Offsetting unabated agricultural emissions with CO2 removal to achieve ambitious climate targets

since stringent climate policies increase food prices in the short run. Achieving the 1.5˚C target without introducing food security support policies could lead to a rise in undernourishment of 80–300 million people in 2050 [30–32]. Given these challenges to achieve rapid reductions in agricultural emissions, a substantial gap between modelled emission pathways and reality appear imminent [15]. If agricultural emissions are not mitigated beyond current levels, the remaining carbon budget to reach the 1.5˚C target is cut by more than half [33–35]. As a result, offsetting unabated agricultural emis- sions could become necessary to achieve climate targets in line with the Paris Agreement. Since technologies to sequester non-CO2 species from the atmosphere exist only at the concep- tual level and the feasibility of their implementation is debated [1], offsetting agricultural emis- sions through additional CO2 negative emissions could become necessary. Carbon Dioxide Removal (CDR) plays an important role in emissions pathways that are consistent with ambitious long-term temperature targets (below 2˚C) [1, 3]. CDR encompasses different techniques with a varying degree of maturity, scalability, and removal potential [36]. Although heavily deployed in emissions scenarios compatible with the Paris Agreement’s tem- perature targets [1], knowledge of how to scale-up and internationally govern CDR as well as on its implications on the planet and society is lacking [36–42]. Existing carbon emission trad-

ing mechanisms have been proposed as main avenues to finance CDR [43–45], yet it is unclear how this would work to guarantee the offset of non-CO2 emissions. Polluters would have, in fact, an incentive to completely eliminating gross CO2 emissions if this results to be more cost- effective rather than paying a price on carbon reflecting the costs of CDR [46]. Offsetting non-CO2 greenhouse gas emissions via removal of CO2 requires defining a con- version metric between different emission species. Parties to the UNFCCC and to the Paris Agreement currently report aggregate CO2-equivalent emissions and removals of greenhouse gases using the 100-year time-horizon Global Warming Potential (GWP100) as a conversion metric [47]. GWP100 is a simple heuristic that accounts for the integrated change in radiative forcing–the perturbation of the Earth’s atmospheric energy balance, which leads to warming– over the 100 years following a pulse emission of a given climate pollutant, relative to the same quantity of CO2. In contrast to explicit radiative transfer modelling, GWP100 does not account for interactions between different species. A large body of literature highlighted shortcomings of assessing the warming effect of short-lived climate pollutants, such as methane, with GWP100 [48–53]. Yet, this inaccurate use of the metric remains widespread and with serious implications: the implementation of the Paris Agreement without adjustments to its emission metrics results in an uncertainty in temperature outcomes as high as 0.17˚C [52]. Alternatives

to GWP100 exist, spanning from Global Temperature Potential (GTP) metric (similarly com- paring emissions pulses, but based on the ratio of warming amounts at the end of the time horizon [54]) to an alternative usage of the GWP metric (denoted GWP�) relating cumulative CO2 emissions to changes in the rate of emissions of short-lived climate pollutants [35, 55]. Huntingford et al. (2015) [56] showed that using GWP100 and GTP100 to substitute methane emissions with CO2 reductions, in the RCP2.6 scenario, leads to higher global mean tempera- tures than an equivalent methane emissions reduction. Lauder et al. (2013) [57] calculated that an one-off sequestration of one tonne of carbon offsets ongoing emissions by 0.9–1.05 kg methane per year. In this study, we explore the use of CDR to offset agricultural emissions of methane and nitrous oxide overshooting the RCP2.6 budget. Although previous studies address the substi- tutability between more or less ambitious reductions of (agricultural) methane emissions and allowable CO2 emissions in general [15, 56], these substitutions have never been explicitly investigated in the context of offsetting via CDR. Reliable estimates of CDR requirements are essential to correctly offset the additional agricultural methane and nitrous oxide emissions. PLOS ONE Offsetting future agricultural emissions with CO2 removal PLOS ONE | https://doi.org/10.1371/journal.pone.0247887 March 17, 2021 3 / 19