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

possible CDR cost by $35/tCO2eq to highest possible cost by $235/tCO2eq). Since it is not pos- sible to reliably predict which technologies will prevail and what their exact cost in each year will be, we use in our analyses a static CDR price. This reflects the rationale [36, 39] that at the beginning higher-cost technologies will have prohibitive costs and limited use, while low-cost options such as afforestation and reforestation will be available at large scales and lower prices. In time, the potential for cheaper options will be exhausted and their price will increase, while technological options will profit from economies of scale and learning effects and their price will decrease as their potential increase [39]. We also consider the cost of CDR per unit of three agricultural products associated with methane and nitrous oxide emissions, multiplying the tax price per ton CO2-equivalent emis- sion by the aggregate methane and nitrous oxide emission intensity of the agricultural prod- ucts. This temporarily reintroduces GWP100 in the analysis, which incompletely accounts for equivalencies between emissions. Yet, real taxes are levied on products of which their disaggre- gated emissions are known. Thus, the use of GWP100 here is undone when calculating the tax per kilogram agricultural products. We perform the analysis for beef, milk, and rice, covering three different types of food (meat, dairy, and grains) closely linked to activities causing the largest share of greenhouse gases within agriculture (enteric fermentation, manure left on pas-

ture, rice cultivation [12, 68]). We use the reported greenhouse gas intensities from Table 2 (expressed in CO2 equivalent per kg product) and first update their value in kg per kg CO2- equivalent to reflect the IPCC AR4 GWP100 used in this study. In a second step, we multiply the greenhouse gas intensity (in kg CO2-eq/kg product) with the proposed tax (in $/kg CO2- eq), yielding a price increase (in $/kg product). Considering the entire data processing pipe- line, our approach thus undoes the usage of GWP100. Finally, we calculate the tax-induced mean increase in price relative to two different agricultural price scenarios, using the average retail prices globally (over 2017–2019 for beef and rice, and in 2017 for milk) and in the United States (over 2018–2020 for beef and milk, and 2017–2019 for rice) reported in Table 2. We exclusively consider price increases due to the policy, although agricultural commodities prices are expected to increase due to reduced land availability and growing global GDP even in the absence of the policy (in the SSP1-2.6 scenario) and because of climate change (e.g. due to water scarcity) [1, 3, 4, 74]. We do not consider the price elasticity of demand for the different agricultural products, or the potential for on-farm production changes, which ultimately relates these increases in price to reductions in agricultural emissions. A general equilibrium Table 2. Methane and nitrous oxide intensities and United Stated and global retail prices for beef, milk, and rice. Beef Milk Rice CH4 N2O CH4 N2O CH4 N2O

GHG intensity (kgCO2eq/kg) 20.33 13.44 1.41 0.83 1.05 0.45 Source Opio et al. (2013) [68] Opio et al. (2013) [68] Brodt et al. (2014) [69] Method Global supply-chain life-cycle assessment Global supply-chain life-cycle assessment Regional supply-chain life-cycle assessment GWP100 used � from IPCC AR4 25� 298� 25� 298� 25� 298� Retail price U.S. $12.50/kg beef $0.85/kg milk $1.58/kg rice Method Average 2018–2020 all beef products Average 2018–2020 Average 2017–2019 Source USDA, 2020 [70] USDA, 2020 [70] U.S. Bureau of Statistics, 2020 [71] Worldwide retail price $4.45/kg beef $1.61/kg milk $0.39/kg rice Method Global average 2017–2019 all beef products Average 2017 price of 14 countries Global average 2017–2019 four types of rice Source World Bank, 2020 [72] Export Action Global, 2018 [73] World Bank, 2020 [72] https://doi.org/10.1371/journal.pone.0247887.t002 PLOS ONE Offsetting future agricultural emissions with CO2 removal PLOS ONE | https://doi.org/10.1371/journal.pone.0247887 March 17, 2021 7 / 19