Carbon-Negative Solutions for Climate Stabilization

Efforts to date to curb greenhouse gas (GHG) emissions have failed to prevent continued increases in emissions, with the rate of emissions growth higher during the 2000s than in the 1990s, and with GHG emissions reaching an all-time high of ~50GtCO2-eq. in 2010. This failure to reduce emissions over the 20+ years since the United Framework Convention on Climate Change (UNFCCC) came into being, now makes limiting increases in global warming to <2◦C (the threshold beyond which climate change is regarded as dangerous) extremely challenging. Since global GHG emissions are now much higher than they were when the UNFCCC was agreed, we are now facing the prospect of requiring “negative emissions technologies” (NETs: i.e., those that result in net removal of GHGs from the atmosphere), to reach atmospheric GHG concentrations consistent with a <2◦C target. NETs include: BioEnergy with Carbon Capture and Storage (BECCS), Direct Air Capture of CO­₂ using Sodium (or Calcium) Hydroxide (NaOH DAC) or monoethanolamine (MEA DAC), and atmospheric CO₂ removal by accelerated weathering of magnesium oxide-bearing rocks (AW-MgO)

The potential of NETs has so far been examined only theoretically through Integrated Assessment Models (IAMs) which have shown considerable cost-competitive potential for GHG mitigation by NETs, even allowing atmospheric CO₂ concentrations to be reduced below current levels. But the technology is controversial since CCS technologies have not yet been demonstrated at scale in major energy conversion applications, and for some options (such as BECCS) the large quantity of biomass required would have significant implications for land use and competition with non-energy uses of land such as food production.

There are risks associated with relying heavily on any technology that has adverse impacts on other aspects of planetary sustainability to achieve our climate goals. Though deep and rapid decarbonisation may yet allow us to meet the challenge of meeting the <2◦C climate goal through emissions reduction alone, this window of opportunity is rapidly closing, so there is likely to be some need for NETs such as BECCS in the future. An over-reliance on BECCS and other NETs in the future, if used as a means to allow continued use of fossil fuels in the present, is extremely risky since our ability to stabilise the climate at <2◦C declines as cumulative emissions increase. A failure of BECCS or other NETs to deliver expected mitigation in the future leaves us with no “Plan B”. “Plan A” must be to reduce GHG emissions aggressively.