Per capita GDP represents the level of economic development.
According to the environmental Kuznets curve (EKC) hypothesis,
there is a non-linear relationship between per capita carbon
emissions and per capita income. In the early stage of economic
development, citizens tend to pay more attention to their income
than to the environment. However, as their income increases, citizens
may become more concerned with environmental quality. In
this paper, per capita GDP was used as an income indicator. At the
same time, the quadratic terms and cubic terms were also introduced
in the regression equation. The provincial GDP and population
data were obtained from the statistical yearbook. To ensure
comparability, the GDP data were converted into real terms using
1995 as the base year.