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

Introduction To limit global warming well below 2˚C, mitigation of CO2 emissions alone will not be suffi- cient [1]. Currently roughly one-third of climate forcing is caused by non-CO2 climate pollut- ants [2] and agriculture causes a large fraction of these emissions [3]. Taken altogether, food systems contribute somewhere between 21–37% of total anthropogenic greenhouse gas emis- sions [4, 5]. Moreover, the relative importance of non-CO2 agricultural emissions will likely increase as CO2 emissions decline to net zero [1, 6–8]. Methane emissions, contributing around a quarter of the 2011 total anthropogenic radiative forcing (cf. Figure 8.15 IPCC AR5 WGI [3]), are largely from anthropogenic sources, with half of them from agriculture (e.g. due to livestock production and rice cultivation) [9–12]. Nitrous oxide, contributing to around 6% of the total greenhouse gases radiative forcing [3], also results primarily from agricultural activities [3], such as the use of inorganic fertilizer and the cultivation of nitrogen-fixing crops [13]. In the absence of transformative policies and technical mitigation options [11], agricultural non-CO2 emissions would reduce the remaining carbon budget so as to result in infeasible emission reduction targets for CO2 [1, 6–8, 14, 15]. Although agricultural productivity shows signs of decoupling from its emissions [16], an ever-growing demand for food production has caused an increase in the absolute agricultural-sector emissions over recent decades [17].

Under business-as-usual scenarios, this growth will continue due to dietary changes linked with increased income per capita, and by 2050 agricultural emissions would be 30–40% higher than today [4, 6, 11, 13, 18]. By contrast, all IPCC scenarios for the agricultural sector that limit warming below 2˚C contain non-CO2 emissions declining by 11–30% relative to 2010 already by 2030 [1, 19]. The IPCC scenarios achieve the needed agricultural non-CO2 emission reductions (1–2 GtCO2-equivalent per year (GtCO2-eq/yr) already by 2030 [1, 11, 19]) through improved agricultural management practices and dietary shifts away from emissions-intensive livestock products [1]. These rates of emission reductions are within the estimates of techni- cally feasible supply-side (1.6–4.6 GtCO2-eq/yr by 2030 [3]) and demand-side (1–8 GtCO2-eq by 2050 [20–22]) mitigation potentials. Additional mitigation could result from novel technol- ogies (e.g. novel plant-based and synthetic proteins, methane inhibitors and vaccines in live- stock, and nitrification inhibitors) [1, 4], although these are sufficiently immature to be excluded from the IPCC scenarios. Albeit technically feasible, curbing non-CO2 agricultural emissions faces several challenges. Plausible agricultural development pathways with mitigation co-benefits can contribute only around 0.2–0.4 GtCO2-eq of avoided emissions by 2030, just 10–20% of what needed to limit warming well below 2˚C [11]. For the livestock sector, it is estimated that only around 10% of

the technically possible mitigation might be economically viable at abatement costs of $50/ tCO2eq [22]. Large up-front investments would be needed to enable technological supply-side mitigation [23], should it prove difficult to fully exploit the large potential of demand-side mit- igation, such as dietary shifts. Although reductions in meat consumption alone could signifi- cantly decrease agricultural emissions, the current trend is the opposite of this [24]. Due to the cultural, affective, and behavioural value of food, people’s willingness to change meat con- sumption behaviour is low [25–27], and policies would be needed to incentivize dietary shifts. Policies to reduce the emission footprint of diets could include the pricing of emissions. Since many food products have a low price elasticity of demand and limited options to reduce emissions through low-cost changes in farm systems, large increases in prices would be neces- sary to achieve modest decreases in emissions [22, 28], exposing this kind of policies to socio- political resistance [24, 29]. Furthermore, applying a globally uniform price to agricultural emissions could lead to trade-offs between ambitious climate targets and food security [30], PLOS ONE Offsetting future agricultural emissions with CO2 removal PLOS ONE | https://doi.org/10.1371/journal.pone.0247887 March 17, 2021 2 / 19 Uniscientia Foundation. The funders had no role in study design data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared

that no competing interests exist.