Greenhouse Gas Removal – and checking it works

According to the Climate Change Committee (CCC) balanced pathway to net zero, we should be capturing and storing 58 MtCO2e/year by 2050 [1]. This is called greenhouse gas removals (GGR) or sometimes carbon dioxide removals (CDR). It is hard to see how we can get to net zero without some GGR, but there is very little of this happening right now. There are several strategies likely to be deployed. Unfortunately, the ones which are most likely to be permanent tend to be most expensive as well, while with the cheap ones it is harder to measure how well they are working.

In the CCC balanced pathway, 90% of GGR is some form of BECCS – bioenergy with carbon capture and storage. That generally means burning biomass for energy and capturing CO2 from the flue gas. Most of the rest is direct air capture (DAC) which means getting carbon dioxide from the air rather than flue gases. The captured gas must then be stored typically in rock formations or depleted oil and gas wells.

 

Figure from [1] CCC sixth carbon budget

Increased woodland will store some carbon too.

What about planting trees? That is counted separately. The CCC proposes increasing woodland cover so that tree growth should provide another 12 Mt CO2e/year by 2050 – only a fifth of the projection for GGR but definitely worth having. Waste wood from plantations will also provide material for GGR. The total removals from GGR and forestry (70 Mt/year) in 2050 in the balanced pathway amount to 13% of 2019 emissions. 

Woodland grows more slowly than energy crops.

You may wonder why we do not rely more on woodland for absorbing CO2. One reason is that forest grows more slowly than energy crops so you need more land area. The CCC balanced pathway increases the area of woodland from 3,200 kha now to 4,400 kha. The energy crops land area proposed is only 720 kha but it is going to absorb as much in the way of CO2 as the forest [16]. 

BEIS is funding innovation in GGR, including DAC with intermittent renewables and biochar.

We know how to do BECCS, even there is not much happening at the moment. However, there are other options and BEIS is funding innovations in this field. The latest round of grants are going to 15 projects of which six are direct air capture using renewable energy or waste heat processes (e.g. powering DAC with intermittent ‘spare’ wind, or with waste heat from a power station); seven involve biochar production on a range of scales; one captures carbon dioxide from sea water and one captures methane from cattle housed indoors [2]. 

Biochar is not in the CCC plans as it was regarded as too uncertain.

The emphasis on biochar is a bit surprising given the CCC plans did not include it at all – at the time the Sixth Carbon Budget report was prepared (it was published in 2020), biochar was regarded as too uncertain to include in the models. Biochar is fairly similar to charcoal; it contains a form of carbon which is biologically stable and can last in soil for centuries, under the right conditions. It also (depending on conditions) acts as a soil improver. 

GGR methods need to be effective, permanent and additional. 

However GGR is achieved, we need robust methods for measuring how much greenhouse gas we have removed. The removals need to be effective, permanent, and additional. We have to take into account the emissions generated during processing and offset these against how much is removed from the air. It is also helpful if removal happens in the same timeframe as the emissions. Considering tree planting:

• Planting trees that die before they mature is not effective 
• In tropical and subtropical areas, half of trees in reforestation projects die within 5 years [3].
• In the UK, 40% of the trees planted around the A14 road upgrade scheme did not survive and had to be replaced [4].
• Woodland is vulnerable to natural disasters such as fire and disease:
• In one summer 2019/2020 Australia lost 20% of its forest to bushfires caused by climate change [5]. 
• Forest fires are increasing in the UK too. Considering 4 year averages, the area burned quadrupled between 2009-2012 and 2019-2022 [6]
• Woodland is also vulnerable to man-made disasters such as construction of new roads or houses. 
• Planting trees removes carbon very slowly at first and it takes 20 years or more to achieve the promised quantity of carbon storage. If I buy carbon offsets for my emissions each year, having them removed in 20 years means 20 years of warming the earth in the interim.
• Planting trees on land that was used for food, triggering forest clearing elsewhere to grow more food, is not additional. This also applies to energy crops.

Engineered GGR is a safer bet for carbon storage.

Do not think I am against planting trees. They are essential, for biodiversity, for flood management, for shade, beauty and serenity and other reasons. However, the impermanence and the long timescale are additional reasons why engineered forms of GGR are a safer bet for climate change, as long as they are monitored and verified. 

Storage in depleted oil and gas wells: leakage possible but should be detectable.

GGR methods vary both on the capture side and the storage side. The DAC projects all capture CO2 as a gas. This is easy to measure but is a bit of a challenge for storage. Storage in depleted oil and gas wells is probably easiest as we already have a lot of the infrastructure required. There are risks of leaks at the injection wells, older well sites, or natural faults. However studies suggest that even in poorly chosen sites leakage should be less than 15% and it would be gradual rather than cataclysmic. This is encouraging. [7]

CO2 is also injected into active oil wells, to increase the pressure and get more oil out. This is called enhanced oil recovery. Some of the CO2 injected remains, some combines with the oil. The oil industry argues that this can make oil recovery carbon neutral [18].

Storing CO2 in basalt formations is permanent but expensive.

Storage in basalt rock is permanent because the CO2 chemically binds with the rock. However, it seems to be rather expensive, energy intensive and also requires a lot of water. CarbFix is the leader in this area and it has its critics [8]. 

Biochar is fairly permanent and can improve soil fertility.

Biochar is an interesting possibility as it requires no new technology or infrastructure. It is not as permanent as geological storage, lasting at most a few centuries rather than millenia, but it Is much cheaper and it can improve soil fertility. Biochar (like charcoal), is prepared by heating biomass (often but not necessarily wood) in a low oxygen environment to get pure(ish) carbon. 

Carbon accounting for biochar is tricky.

• Biochar is not immune to carbon leakage and the carbon accounting is not trivial.
• While making the biochar, it is very important that the heating process is managed carefully using a retort to burn the gases that are released. Otherwise you can get methane emissions which are far worse than CO2 for greenhouse impact.
• When you have made the biochar, the carbon content is highly variable, depending on both how you make it and what you make it from (see chart) [13].
• After adding biochar to the soil, it can increase methane emissions from soil, though mainly if it is waterlogged (e.g. rice paddy) or if the manufacturing stage was not hot enough. It should be hotter than 300°C [10].
• This is to do with getting different molecular structures at different temperatures – think diamond, graphite, graphene, bucky balls … all different molecular structures of carbon.
• Biochar does break down slowly. After a century it is possible only half is left but if it is made at high enough temperatures there could be 90% or more still present [13].

The latest changes to IPCC carbon accounting guidance require monitoring for methane emissions when producing biochar and from treated soil [15]. The IPCC has published provisional carbon factors for using biochar for GGR, albeit with wide uncertainty ranges [13]. The chart below shows the proportion of carbon from different sources. Wood is the best.

Mass of organic carbon in biochar, as a proportion of biochar product, with uncertainty ranges. Data from the IPCC [13]. Pyrolysis is like gasification but at a higher temperature and with no oxygen present.

Biochar can be used as soil improver.

Biochar is often (usually?) good for the soil. It can improve the structure, both allowing drainage and storing water in tiny pores [11]. However, biochar is not pure carbon and the other components are a bit variable. Usually they are alkaline (like wood ash is alkaline) and this can also help soil if it is acidic to start with. If the source biomass has toxic metals in it so will the biochar. Biochar can also contain toxic organic compounds created during the heating process [12]. Quality is very important.

Enhanced weathering is even trickier to monitor but requires no biomass.

If you thought the carbon accounts for biochar were painful, enhanced weathering is even worse. This uses rock such as basalt which reacts with CO2 to form carbonates such as limestone. You have to grind up the rock and scatter it over a large area to expose it to the air. You can use farmland for this. In time ( a few years) it should react with CO2 in the air but the rate depends on conditions. Measuring how much you have made means repeated analysis of soil samples over the treated area [14]. This is much harder than with biochar where you can measure how much you have made at the factory stage. 

Enhanced weather needs no biomass – but does mean mining.

Enhanced weathering does have the major advantage that you do not need to grow biomass first, which means it does not compete with crops or biodiversity for land area. The scale required will mean mining which also has environmental impacts, but relatively localised.

As always with climate change solutions, there are no silver bullets and we should progress all these technologies as far as we can. However, it is very important that we keep robust accounts of how effective they are in practice, even when this is difficult. When we calculate costs we need to include the cost of monitoring as well as process cost.

[1] The sixth carbon budget. The UK’s path to net zero (CCC) 2020

[2] Direct Air Capture and other Greenhouse Gas Removal technologies competition (BEIS) 2020

[3] Half of replanted tropical trees don’t survive, new study finds (UK centre for ecology and hydrology) 2022

[4] A14 Cambridgeshire: 'Large proportion' of million trees dead (BBC) 2021

[5] Unprecedented' globally: more than 20% of Australia's forests burnt in bushfires (Guardian) 2020

[6] Area burned by wildfires in the United Kingdom from 2009 to 2022 (Statista.com)

[7] World can ‘safely’ store billions of tonnes of CO2 underground (CarbonBrief) 2018

[8] Carbfix and Climework’s large-scale plans to capture CO2 and inject it into basalt formations in Iceland involve high consumption of scarce resources and potential risks (Geoengineering monitor) 2021

[9] Carbon Sequestration through accelerated carbonation of concrete aggregate (Gold Standard) 2022

[10] Nan, Q., Xin, L., Qin, Y. et al. Exploring long-term effects of biochar on mitigating methane emissions from paddy soil: a review. Biochar 3, 125–134 (2021). https://doi.org/10.1007/s42773-021-00096-0

[11] Biochar (RHS) 

[12] Paulina Godlewska, Yong Sik Ok, Patryk Oleszczuk, THE DARK SIDE OF BLACK GOLD: Ecotoxicological aspects of biochar and biochar-amended soils,

(Journal of Hazardous Materials) 2021

[13] Method for Estimating the Change in Mineral Soil Organic Carbon Stocks from Biochar Amendments: Basis for Future Methodological Development (IPCC) 2019

[14] Carbon Accounting for Enhanced Weathering (Frontiers in Climate) 2022

[15] 2019 Refinement to the 2006 IPCC guidelines for National Greenhouse Gas Inventories (IPCC) 

[16] The Sixth Carbon Budget Agriculture and land use, land use change and forestry (CCC) 2020

p40 indicates 6.4Mt biomass from energy crops in 2050. This equates to approximately 3.2 Mt carbon or 12 Mt Co2).

[17] Can CO2-EOR really provide carbon-negative oil? (IEA) 2019