The environmental consequences of climate-driven agricultural frontiers

depends on species’ ability to occupy newly suitable areas. Species potentially impacted by frontiers are most numerous in Central America and the Northern Andes, with secondary concentrations in the Himalayas and highlands of New Guinea. Water quality impacts The potential impact of climate-driven agricultural frontiers on downstream water quality has may affect large numbers of people and their water infrastructure. The agricultural water qual- ity (AWQ) footprint of frontiers encompasses the homes of between 0.4–1.0 billion people (RCP 4.5, ensemble minimum and maximum) and 1.2–1.8 billion people (RCP 8.5, ensemble minimum and maximum), of whom 900 million-1.6 billion (RCP 8.5, ensemble minimum and maximum) live in areas in which more than half of the water supply is projected to be impacted (Table 4). Water quality changes in these downstream areas from fertilizer and bio- cide runoff may affect human health, ecosystem health, production of fisheries and the cost of water treatment. Table 4 shows that agricultural frontiers increase the amount of land potentially affected by changes in AWQ 9% to 16% (median 12%) compared to current impact (RCP 8.5, ensemble minimum and maximum). Given that some of this new farmland (in drylands) will not gener- ate significant runoff, under RCP 8.5 the land area with AWQ varies from a maximum addi- tional 7–12% (median 9%) with the maximum additional global population affected varying from 9–10% (median 9%). Elevated levels of AWQ (>50%), affecting 3–6% of additional land

surface (median 4%), impacting an additional 2–3% (median 2%) of the current global popula- tion. The additional AWQ per unit land area of new cropland varies between ensemble mem- bers and reflects the distribution of cropland in runoff generating areas vs not, as well as the downstream differential mixing of runoff from agricultural and non-agricultural land under different spatial frontier outcomes. Hydrologic infrastructure, including the global estate of reservoirs created by dams that are essential for urban water supply, irrigation and hydropower are also potentially affected by the AWQ footprint of agricultural frontiers. 6.4–8% (median 7.3%, RCP8.5) or 5.5–6.9% (median 6.3% RCP4.5) of global reservoirs would experience increased AWQ impacts as a result of agri- cultural frontiers and 2.0–3.8% (median 2.9%, RCP 8.5) or 1.7–3.1% (median 2.4% RCP4.5) of reservoirs would be exposed to elevated impacts (AWQ >50%). These are in addition to the 63.3% of reservoirs already with AWQ>0 under the current distribution of crop suitability (50.2% at >50% AWQ) (Table 4). Table 3. Environmental impacts of agricultural frontiers under both RCP8.5 and RCP4.5 2060–2080 climate pro- jections. Areas of significant biodiversity resources assessed are biodiversity hotspots; endemic bird areas (EBA); key biodiversity areas (KBA). Numbers presented for biodiversity resources are the median [range] number of areas that intersection with frontiers across all GCMs. Potential impacts on restricted range bird species are presented as the

median [range] number of species with modeled range intersection with frontiers in current and 2060–80 climate pro- jection. Modeled future ranges are assessed under an assumption of no-dispersal and a 10 km/decade dispersal rate. Global Conservation Priority/Ecosystem Service Potential Impact of Climate-Driven Agricultural Frontiers Important Biodiversity Areas RCP8.5 RCP4.5 Biodiversity Hotspots (n = 34) 19 [18–22] 19 [17–21] Endemic Bird Areas (n = 218) 48 [43–52] 49 [43–54] Key Biodiversity Areas (n = 11,824) 1590 [1361–1810] 1601 [1416–1788] Restricted Range Birds Current Range 409 [375–436] 409 [380–442] 2060–80—No Dispersal 385 [344–410] 228 [185–295] 2060–80—Full Dispersal 491 [452–525] 362 [294–432] https://doi.org/10.1371/journal.pone.0228305.t003 The environmental consequences of climate-driven agricultural frontiers PLOS ONE | https://doi.org/10.1371/journal.pone.0228305 February 12, 2020 9 / 19