2.1. The IPAT model pioneers
Today, the protection of the environment has become a priority issue in the world. The existing literature reveals that most studies are interested in examining the key factors underlying the degradation of the quality of the environment using a very simple model called the "IPAT model" since the 1970s. The pioneers of this model are the Americans Paul Ehrlich and John Holdren (1971). This is why, the existing literature reveals that most studies have been interested in examining the key factors of the environmental quality degradation since the 1970s relying on the very simple "IPAT” model. This model takes into account three major descriptive impact factors of human populations, as well as their interactions and retroactions, to protect ecosystems and promote ecological transition not only in developed countries but also in developing countries.
The IPAT model can be used utilized to evaluate the influence of economic activity on the environment and the used amount of energy. The model puts into use a random effect regression to evaluate the environmental pressure brought on by demographic variables, wealth, and technological advancement. The model is can be expressed as follows: I = P.A.T where I is the human impact on the biosphere expressed as the product of components namely, P; that stands for population, A for affluence and T for technological aspects.
However, several recent works have conferred a stochastic content to the equation IPAT, while replacing T with a random term, allowing a statistical processing. To address this shortcoming the STIRPAT (Stochastic Impacts by Regression on Population (P), Affluence (A) and Technology (T)) model was suggested by Dietz and Rosa (1994).
It is most widely used by economists to assess the effect of population, wealth, technology and other factors on carbon dioxide (CO2) amounts. It is another stochastic reformulation of the IPAT model. The STIRPAT model makes the application of statistical tools to social research easy. Noteworthy, the model was further developed by York, Rosa and Dietz (2003). Thus, when the T component is considered as a random term, other variables, assumed to be explanatory of environmental impact, can be incorporated into the IPAT equation.
The specificity of the STIRPAT model can be expressed as follows:
$$I=a{P}^{b}{A}^{c}{T}^{d}e$$
With I denotes the environmental impact, P expresses the population value, A is the economic activity variable, T is the technologies used while e is the random term, b is the ecological elasticity value of P, c is the ecological elasticity value of A and d denotes the ecological elasticity value of T.
Using the extended STIRPAT model, the equations give particular importance to the demographic and activity variables. However, as allowed by the STIRPAT model, other potential explanatory variables may be examined, based on their statistical significance and their relevance to explaining the endogenous variable, i.e. environmental quality.
2.2. Empirical background
Over the previous few decades, a good amount of the literature has examined several key factors in the environmental quality degradation relying on the extended STIRPAT model.
2.2.1. The relationship between population and CO2 emissions
Previous theoretical and empirical contributions have extensively studied the relationship between population and CO2 emissions and achieved various results.
The results reached in Shi’s study (2001) show that the increase in CO2 emissions is caused by the rising energy consumption, emphasizing that there are no impacts from population or technology. Other studies, however, (Cole et al., 1997; Schmalensee et al., 1998) have shown that population and technology are among the key factors in CO2 emissions. Cramer (1998, 2002) and Cramer and Cheney (2000) examined the impact of population on air pollution in California and proved that population is associated with several sources of CO2 emissions.
In the context of the IPAT model, Dietz and Rosa (1997) and York, Rosa and Dietz (2003), focused on CO2 emissions and energy use and examined the roles played by population, average resource and energy consumption per individual (affluence) and the used technologies. The authors found a positive linear relationship between population and CO2 emissions.
Similarly, and relying on the context of a STIRPAT model, York et al., (2003) proved the existence of a positive impact of population on CO2 emissions in their study of a panel of 146 developed and developing countries.
However, using the STIRPAT model over the period 1975–2000 in countries with different income levels, Ying et al., (2006) show that the 15–64 age group of the population has a negative impact on CO2 emissions in high-income countries, but the effect is rather positive at other income levels.
Using an extended version of the STIRPAT model, Wang et al., (2013) examined the impact of several factors such as population, economic level, technology, urbanization, energy consumption structure and degree of foreign openness on environmental quality in their study. The achieved results show that population is the most important factor influencing CO2 emissions in Guangdong, China.
2.2.2. The relationship between economic growth and CO2 emissions
It should be recognized that Grossman and Krueger (1991) were pioneers in the study of the relationship between economic activity and the environment. Their study is based on the Kuznets environment curve, and described the relationship between a country's level of wealth (measured by GDP per capita) and its level of inequality.
The Kuznets environment curve can be referred to in the environmental field (Grossman and Krueger, 1991), where many health and environmental indicators deteriorate with economic growth. This has inspired a number of studies investigating whether economic growth is accompanied by environmental degradation. A review of the literature shows us that there are works that confirm the CKE hypothesis while others do not.
Among the studies that showed an inverse relationship between economic growth and environmental quality we can cite that of Ying et al., (2006). The authors used the STIRPAT model over the period 1975–2000 in countries with different income levels and their results showed a very high impact of GDP per capita on total CO2 emissions in low-income countries.
On the other hand, Shoufu et al., (2009) analyzed the effect of GDP per capita alongside with population, urbanization level, industrialization degree and energy intensity on environmental quality in China over the period 1978 to 2006. Their results show that GDP per capita has a negative impact on environmental quality. In addition, it proved that population, urbanization level, industrialization degree and energy intensity have negative effects on the environment.
Other studies show an inverse relationship between economic growth and environmental quality. In fact, Shafik's study (1994) shows that economic growth is associated with environmental improvements where there are generalized local charges and substantial benefits. But if the costs of environmental degradation are borne by others (the poor or other countries), there is little incentive to change damaging behavior.
Using a STIRPAT model, Yeh and Liao (2017) show that the relationship between GDP per capita and CO2 emission is inverse during the period 1990–2014 in Taiwan. This means that Taiwan has reached a stage of economic strength that allows it to reduce carbon emissions.
As for Bensbahou and Seyagh (2020), they examined whether economic growth affects environmental quality in Morocco for the period 1980–2018. The study used the ARDL method and considered CO2 emissions (metric tons per capita) as an indicator to measure the environmental status. Their results showed the existence of an inverted U-shaped relationship between GDP per capita and CO2 emissions. In other words, if CO2 emissions rise, GDP goes up until the inflection point, where CO2 emissions gradually fall, which reflects the importance of the environment.
2.2.3. The relationship between technology, energy consumption and CO2 emissions
Most of the existing studies in the literature reveal their unprecedented interest in examining the key factors in the environmental quality degradation. This obviously proves that environmental protection has become a priority issue worldwide.
The study by Ghazali and Ghulam (2019) examined the influence of different factors on environmental degradation for ten newly industrialized countries (NICs), applying the STIRPAT model for the period 1991–2013. The authors used the "dynamic common correlated estimator" (DCCE) to analyze the effect of technology alongside population, GDP per capita, labor productivity, trade openness and energy intensity among others on CO2 emissions. The results show that technology, population, GDP per capita and energy intensity are the major factors in the long-term degradation of environmental quality in the ten studied countries.
Similarly, a wealth of literature has explored the direction of causality between energy consumption and CO2 emissions. Most of these studies have shown that energy consumption has a considerable impact on the environment and is a source of climate change. Kraft and Kraft (1978) were the pioneers to show that there is a relationship between energy consumption and growth, which is one of the driving factors of environmental degradation (Grossman and Krueger, 1991; Jumbe, 2004; Ying et al., 2006 among others).
Using the ARDL method, the results of the study by Kivyiro and Arminen (2014) confirmed the Kuznets Curve Hypothesis (KCH) that energy consumption has a negative impact on environmental quality in the Democratic Republic of Congo, Kenya, and Zimbabwe over the period 1971–2009 in the long term.
Esso and Keho (2016) performed a study on a sample of 12 Sub-Saharan countries covering the period 1971–2010. Relying on annual data collected from the countries under study, they showed that energy consumption and economic growth negatively influence environmental quality in Benin, Ivory Coast, Nigeria, Senegal, South Africa and Togo over the long run.
In a more recent study, Chontanawat (2020) used the co-integration and causality method to examine the dynamic relationship between energy consumption, economic growth and CO2 emissions in ASEAN2 countries over the period 1971–2015. Their findings show that increasing energy consumption, which has a positive influence on economic growth, degrades the quality of the environment (CO2 emissions).
2.2.4. The relationship between waste recycling and CO2 emissions
Most of the studies empirical results provide useful information in terms of policy implications. Policies that necessarily aim to improve renewable energy consumption, should reduce energy intensity and encourage waste recycling. This would help cut down CO2 emissions, without significantly affecting economic growth. In fact, several studies have shown the significant contributions of waste recycling to environmental sustainability. Ayodele et al., (2018) showed that municipal solid waste recycling in some Nigerian cities contributed positively to the energy sector as well as many economic and environmental fields relying on a population model covering the 2017–2036 period. The environmental benefit of municipal solid waste recycling is determined based on the assessment of its greenhouse gas emission potentials. Similarly, Cudjoe et al., (2021) analyzed the energy (electricity) and environmental (GHG and air pollutant emissions) benefits of recycling steel, non-ferrous metals, plastic and paper waste from 2005 to 2017 in China. The results show that solid waste recycling saved 43.2% electricity and avoided a significant amount of GHG emissions and air pollutants. In addition, Razzaq et al., (2021) tested the effect of municipal solid waste (MSW) recycling on CO2 emissions and economic growth in the United States using quarterly data during the period 1990–2017 and relying on the ARDL method. The results show that a 1% increase in MSW recycling leads to 0.317% (0.157%) increase in growth in the long term (short term) and reduces CO2 emissions by 0.209% (0.087%) in the long term (short term). These results also mean that any MSW recycling-related policy generates significant changes in economic growth and carbon emissions.
In contrast, Bayar et al., (2021) examined the effects of municipal waste recycling and renewable energy on environmental sustainability indicated by CO2 emissions in EU member states during the period 2004 to 2017 and using cointegration and panel causality analyses. The authors find that the results are mixed. In other words, the analyses show a negative impact of recycling and the use of renewable energies on CO2 emissions, which varies from one country to another. In addition, Ramos-Meza et al., (2022) analyzed the effect of municipal solid waste recycling and other key variables on carbon emissions over the period 1975–2020 in China, using the ARDL "bounds testing approach" proposed by Pesaran et al., (2001). The results show a positive relationship between municipal solid waste recycling and CO2 emissions. This contribution means that the recycling process is inefficient due to the considerable municipal solid waste production, which calls for a sustainable method of municipal solid waste disposal and recycling in the country.
From our literature review, we can deduce that most studies are interested in identifying the key factors influencing CO2 emissions. These factors have considerable negative impacts on the environment and are a major cause of climate change. Accordingly, our study opted for an integrated analysis to examine the dynamic relationship between municipal solid waste recycling and CO2 emissions using the extended STIRPAT model in France.