Abstract
Public debt can affect environmental quality in world countries through various mechanisms, including control of corruption and natural resource rent. In this regard, this study uses a threshold panel regression model and the load capacity factor (LCF) as an environmental sustainability assessment index to analyze the effect of public debt on the environment quality in 67 countries between the years 2002 and 2022 and according to the threshold effects of natural resource rent and control of corruption. The results show that the threshold level of the total natural resources rents as a share of the GDP is 12.385 percent and the threshold level of the control of corruption index is 0.882. In the model with the threshold variable of natural resource rent, the effect of public debts is insignificant in the first regime, but negative in the second regime. In the model with the threshold variable of control of corruption, the effect of public debts is negative in the first regime, but positive in the second regime. Based on this, it can be said that the natural resources rent and control of corruption play an important role in the type and intensity of public debt's impact on environmental sustainability. Based on other results, per capita energy consumption and population density have a negative and significant effect on LCF and the environmental hypothesis of load capacity curve (LCC) is also confirmed.
Introduction
The recent history emphasizes two distinguishing features of unsustainable development, stemming from global environmental degradation and from rising sovereign indebtedness.According to the Global Footprint Network (GFN) Annual Report (2022), the per capita level of ecological footprint (in global hectares) in the world in 2022 has exceeded its biological capacity by about 70%. This year, the per capita ecological footprint was about 1.51; while the amount of biological capacity per capita is estimated at 2.58, which shows the ecological deficit per capita at about 1.07. Also, more than 85% of the world's population lives in countries that are facing the issue of "environmental deficit" due to the emission of pollutants


 
exceeding the absorption capacity of nature. On the fiscal policy side, the post-1970s oil shocks era is characterized by persistent deficits, which fueled an increasing trend in the public debt-to-GDP ratios worldwide and resulted into historically-exceptional indebtedness nowadays.
A fundamental debate concerns the environmental and climate consequences of these large indebtedness levels. The pessimistic view in this regard believes that high public debt may hurt economic growth and damage the natural environment by hindering the implementation of environmental protection programs. This could undermine the ability of indebted economies to engage in sustainable energy transition. In contrast, the alternative view holds that public debt can enhance environmental quality by financing investments in low-carbon technologies, clean energy projects, or environmental research and development activities that primarily benefit future generations. Based on the combination of the above two views, another view (the third view) can be imagined. In this way, the effect of public debt on the quality of the environment depends on various factors such as the degree of institutional quality and the level of natural resources. Good institutions use government debt effectively to finance environmentally friendly projects and research funds to develop clean energy technologies such as renewable energy projects. In addition, debt repayment can force highly indebted economies to increase pressure on the natural environment and environmental degradation to raise public revenues.
 Based on the above explanations, the main purpose of this research is to empirically analyze the effect of public debt on the environmental sustainability in 67 countries between 2002 and 2022 and considering the threshold effects of natural resource rent and control of corruption (institutional quality index).
Methods and Material
This study builds the following threshold regression model based on Hansen’s threshold regression model and assumes the existence of a single threshold effect to specifically examine the threshold effects of natural resource rent, and control of corruption on public debt on environmental quality (Eq. (1) and Eq. (2)):
 




(1)


 
 
 
 




 




(2)


 
 
 
 




 
Where lnLCF represents natural logarithm of load capacity factor, which is equal to biocapacity (per capita) / ecological footprint (per capita). This indicator measures environmental quality. lnGDP (natural logarithm of real gross domestic product per capita, constant 2015 dollars), lnEC (natural logarithm of Primary energy consumption per capita; kWh/person), and lnPD (natural logarithm of population density; people per sq. km of land area) are control variables other than lnDebt that have an impact on the functioning of the LCF. lnDebt is the natural logarithm of public debt % of GDP of each country. NRR (total natural resource rents % of GDP) and COC (control of corruption) are the threshold variables. , , , , ,  are the coefficients. and  denote the threshold values.  is an individual effect, which means it is a constant influencing factor.  is a term for random interference. The pertinent variables are logarithmically treated to remove heteroscedasticity.
Results and Discussion
This study first examines the possibility of a threshold effect. The demonstrate that there is a single threshold for Debt to LCF, with thresholds of 12.385 and 0.882, and that there is neither a double nor a triple threshold when NRR and the control of corruption are the threshold variables. This demonstrates that there are two levels of differences and a threshold effect between Debt and environmental quality.In the model with the threshold variable of natural resource rent, the effect of public debts is insignificant in the first regime, but negative in the second regime. In the model with the threshold variable of control of corruption, the effect of public debts is negative in the first regime, but positive in the second regime. Based on other results, per capita energy consumption and population density have a negative and significant effect on LCF and the environmental hypothesis of load capacity curve (LCC) is also confirmed.
Conclusion
Based on the results, it can be said that the natural resources rent and control of corruption play an important role in the type and intensity of public debt's impact on environmental sustainability. Based on this, a serious effort to control corruption and apply anti-corruption policies in the fields where corruption leads to the reduction of financial resources and investment in clean energy and environment-friendly projects by diverting and wasting public debts, is suggested. Also, since public debt in countries with high levels of natural resources through the rapid extraction and sale of these resources leads to a decrease in environmental sustainability, public debt control and proper management of the ratio of debt to GDP is necessary in order to reduce the speed of natural resource extraction. In addition, it is necessary to allocate an optimal share of the rent resulting from the sale of natural resources to green funds in order to invest in appropriate and environmentally friendly technologies.