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!