources and investment would be required due to the shear spatial extent at which such methods would need to be implemented.  This also poses another problem, there are limitations to how far the earths surface albedo can be significantly altered (arguable some of the above options would exceed what is reasonable!), as such, with climate continuing to warm, this method, like cloud whitening, has a use-by date dicatated by our climbing CO2 levels.

The literature surrounding these methods falls short of some of the previous SRM approaches.  Perhaps this should be considered to be representative of the lack of support in these areas due to their ineffective and unfeasible nature.

Conclusion

Regardless of the individual environmental impacts of each of these methods (which regionally would be quite significant!), these methods are fundamentally limited by their inability to effectively and fully achieve the desired level of global climate cooling.

As promised: stratospheric sulphate aerosols vs. cloud brightening (Jones et al., 2010)

Jones et al. (2010):

Employing the HADGEM2-AO GCM to conduct 60 year simulations, this study sought to establish how these two methods compared in terms of climate impacts.

In summary:

Both techniques were able to sufficiently reduce global temperatures, though Stratospheric sulphates were considered more effective.  The variation between the two was most evident in the geographic distribution of these changes ...

Largely it would be considered that the stratospheric method produced more uniform changes in forcing, and thus the distribution of climatic effects deviated far less from the temperature equivalent natural conditions. The diagrams below the deviation of rainfall for the two methods, with each varying in terms of distribution and magnitude, though it is clear that the marine cloud method produces the most severe deviation. 

                                          (a) SO2

                                         (b) Marine clouds

The uneven and very regionally focussed climate perturbations induced by marine cloud brightening, is also reflected in changes in sea ice extent.  The increase in sea ice extent was far less for this technique than when compared to the more globally distributed effects of stratospheric sulphate injections.  

What the authors draw from these results is that the effectiveness and nature of climatic changes induced by these methods is largely dependent on distribution and location of application.  

Conclusions

The marine cloud method in this study used what is considered to be a reliable and efficient seeding regime, but it restricts application to the eastern coasts of the Pacific ocean and the south Atlantic and therefore more anomalous climatic outcomes and inefficiency.  Though marine cloud seeding regimes could be more global, they are still restricted and such regions (as the ones used in this study), if more efficient, would still dominate the forcing.  Comparatively stratospheric aerosol applications can be much more widely applied, and the nature of stratospheric circulation aids such distribution, yielding in this model more desirable climatic outcomes. 

SRM: Marine cloud brightening (MCB)

So marine cloud brightening brings SRM into the tropopause.  It consists of smaller scale, regional changes in albedo by increasing the concentration of cloud micro-droplets in localised marine cloud systems, basically making clouds whiter and therefore reflect more solar radiation.

So how would it work...?

Increased micro-droplet concentration can occur by increasing the concentration of particles that act as cloud condensation nuclei (CCN); though if too large these particles will increasing precipitation, thus reducing micro-droplet concentration.  Subsequently to minimise research and engineering costs, naturally occurring CCN have been proposed in the form of sea salt - a highly abundant and readily, not to mention freely, available resource!  

The complicated bit about implementing this would be distribution, how do we go about injecting sea salt into the lower atmosphere? This could be possible via aircraft or being projected from ocean vessels.  Though, due to very short residence time (<10 days), it has been calculated that 1500 ocean vessels, each with 28 billion nozzles, distributing >50 cubic meters of sea water droplets per second,  would be required to counteract a doubling of natural levels of atmospheric CO2.  Thats a lot of energy required in a time when we are supposed to be making cutbacks! Fortunately, those clever scientists (Salter et al., 2008), concerned about further exacerbating CO2 emissions, have proposed a low carbon option in the form of 'Flettner rotor ships' (see below); these are wind-powered, unmanned, remote controlled ships that can be moved with the seasons to desirable locations (allowing targeted cooling!) whilst spraying sea water droplets into the lower atmosphere. However, the plans aren't as comprehensive as they may seem, the technology required between drawing on sea-water and it's effective utilization in cloud seeding is far from complete, the intricacies involved are beyond the needs of this blog, but they would sufficiently prevent efficient and effective implementation in the near future.  The overall feasibility the  Flettner vessels is still unknown.

The capacity for MCB to mediate climate warming is limited to the doubling CO2, beyond this this method could only feasibly be used to stabilise climate.  As such, in the case of a climate emergency (which at our current increasing rate of CO2 emissions will hopefully occur  beyond doubling natural levels of CO2), this method is unlikely to be a considered option in returning us to pre-industrial temperatures, though it could be used to keep temperatures below a critical threshold.

Cost

There has been little in the way of analysing cost-effectiveness of this method, but ocean water is far from expensive so that reduces costs significantly.  It has been suggested that the fleet of Flettner vessels could cost in the region of £2 billion (Salter et al., 2008), which depending on the cost of maintenance could make them a cost-effective long-term investment in comparison to the high annual costs of the previous SRM methods.  But, how much additional investment is required for research and development of the appropriate technology?

Environmental impacts 

So lets begin with the positives:

• Global temperature reduction: Various models indicate that the potential to increase albedo will be substantial enough to balance the radiative forcing of doubling CO2 concentrations (Latham et al., 2008; Rasch et al., 2009; Jones et al., 2010 ... to name but a few!).  As such it is possible for us to counteract global warming, excellent this is a good start!  Latham et al. (2011) have established that such temperature reductions will also address ice loss in the poles.
• Targeted regional temperature reductions: Unlike stratospheric aerosols, which are difficult to confine regionally due to stratospheric circulation, this method could enable us to address just those areas most susceptible to the effects of global warming, should we be hesitant to implement engineering on a global scale!  Latham et al. (2008) were able to demonstrate, through modelling, that focussing on cloud modification in the North Atlantic we may be able to reduce summer ice retreat.  
• Reduce hurricane spawning and intensification: The links between extreme weather and climate change are still being explored (Check out Andy's blog for more on this!).  It is still being established whether intensification of hurricanes is a likely symptom of increased sea surface temperature due to global warming.  Due to the potential for regional application of cloud brightening, it would be possible to focus cooling sea surface temperatures in areas of hurricane spawning and intensification.

... If only it was that simple! If you play with clouds, you inevitably affect precipitation.

 Now the negatives;

• Precipitation patterns: despite different models using different strategies (location, intensity, season) the message is largely the same, that precipitation will largely decline and patterns will change.  The amazonian basin is subject to significant precipitation reductions which are essential for the productivity of the rainforest, though it is difficult to gauge the impact as the magnitude of precipitation decline is imprecise.   
• Localised changes in albedo:  Though previously put as an advantage, this could also be a huge disadvantage due to the interconnected and non-linear nature of the planet's climate and weather systems.  Little is understood about how this would firstly affect regional climate, but also how these changes may perturbate the rest of the global system.  
• Unbalanced effects: Highlighting the non-linear nature mentioned in the precious point, the link between temperature reduction and resolving loss of polar ice is somewhat out of sync (Latham et al., 2008).  Correcting global temperatures doesn't fully resolve ice loss at the poles (possibly partially due to reductions in precipitation), similarly, the forcing required at each the poles appears to also be different. 
• Not directly an environmental impact, but knowledge and modelling limitations make it difficult to fully anticipate the extent of these impacts.  Cloud seeding not only increases cloud albedo, but it will also reduce precipitation as well as increasing cloud cover and longevity, the relationships and climatic responses of which are little understood.  As such they are not incorporated into modelling and the regional and global effects are unknown.   These undesirable effects fo cloud seeding are likely to exacerbate the negative impacts on global and regional climate. 

Conclusion

Much like the other techniques this has its pros and cons. It lacks the ability for immediate implementation, the climatic intricacies appear to be little understood and it may not have the capacity to fully rectify anthropogenic climate warming.  We don't have significant examples to draw on to really understand the effects (such as volcanic eruptions and stratospheric aerosols) and much more research is needed to really gauge the feasibility of this method as the current models are significantly lacking.  Jones et al. (2011) compare the modelled impacts of sulphate aerosols and MCB ... it looks as though sulphate aerosols comes out as the overall winner (but I'll save that for the next blog!)

"not exactly disinterested science"

Also on the Guardian:


Following up on their earlier article and my blogs about public support for geo-engineering, the focus has shifted onto the short-fallings of the Mercer et al. (2011) study.  Apparently, the study was "not exactly disinterested science" ... I think I may have mentioned this, but if the experts agree then it must be true!

Did nobody tell them about geo-engineering?

So the most recent predictions are out!  This week, a Guardian article highlighted that the International Energy Agency (IEA) study suggesting that at our current rate we have only 5 years left before reaching a critical climatic threshold and rendering ourselves to an irreversible climate.  Could such predictions see a push towards geo-engineering?

SRM: In the stratosphere ... more on the environmental impacts

And now for the environmental supplement of stratospheric aerosol injections…

Climatic patterns

Where sulphate aerosol can combat global warming they pose severe diruption global precipitation.  Higher temperatures cause more evaporation (and precipitation), however aerosol cooling not only reduces evaporation, but also alters global temperature gradients and therefore precipitation distributions; Goldstein et al. (2010) highlights the numerous studies predicting aerosol induced precipitation reductions by up to 10%.  There would likely be disproportionate declines in some regions, putting already hydrologically sensitive areas at further risk. Primarily studies have indicated that climate in the tropics would be most significantly prone to such precipitation declines, most significantly both Asian (Bollasina et al., 2011) and African Summer monsoons would reduce in intensity.  The consequences of such alterations could put billions of lives at risk in regions where water and food supplies are already at a tipping point. 

Alternative strategies have been proposed.  One of the effects of global warming already in motion is that of Arctic warming.  Melting ice sheets and reductions of summer ice extent are a reality, not just a projection of the future.  Some have proposed the injection of sulphates at the poles with the intention of addressing Arctic warming whilst progress is made toward CO2 mitigation, without reducing tropical precipitation.  These methods raise as many doubts as tropical injections, though they are arguably cheaper to implement as sulphur injections would not be required in winter (3Tg per year instead of 5Tg), significant impacts to tropical summer monsoons appear to be unavoidable (Robock et al., 2009).

Acidification

The inevitable fallout and deposition of sulphate aerosols has been raised as a potential issue.  However,  a study by Kravitz et al. (2009) has indicated that the although this acidic deposition would increase.  The midlatitudes would experience the largest absolute increase, though such level would be masked in such regions by urban deposition; comparatively the largest relative to its baseline values would be experienced in pristine environments such as Antarctica.   The magnitude in both instances is relatively insignificant, however ...  In relation to the ocean acidification induced by CO2 over recent decades, sulphate deposition would be 2 orders of magnitude less.  Similarly, relative to the critical loading values associated with the most sensitive terrestrial areas (Kuylenstiernaet et al., 2001), the maximum value of point deposition calculated was at least an order of magnitude less.  Subsequently it appears that acidification in relation to sulphate aerosols SRM would be of minimal consequence.  However, the study recognises that the spatial and temporal resolution of the study may potential disguise larger loading in smaller scaled regions, whilst the effects on terrestrial water bodies couldn’t be assessed in this study, a significant short-falling in-light of the already declining water quality experienced globally. 

Ozone

Though global surface temperatures are cooled but sulphate aerosols, the stratosphere is in fact warmed.  Such warming has the potential to inducing lofting of the ozone layer, raising it to altitudes that will increase photochemical destruction of the ozone (McCormick et al., 1995).  A double edged sword;  by reducing solar radiation passing through the earths atmosphere, the photochemical reactions required for ozone production are also liable to be reduced.  This impact is further supported by studies indicating that volcanic eruptions such as Pinatubo have shown significant reductions in the ozone (Brasseur and Granier, 1992).  Tilmes et al. (2009) concluded that should stratospheric sulphate aerosol injections be implemented that this could set back ozone hole recovery by 30 years. 

Primary productivity

With crop growth and other plants being photosynthetic it is reasonable to assume that reductions in solar radiation would be detrimental.  However, research has suggested that plant productivity would conversely increase due to increased light use efficiency associated with more diffuse radiation (Mercado et al., 2009).  After the atmospheric sulphate loading of the Mount Pinatubo episode in 1991, deciduous forest productivity increased by up to 23% (Gu et al., 2003).   That said, should drought conditions be initiated by sulphate injections, then plant productivity will decline in affected regions regardless of the influence of reduced solar radiation levels.

Also...

Two other issues, not as directly significant but important nonetheless, whitening of the skies and energy capture.   Declining blue skies is not only a problem aesthetically, but also could have severe psychological implications (Robock,2008).  Whilst the energy capture point refers to solar power, should we reduce solar passage through the atmosphere we subsequently compromise a key source of renewable energy in some countries, Murphy (2009) indicated that in California, peak capacity for solar energy generation decline form 90% to 60% peak capacity in summer of 1992 proceeding Mount Pinatubo's eruption.

Depending on the technique implemented, there will also be varying degrees of environmental compromise associated with implementation.

Conclusions

Though some perceived problems have been remedied by certain studies, namely ideas about plant productivity and acidification, other problems are still remain; each of these issues has a significant weighting in opposing implementation.  The changes in precipitation patterns could have vital implications on livelihoods, economies and biodiversity for example, outweighing the value of diffuse radiation for primary productivity.

SRM: In the stratosphere

One of the hottest topics in geo-engineering is that of injecting aerosols into the stratosphere, primarily sulphate aerosols.  The idea behind such proposed methods was first mentioned by Budyka in 1974 and is inspired by nature itself; the stratosphere contains a layer of aerosols that scatter solar radiation away from the earth and are therefore contributing to the planetary albedo.  There are two options for this method, engineered aerosols or sulphate particles.  Despite the potential offered by the engineered approach, sulphate particles are considered a more viable option at present as they require somewhat less research; they would be simple to implement and we already know a fair amount about the effects of dramatically increasing sulphate aerosols due to explosive volcanoes.

Volcanic climate forcing …

Volcanic eruptions project vast volumes of debris into the atmosphere; sulphur dioxide (SO2) emitted during the eruption oxidises in the atmosphere to form Sulphuric aerosols.   If larger enough eruptions can deposit SO2 into the stratosphere where it is retained long enough to subsequently increase the global albedo and reduce solar radiation reaching the surface, inducing a net cooling. Due to a combination of instrumental and palaeo- ice core records we have a good record by which to correlate volcanism with climate:

Mount Pinatubo (McCormick et al., 1995)

The infamous Mount Pinatubo eruption of 1991 produced 20 x 1012 g  (20 Tg) of SO2.  Compared with an average of 1958-91, there was a cooling of 4.3Wm-2 between 40 N and 40 S and average decline of 0.7 degrees C globally.  By 1993, however, the radiation balance was reinstated, thus the perturbation was short-lived.

Palaeo-records

However, the palaeo-record has enabled us to extend our understanding back beyond this instrumental period, during which time has been very explosive volcanism has been limited.  GISP2 ice-core records have revealed that not only are volcanic injections of sulphate aerosols capable of inducing short term climate cooling, but that the cumulative effect of frequent, large scale eruptions can result in persistently cooler global temperatures.  Such frequent and explosive volcanism is coincident with, and thus considered to have at least partially been the cause of cooling in the early Holocene (Zeilinski et al., 1994; Bradley, 2003) and the latter half of the Maunder Minimum (Bradley, 2003). As such, it can be established that, although the cooling effect of sulphate injections are relatively short-lived, continual or frequent contributions into the stratosphere can induce persistent cooler global temperatures.

It can be concluded therefore that based on current knowledge, this appears to be a highly effective option and efficient which would demonstrate results in little over a year.

How?

There are a whole variety of ideas being explored.  The main ideas include distribution into the stratosphere via planes, artillery fire and stratospheric balloons.  At present the most viable option would be that of the plane, as the military already use the necessary specification of aircraft; some modifications would be required but near immediate implementation could occur.   Other ideas, which at this stage in their development sounds a bit far-fetched to say the least, include a tall tower potentially reaching upto 114km, using balloons for support and  a space elevator running on solar power  (Robock, 2009)…

But what are the risks?

• Change in regional precipitations leading to drought in Africa and Asia
• H2SO4 deposition 
• Ozone depletion
• No more blue skies
• Less solar power
•  Environmental impact of implementation

And the cost:

Table 1. Adapted from Robock (2009).  The costs for different methods of injecting 1Tg of a Sulphur gas per year into the stratosphere

Method	Annual Cost ($)
planes	225,000,000 – 4,175,000,000
Artillary fire	30,000,000,000
Balloons	21,000,000,000 – 30,000,000,000

Conclusion

The efficiency and feasibility of this method renders it a strong possibility.  Modes of implementation are available, thus there would be no delay reliant on developing the necessary technologies.  And though the costs are large, they are substantially reduced in comparison to those proposed through space-based SRM, and are cheaper than investing in technologies as an alternative to CO2 emissions.  The next blog will be looking at the environmental risks more closely.