The environmental consequences of climate-driven agricultural frontiers

Uncertainty To account for the uncertainty of future climate projections, all impacts of climate driven agri- cultural frontiers were assessed on an individual GCM/RCP/time period basis and results are presented as ensembles across all climate projections. The choice of binary threshold is a possi- ble source of uncertainty, but in this analysis that uncertainty is constrained by choosing a threshold that is conservative from the perspective of frontiers. For instance, in EcoCrop a threshold of 20 (“very marginal to marginal”) includes areas that are possible but not optimal for cultivation [22–23]. The total area of frontiers is largely insensitive to adjustments to the choice of threshold across all methods used, because under a more permissive threshold the currently suitable area will expand, but there will be an accompanying expansion of frontiers poleward—and vice versa for a less permissive threshold. Uncertainty is more difficult to con- strain in precipitation-driven frontiers where there is high disagreement on sign of change in GCMs. This makes Sahelian and Australian precipitation-driven frontiers much more uncer- tain than other frontiers, as noted above. The greatest uncertainty is in actual cultivation of frontiers, as discussed below. Comparison of the modeled crop distributions for both current and future climates including the possible reduction of frontier areas due to soil constraints as defined by the union of GAEZ soil resource classifications are shown in S5–S8 Figs.

Table 4. Agricultural water quality (AWQ) impact of climate-driven agricultural frontiers under RCP 8.5 and RCP 4.5. Elevated AWQ is >50% of water supply with AWQ impacts. AWQ footprint (million km2) Elevated AWQ1 footprint (million km2) Current population affected by AWQ (billion) Current population affected by elevated AWQ1 (billion) Potential population affected by AWQ (billion) Potential population affected by elevated AWQ (billion) Footprint per land area of cropland Global dam estate affected by AWQ (%) Global dam estate affected by elevated AWQ1 (%) Frontiers RCP 8.5 2060–2080 Median 11.7 5.2 0.7 0.1 1.4 1.1 0.79 7.3 2.9 Frontiers RCP 8.5 2060–2080 Max 15.6 7.8 0.8 0.2 1.8 1.6 0.87 8 3.8 Frontiers RCP 8.5 2060–2080 Min 9.1 3.9 0.7 0.1 1.2 0.9 0.67 6.4 2 Frontiers RCP 4.5 2060–2080 Median 6.6 2.2 0.5 0.07 0.7 0.662 0.82 6.3 2.4 Frontiers RCP 4.5 2060–2080 Max 8.9 3.6 0.52 0.1 1.017 1.017 0.95 6.9 3.1 Frontiers RCP 4.5 2060–2080 Min 4.7 1.2 0.4 0.05 0.407 0.407 0.65 5.5 1.7 Global (reference period 1960– 90) 53.2 37.6 7.5 4.1 9.4 7.5 0.71 63.3 50.2 https://doi.org/10.1371/journal.pone.0228305.t004 The environmental consequences of climate-driven agricultural frontiers PLOS ONE | https://doi.org/10.1371/journal.pone.0228305 February 12, 2020 10 / 19