The environmental consequences of climate-driven agricultural frontiers

The release of carbon following tilling from previously untilled soils is believed to occur rapidly and estimates suggest that 25–40% of total soil carbon is released within five years of plowing [54]. Therefore, an upper bound estimate of the total amount of carbon that might be released from the cultivation of climate-driven frontiers would be on the order of 177 GtC, which is equivalent to 119 years of current CO2 emissions of the United States [55]. The actual area affected would be smaller than the frontier due to economic and physical factors, but emissions might be greater because many of the potentially affected soils are peat, which may degrade when disturbed, releasing more and deeper carbon. In either event, the magnitude of the potential release indicates that policies directed at constraining development of these areas are vitally important. From a global perspective, 177 GtC is more than two-thirds of the 263 GtC within which total future emissions must be constrained to limit global mean temperature increase to the internationally agreed Paris agreement target of 2˚C global mean temperature increase above pre-industrial levels [56]. One way to address the challenge posed by cultivation of frontiers is through promoting agricultural management practices that conserve soil-bound carbon. In particular, policies that incentivize leaving peat soils intact and promoting conservation tillage could significantly reduce the quantity of carbon released and slow the speed at which it is released [57–58]. Thus,

while specific estimates as to the speed or extent to which these carbon sources might affect the Table 2. Accounting of soil organic carbon in top 1m in areas of agricultural frontiers and resulting potential carbon emissions under RCP 8.5 and RCP 4.5. Rows with grey shading apply GAEZ general soil suitability constraints and soil requirements for each crop to the climatically suitable frontier areas. Landcover classification Soil organic carbon stock (GT) 1m Low-end IPCC method estimate C release (GT) [forest 25%; grass/ shrubland 10%; wetlands 20%] High-end IPCC method estimate C release (GT) [forest 25%; grass/ shrubland 10%; wetlands 110%] 1m estimate C release (GT) [all land classes 25%] 1m estimate C release (GT) [all land classes 40%] Forests (RCP 8.5) 267.5 [192.6– 355.4] 66.9 [48.1–88.9] 66.9 [48.1–88.9] 66.9 [48.1–88.9] 107.4 [77.0–142.2] 243.3 [174.6– 328.6] 60.8 [43.7–82.2] 60.8 [43.7–82.2] 60.8 [43.7–82.2] 97.3 [69.9–131.5] Shrubland/ Grassland (RCP 8.5) 286.6 [133.6– 564.0] 28.7 [13.4–56.4] 28.7 [13.4–56.4] 71.6 [33.4–141.0] 114.6 [53.5–225.6] 265.9 [118.7– 532.5] 26.6 [11.9–53.3] 26.6 [11.9–53.3] 66.5 [29.7–133.1] 106.4 [47.5–213.0] Permanent wetlands (RCP 8.5) 73.6 [56.6–74.3] 14.7 [11.3–14.9] 81.0 [62.3–81.7] 18.4 [14.2–18.6] 29.4 [22.7–29.7] 54.2 [54.8–38.5] 10.8 [7.7–11.0] 60.3 [42.3–59.6] 13.5 [9.6–13.7] 21.7 [15.4–21.9] Total soil organic carbon (RCP 8.5) 632.4 [400.0– 991.5] 110.2 [72.8–160.1] 176.5 [123.8–226.9] 158.1 [100.0–247.9] 251.1 [153.1–397.5] 569.2 [347.6– 914.0]

98.2 [63.2–146.4] 147.0 [97.8–195.7] 142.3 [86.9–228.5] 225.3 [132.7–366.4] Forests (RCP 4.5) 271.9 [197.4– 353.8] 68.0 [49.3–88.4] 68.0 [49.3–88.4] 68.0 [49.3–88.4] 109.8 [78.9–141.5] 248.3 [178.9– 326.7] 62.1 [44.7–81.7] 62.1 [44.7–81.7] 62.1 [44.7–81.7] 99.3 [71.5–130.7] Shrubland/ Grassland (RCP 4.5) 196.5 [77.2– 434.9] 19.7 [7.7–43.5] 19.7 [7.7–43.5] 49.1 [19.3–108.7] 78.6 [30.9–174.0] 178.1 [67.6– 401.3] 17.8 [6.8–40.1] 17.8 [6.8–40.1] 44.5 [16.9–100.3] 71.2 [27.1–160.5] Permanent wetlands (RCP 4.5) 71.5 [39.6–74.3] 14.3 [7.9–14.9] 78.6 [43.6–81.7] 17.9 [9.9–18.6] 28.6 15.9–29.7] 52.4 [26.2–54.8] 10.5 [5.2–11.0] 57.7 [28.9–60.3] 13.1 [6.6–13.7] 21.0 [10.5–21.9] Total soil organic carbon (RCP 4.5) 539.9 [314.2– 862.9] 101.9 [65.0–146.8] 166.2 [100.7–213.6] 135.0 [78.5–215.7] 215.9 [125.7–345.2] 470.8 [272.8– 771.3] 90.4 [56.7–132.8] 137.5 [80.3–182.1] 119.7 [68.2–195.7] 191.5 [109.1–313.1] https://doi.org/10.1371/journal.pone.0228305.t002 The environmental consequences of climate-driven agricultural frontiers PLOS ONE | https://doi.org/10.1371/journal.pone.0228305 February 12, 2020 7 / 19