As the Paris climate deal takes legal effect, it is necessary to assess the technical aspects and challenges to limit the global temperature increase. Given the problems in completely eliminating greenhouse gases (GHGs) emissions from human activities, one of the possible solutions is using Negative Emission Technologies (NETs) as a way of compensating for those emissions. As the UK has recently stated a target of net zero emission, Smith and colleagues took on a preliminary assessment of land-based NETs in this country in order to estimate their potential and impact.
There are a number of ways negative emissions could compensate for CO2 emissions:
1) Bioenergy with Carbon Capture and Storage (BECCS), using crops to extract CO2 and then burning them for energy and sequestering the result emissions, thought to hold the most potential to bring down CO2 levels
2) Direct Air Capture of CO2 (DAC) from ambient air and either burying it underground or using it in chemical processes
3) Enhanced Weathering of minerals (EW), by spreading pulverised rocks onto soils to increase the natural weathering process that takes up CO2
4) Afforestation and Reforestation (AR)
5) Soil Carbon Sequestration (SCS), which uses modern farming methods to reverse past losses of soil carbon and sequester CO2
6) Biochar, that converts biomass into biochar for use as soil amendment
Smith and colleagues considered the use of UK land specifically and only technical aspects of these technologies. Other factors, e.g. of a socio-political nature, were not considered and are thought to lower the potential of the NETs considerably.
Regarding land availability, BECCS and AR use land that can no longer be used for food production, assumed to be 1.5 Mha. The same value is assumed for biochar, since growing feedstock for it cannot be done in the same land used for food. SCS and EW can be practised on land without changing its use, here assumed to be 8.5 Mha. Finally, DAC has no land footprint so it is not constrained by land availability.
Negative emission potential for BECCS, AR and biochar are 4.5‒18, 5.1 and 1.73‒11.25 Mt C eq. per year, respectively. SCS would deliver 0.255‒8.5 Mt C eq. per year and the combined potential for EW would be 22.5 Mt C per year. DAC is compared at the same level of BECCS, i.e. 4.5‒18 Mt C eq. per year.
In the UK, total emissions of GHGs are equal to an average of 153 Mt C eq. per year. Considering that not all NETs can be applied at the same time and assuming no interaction between practices, the maximum aggregate potential of land-based NETs is estimated to be 12‒49 Mt C eq. per year (BECCS plus SCS plus EW). This represents only 8‒32% of current UK GHGs emissions. DAC, however, could increase this number further.
This maximum aggregate potential is limited by a number of factors, including cost, energy, environmental and socio-political constraints. More studies are needed to fully understand and hopefully overcome the barriers to implementation and reach the target of net zero emission.
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Pete Smith, R. Stuart Haszeldine and Stephen M. Smith
Environ. Sci.: Processes Impacts, 2016, 18, 1400-1405
About the webwriter
Luiza Cruz is a PhD student in the Barrett Group at Imperial College London. Her work is towards the development of new medicines, using medicinal and natural products chemistry.
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