Figure 4. P surplus rate (kg P ha-1yr-1) by region. (a) P surplus rate (kg P ha-1 yr-1) estimated in 2010 (average of 2005-2014). (b) P surplus rate (kg P ha-1yr-1) projected in the 2050 BAU scenario. (c) P surplus rate (kg P ha-1 yr-1) projected in the 2050 MPA scenario. (d) P surplus rate (kg P ha-1 yr-1) projected in the 2050 HPA scenario. Grey area: missing data. Orange area: P surplus rate higher than the upper planetary boundary (6.9 kg P ha-1yr-1, rounded to 7 kg P ha-1yr-1). Blue and white areas: P surplus rate lower than the upper planetary boundary. Rates larger than 20 kg P ha-1 yr-1 are shown in red. Rates lower than -2 kg P ha-1 yr-1 are shown in blue.
Based on our estimation, the global planetary boundary of P surplus is at 4.5-9 Tg P yr-1. To keep the global P surplus under the boundary in 2050, we need to improve the global PUE from the current 60% to somewhere between 69% and 82%. Dividing the global P surplus planetary boundary (4.5-9 Tg P yr-1) by global total harvest area in 2010 (average of 2005-2014, \(1.3\times 10^{9}\) ha), we estimated the regional planetary boundary of P surplus rate at 3.5-6.9 kg P ha-1 yr-1. With this threshold, only in the HPA scenario or even more ambitious scenarios could the P surplus in major countries fall into this “safe” range (Fig. 4). In the BAU and MPA scenarios, 62 and 30 countries would still transgress the upper planetary boundary of the regional surplus rate (6.9 kg P ha-1 yr-1) in 2050. These countries occupied 46% and 19% of the harvested area in 2010 (average of 2005-2014), respectively.