Going below zero

With the advent of the Paris Agreement, there is a new focus on net zero emissions. This is largely driven by a better understanding of climate science (the importance of cumulative emissions), but also by a line in the Agreement itself which calls for a ‘balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century’. This potentially brings into play a set of technologies known as negative emissions technologies or NETs. A NET is a technology which draws down on atmospheric carbon dioxide; perhaps the simplest implementation of this is planting a tree.

NETs are required for two reasons over the long term;

  1. Be it local or global, a requirement for net zero emissions will inevitably mean a balance between remaining sources of emissions and the removal of carbon dioxide from the atmosphere as an offset, rather than a world of no emissions at all. Remaining sources of emissions could include some continuing use of fossil fuels but without dedicated carbon capture and storage (e.g. aviation) or very difficult to manage emissions such as from the agriculture sector. This requirement may only need NET deployment on a modest scale, simply to match the remaining emission sources. However, if those sources remain significant, then NET deployment would have to be scaled to match.
  2. At a global level, cumulative emissions may have exceeded a desired level for a certain temperature goal, in which case there is a need for an overall drawdown on atmospheric carbon dioxide, beyond that which natural sinks might deliver (e.g. continued ocean uptake). This is likely to require very significant deployment of NETs, certainly on the many gigatonnes per annum scale.

Even before the Paris Agreement, an in-depth look at the IPCC 5th Assessment report would have shown that many of the scenarios consistent with the 2°C goal included a period in the second half of the century when global emissions were negative to achieve a net drawdown on atmospheric carbon dioxide. The reason for needing such a period is that under these scenarios it doesn’t prove possible to limit emissions sufficiently, given the time it takes to re-engineer the energy system in the face of rising demand and legacy infrastructure.

The Paris Agreement has only strengthened the need for negative emissions technologies. With a goal of somewhere between 1.5 and 1.8C (‘well below’, as the Agreement states, could be interpreted as at least 10% below 2°C), the cumulative emissions of carbon should be some 175 billion tonnes of carbon lower than for a 2°C scenario, or 640 billion tonnes CO2. At current levels, that is the equivalent of 15 years emissions. As I illustrated in a pre-Paris post, decades of NET deployment and use may be required to meet this stringent carbon budget.

A recent article in Nature Climate (Biophysical and economic limits to negative CO2 emissions, Nature Climate Vol 6, January 2016) looks more deeply at the set of technologies that society may come to depend on in the coming decades. The article neatly categorises them with yet another set of acronyms (with OU, AS and BC ascribed by me);

  • BECCS: bioenergy with carbon capture and storage.
  • DAC: Direct air capture of carbon dioxide from ambient air by engineered chemical reactions. This would then become DACS (or DACCS) if geological storage were involved.
  • EW: Enhanced weathering of minerals, where natural weathering to remove carbon dioxide from the atmosphere is accelerated and the products stored in soils, or buried deep in land or deep-ocean.
  • AR: Afforestation and reforestation to fix atmospheric carbon ion biomass and soils.
  • OU: manipulation of carbon uptake by the ocean, either biologically or chemically.
  • AS: Altered agricultural practices, such as increased carbon storage in soils.
  • BC: Converting biomass to recalcitrant biochar, for use as a soil amendment.

The article focusses on BECCS, DAC, EW and AR and gives a detailed breakdown of the global impacts of these technology areas in terms of water, energy needs, land use and so on. It is clear that there is no silver bullet to rely on. While BECCS and DAC can potentially be deployed at scale and make a material difference to atmospheric carbon dioxide (>3 GT Carbon per annum by 2100, or 10+ GT CO2), BECCS requires significant land and water use (but is a net energy producer), whereas DAC is a big energy user. The latter is also deemed to be very expensive to implement. EW, on the other hand, just doesn’t make the grade in terms of scale. That leaves AR, which is certainly scalable but only very large scale deployment occupying huge swathes of land will make a significant difference in atmospheric carbon dioxide.

The paper ends with the rather sobering recognition that a failure of NETs to deliver expected mitigation in the future due to any combination of the biophysical and economic limits examined, leaves the world with no ‘Plan B’. Clearly there is much more to be done to commercialise and deliver a sustainable pathway for this family of technologies.