3.2.1 Economic Development
On the regional scale, the PUE of 70 out of 113 countries had a significant linear or quadratic relationship with GDP per capita. Among these countries, 47 of them suggest the expected U shape relationship between PUE and GDP per capita (Table 4), though the turning points and slopes vary. These 47 countries accounted for over half of the global harvested area and total P in crop production, and over 80% of P fertilizer consumption at cropland in 2010 (61%, 70%, and 84%, respectively). They include major crop-producing and fertilizer-consuming countries such as the U.S., Brazil, China, and India.
EKC hypothesis was not rejected across the 113 countries when the independent variables only include the linear and quadratic terms of GDP per capita (Table S10). After considering country and time effects and adding more variables to the regression model, only the linear term has a significantly positive relationship with PUE (Table 4, Table S10, and Table S14). These results indicate synergy between global economic development and PUE (the second half of the EKC curve) between 1961-2014 in these 113 countries.
Note that the EKC hypothesis does not tell the future of a country or define the pathway that all countries should follow. Instead, EKC suggests potential reasons behind PUE patterns and informs political strategies to improve PUE. China has been experiencing an increasing GDP per capita and decreasing PUE (Fig. S3). According to the EKC hypothesis, this tradeoff may no longer continue, as China’s PUE is approaching a lower asymptote between 30% and 40% (Table 3). However, China’s PUE would not inevitably reach a turning point and then increase without any technological innovation or political interventions. Improved technologies and proper policies can help a country like China turn the corner on the EKC and reach a sustainable PUE in an even shorter time.