Large scale deployment of CO2 removal to achieve 1.5-2C global warming: A review of methods, technical feasibility and socio-environmental impacts

Background

The UN Paris Agreement (2015) binds the UK to contribute to limiting global warming to 1.5-2C above pre-industrial levels. Though in 2018 UK net emissions of carbon dioxide CO2 were estimated to be 2.4% lower than 2017, CO2 accounts for 81% of total UK GHG emissions. Integrated assessment models suggest that consistency with 1.5C requires global net negative CO2 emissions by around 2050 (baseline pre-industrial levels) (and 2075 for 2C): human-caused CO2 emissions must be matched by the amount removed via industrial CO2 removal (CDR).

Carbon Dioxide Removal (CDR)

CDR involves the extraction of CO2 from the atmosphere and its permanent storage (in the ocean, geological formations or subsurface).
Four pathways (P1-P4) are illustrative of how CDR can help reach global commitments, ranging from the most GHG-reliant (P4), to the most reliant on a decarbonised energy system (P1).
This report assesses the available methods and technical feasibility of large scale deployment of CDR and evaluates its socio-environmental impacts.

IPCC3.png
All four illustrative mitigation pathways indicate the timing of net zero emissions for the 1.5C limit is around 2050.

 

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G1: (TRL 1-2) Basic principles observed and reported, application formulated
G2: (TRL 3-5) Experimental working demonstrated, validation in laboratory and relevant environment
G3: (TRL 6-7) Prototype demonstration in relevant or operational environment

Bioenergy with carbon capture and storage (BECCS)

BECCS combines bio-energy systems and carbon capture and storage. Bioenergy feedstock captures CO2 in the atmosphere and is then turned to energy products via combustion. The CO2 emitted from this industrial process is captured and stored underground (e.g. in depleted oil and gas fields or aquifers), without being released into the atmosphere. BECCS both generates negative emissions of CO2, while it produces commercially available energy products e.g. heat, electricity and fuels.
BECCS is the only CDR in operation over extended periods and showing stability, e.g. Drax Power Station pilot in the UK. Even if the UK increases use of BECCS (as committed to in the Carbon Plan), then current rates of deployment may lead to future reliance on international availability and market prices to import bioenergy.

Impacts (Henceforth, relative to large-scale deployment.)

  • Conversion of estimated 26\% of the currently globally available agricultural land by 2100 needed. Land prices would rifle up, increasing food prices. Given an estimated increase of 2bn people by 2050, rising food prices may increase undernourishment amongst 150mio people. In the UK, conversion of 7\% to 61\% of agricultural land for production of biomass would be needed, resulting in competition for land.
  • Transformation of agricultural land into fast-growing monoculture plantation may threaten biodiversity and as nutrients are removed alongside feedstock, land degradation may result. During conversion to feedstock, soil could release carbon into the atmosphere. In areas with carbon-dense forests (e.g. tropics), it may take 10-100 years for the land to recover its ability to store carbon.
  • Water for irrigation could lead to pressure on water scarcity, use of pesticides could lead to increased pollution.
  • Best practice scenario (BPS): deploying BECCS at smaller scale may promote preservation of biodiversity, e.g. using native plant species for feedstock. Planting feedstock in degraded land and desertified areas could counteract land degradation. Food security could be counteracted by involving local communities in the implementation of BECCS and using feedstock as food source.

 

Reforestation, forest management and afforestation (FM)

Reforestation: Land, which was once a forest, yet had been used as agricultural land, is transformed into forest.
Forest management: Degraded forest being revitalised, e.g. by planting more trees where some where lost.
Afforestation: Converting land, which never hosted a forest, to a forest.

As FM is desirable for conservation of biodiversity, contrasting soil erosion etc., some countries are already employing them, e.g. Brazil’s 12mio hectares Atlantic reforestation.

Impacts:

  • Competition for land and consequently increasing food prices by 80\% in 2050 may likely result. This could cause undernourishment of 80-300mio people
  • Roughly 1-2\% of the total renewable freshwater on land appropriated by humans per year would be required. Given the increase in global population, the intensity in water demand increase water scarcity.
  • Forests reflect less than bionenergy feedstock. Hence, they absorb more solar radiation and contribute more to net climate warming.
  • BPS: using native plant species may help preservation and restoration of biodiversity. Soil erosion may be counteracted due to the growth of root networks and nutrients retention could result.

Biochar

Biochar is charcoal obtained by heating biomass in conditions of low oxygen, and is distributed over soil. As the carbon absorbed by biomass before it was burned is very slow to break down, carbon is fixed in charcoal.
Technology relevant to large scale processing is not given in the UK. Deploying Biochar at large scale is relatively costly, but price could be reduced by selling Biochar as a substitute for feritliser.

Impact:

  • 20\% of global cropland needed to provide biomass to produce Biochar. Pressure on food security likely to result.
  • Biochar may enhance the soil’s ability to retain water and provide carbon for plants. This could decrease the agricultural the necessity, hence the pollution, from fertilisers. The improvement of crop yield due to Biochar may have a positive effect on food security.
  • Biochar is black, so largely absorbs sun radiation and may contribute to global warming.
  • BPS: estimated 5-9mio km^2 of land could be utilised for biomass production without competing with food security, by utilising degraded land.

 

Direct Air Capture (DAC)

Direct air capture sequestrates atmospheric CO2 by filtering air through a chemical, which absorbs CO2. When saturated with CO2, the chemical hydrated such that the absorbed CO2 is released. The released CO2 is compressed to high pressure and stored or distributed. There are two commercially available DAC types: high temperature water based and low temperature solid sorbent-based.

Impact:

  • Per unit of carbon removed, DACs require least amount of land. Yet, high water demand could create pressure on water availability
  • Deploying DACs in CO2-poor areas could increase capture costs, but local CO2 depletion could affect vegetation
  • Sorbents used to capture CO2 could have impacts on pollution and there is uncertainty regarding how leak-proof CO2 storage is.
  • The compression of CO2 is highly energy intensive, which could increase electricity prices.
  • BPS: Utilising waste heat could lower supply costs, lowering capture costs from $100 to $45 by 2050, while promoting waste management.

 

Enhanced weathering (EW)

EW accelerates a natural process by which rock forming materials capture carbon from the atmosphere. The rocks are crushed and spread in locations with good exposure to air (coastlines), and water (oceans).
EW has not been widely tested, but the UK has wide resources of suitable rock. It is estimated that 430GtCO2 could be captured in the UK, at a cost of between £15 and £361/t.

Impacts:

  • If ground rocks are distributed on agricultural soils, the latter may become more productive as carbon acts as a fertiliser.
  • Due to the energy input needed for the chemical reactions involved in EW and mining of the minerals, EW could increase electricity prices.
  • Ground rocks could end up in the ocean, where they would subtract CO2 from water and thereby counteract ocean acidification (OA), which results from the ocean absorbing CO2 from the atmosphere.
  • EW is a way of accelerating a natural geological process: it does not face any challenges relative to storage and potentially leakage.
  • BPS: if applied on agricultural land, EW may not compete with land for other uses.

Ocean liming (OL)

As part of a natural process, the ocean traps great amounts of carbon, which leads to OA. OL refers to adding lime to oceans to increase their capacity to trap CO\(_2\). Lime is produced by heating limestone, which is a common industrial process.

Impacts:

  • OL requires approx. 30\% of the lime produced by the global coal industry per year for every GtCO\(_2\) .
  • OA may threaten coral reefs, having adverse socio-economic consequences, e.g. on commercial fisheries and alter the attraction of coral reefs for tourism. £87mio total net economic loss from changes in catch due to OA by 2050 in the UK are estimated.
  • Sea dumping is restricted by law (UN Convention on law of the sea (1982)). However, models suggest that even with extensive use of CDR, consequences of OA are already determined.

Disclaimer: Here’s a screenshot of the references I have made throughout this text. I had to transcribe the text from a LateX document, hence there are no direct references in the text as the references were in the .bib file.

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