The estimated results of the PVAR model for high-income countries are shown in Table 8. Firstly, in the lagging period, economic indicators (FDI, trade openness, and economic growth), renewable energy consumption, human development, and CO₂ emissions are vulnerable to its own impact at the 10% and 1% significance levels, but industrialization does not have obvious effect on itself Table 5.

Secondly, from the impact of economic indicators on CO₂ emissions, it can be seen that economic growth and industrialization have positive effects on CO₂ emissions at the 1% and 10% significance levels, and vice versa, supporting the feedback hypothesis (Banday and Aneja, 2020). However, CO₂ emissions have a significant negative impact on FDI at the 10% significance level, and the positive impact of CO₂ emissions is much greater than the negative impact of economic indicators, which shows that some high-emission industries in high-income countries are being replaced by low-carbon industries under the dual effect of energy saving and emission reduction. In addition, economic growth has a positive impact on industrialization, FDI, and trade openness at the 1% significance level. This indicates that economic growth not only attracts large inflows of FDI and the improvement of industrialization level for those countries, but also has a positive impact on technological innovation, employment and income. Moreover, it plays an important role in improving productivity. However, industrialization has a negative impact on FDI and trade openness at the 5 and 10% significance levels, and vice versa. This result implies that those high-income countries have increased awareness of environmental pollution and quality issues in life. This result is supported by the studies of Mongo et al. (2021) (Mongo et al., 2021), Eller et al. (2005) (Eller et al., 2005) and Rahman (2015) (Rahman, 2015). Conversely, when CO₂ emissions increase by 1%, industrialization and FDI will also increase, but the increase relative to economic growth is almost insignificant, especially at the 10% significance level, which is statistically weak. This is consistent with the results of Koengkan (2019) (Koengkan, 2019) and Soukiazis et al. (2019) (Soukiazis et al., 2019). They believe that the increase in CO₂ emissions is positively correlated with fossil energy consumption. As expected, the economic growth of high-income countries promotes the increase of CO₂ emissions.

Thirdly, from the impact of renewable energy consumption on CO₂ emissions and economic indicators, it can be seen that renewable energy consumption has no significant impact on CO2 emissions, industrialization, trade openness, and FDI in high-income countries. This supports the neutral hypothesis. This means that any renewable energy consumption policy can be adopted independently of economic growth. Chiu et al. (2009) (Chiu and Chang, 2009) suggest that this result may be due to the fact that those countries have not yet reached the threshold point where renewable energy consumption starts to significantly reduce CO₂ emissions. It is in line with the conclusion of a study by Amer (2020) (Amer, 2020), which believes that only when renewable energy supply accounts for about 8.3889% of the total energy supply can it have an impact on reducing CO₂ emissions. Thus, according to the BP Statistical Review of World Energy (2020), the renewable energy consumption as a percentage of total energy consumption in some selected countries are: Saudi Arabia 0.144%, Singapore 0.239%, United Arab Emirates 0.771%, Hungary 4.017%, Luxembourg 4.061%, Belgium 6.937%, United States 8.912%, Japan 9.313%, France 11.733%, United Kingdom 14.45%, Germany 17.485%, Canada 27.638%, etc. However, according to Menyah et al. (2010) (Menyah and Wolde-Rufael, 2010) and Bilan et al. (2019) (Bilan et al., 2019), we argue that in the case of global warming, these countries must reduce the share of fossil fuel consumption in their productive lives, while strengthening their support policies to promote a faster development of the renewable energy sector. In addition, economic growth has a negative impact on renewable energy consumption at the 1% significance level, supporting the protection hypothesis, which means that conservative policies on renewable energy consumption will not have a negative impact on economic activities. This finding is similar to existing energy literature such as Destek et al. (2017) (Destek and Aslan, 2017) for the case of Emerging economies and Marques et al. (2012) (Marques and Fuinhas, 2012) for the case of Europe countries.

Fourth, from the impact of HDI on economic indicators, it can be seen that HDI has a negative impact on industrialization at the 10% significance level, while industrialization has a positive impact on HDI at the 5% significance level, but HDI has a negative impact. The positive impact is greater than the positive impact of industrialization, which shows that to some extent HDI has suppressed the decline in the unemployment rate in high-income countries. In addition, trade openness has a significant negative impact on HDI at the 1% significance level. This result is supported by the research of Wang et al. (2018) (Wang et al., 2018) and Khan et al. (2019) (Khan et al., 2019) argue that trade openness will lead to a decline in HDI. Conversely, the increase in HDI will inhibit more external economic activities. However, Amer (2020) (Amer, 2020) argues that higher levels of HDI helps stimulate economic activity in the outside world, and that as standards of living increase, high-income countries can have the opportunity to consume different kinds of goods and services that are not available domestically or are produced relatively cheaply.

Finally, there are not many studies on the impact of HDI on CO₂ emissions in previous literature, and there are relatively few empirical studies on this relationship. In summary, HDI has a significant negative impact on CO₂ emissions at the 10% significance level, but this negative impact is far greater than the positive impact of CO₂ emissions at the 1% significance level (Amer, 2020). Therefore, the positive impact of CO₂ emissions on HDI is negligible. With the high-quality economic development of high-income countries, to a certain extent HDI has led to a reduction in CO₂ emissions, which also reflects those high-income countries are increasingly aware of the important role of high-quality human development in order to achieve the expectations of reducing CO₂ emissions. However, this is contrary to the findings of Soukiazis et al. (2019) (Soukiazis et al., 2019) for OECD countries, who concluded that CO₂ emissions have a negative impact on HDI and do not have any statistical correlation.

Table 9 shows the estimation results of the PVAR model for upper-middle-income countries. Firstly, in the lagging period, economic indicators (industrialization, FDI, trade openness, and economic growth), renewable energy consumption, and CO₂ emissions are vulnerable to its own influence at the 1% significance level, but HDI is not significant to itself.

Secondly, by analyzing the impact of renewable energy consumption and economic indicators on CO₂ emissions, it can be seen that CO₂ emissions have a negative impact on industrialization and trade openness at the 1% significance level (one-way causality), and the impact coefficients are all over 0.03. This shows that there is an inverted U-shaped relationship between industrialization, trade openness, and CO₂ emissions in upper-middle-income countries, and supports the EKC hypothesis that CO₂ emissions initially increase with the economic development of upper-middle-income countries until they reach a stable point and then decline. In the case of China, studies such as Jalil et al. (2011) (Jalil and Feridun, 2011), Riti et al. (2017) (Riti et al., 2017) and Hao et al. (2021) (Hao and Cho, 2021) also confirm the inverted U-shaped relationship between income and environment performance for other pollutants. Surprisingly, FDI has a negative impact on industrialization at the 5% significance level, which shows that FDI has not only failed to promote the growth of the industrial sector in upper-middle income countries, but has hindered the economic growth of the industrial sector. At the same time, FDI has a positive impact on trade openness at the 1% significance level, which shows that the large inflow of FDI has greatly promoted the trade exports of those countries, and the trade exports of those countries have also indirectly hindered the development of industrialization and vice versa. This may be due to the extensive economic development of upper-middle-income countries in recent years, which has prompted a large inflow of FDI, which will lead to the continuous deterioration of the environment in those countries. Therefore, investors from these countries pay more attention to environmental regulations and clean technologies in terms of FDI inflows, thereby improving energy efficiency and reducing CO₂ emissions (Zhang and Zhou, 2016). In addition, there is no statistical impact between economic growth, FDI, renewable energy consumption, and CO₂ emissions in upper-middle-income countries, and vice versa. Therefore, one may conclude that the EKC hypothesis and the PHH are invalid for these countries.

Thirdly, from the impact of HDI on economic indicators, it can be seen that there is a significant impact relationship (two-way causality) between HDI and FDI. From the perspective of the impact coefficient, the positive impact of HDI on FDI (9.633) is far greater than the negative impact of FDI on it (−0.002), which indicates that upper-middle-income countries have begun to pay attention to environmental pollution and quality issues in life, and are raising their requirements when attracting foreign direct investment projects, especially in environmental regulations. These results were supported by studies undertaken in relation to China by Hung (2021) (Hung, 2021) and in BRICS countries by Wang et al. (2020) (Wang et al., 2020). However, this result was contrary to those of Reiter et al. (2010) (Reiter and Steensma, 2010), who pointed out the positive relationship between foreign direct investment and HDI for upper middle-income countries, which supports the feedback hypothesis. In addition, economic growth has a positive impact on HDI at the 1% significance level, and its impact coefficient is positive, which means that economic growth promotes HDI in the long run. The similar findings are found in the studies of Niu et al. (2013) (Niu et al., 2013), and Sasmaz et al. (2020) (Sasmaz et al., 2020). However, this finding differs with the conclusions of Adekoya et al. (2021) (Adekoya et al., 2021), Wang et al. (2018) (Wang et al., 2018) and Mustafa et al. (2017) (Mustafa et al., 2017). With the improvement of the level of economic development, upper-middle-income countries pay more attention to environmental pollution and quality issues in life, so that they can better obtain social services such as medical treatment and education and improve the quality of life of national. Meanwhile, upper middle-income countries are the middle force of global economic development, and FDI inflows play an important role in economic development, increasing productivity, creating jobs and income, so FDI inflows contribute to the human development of these countries.

Finally, from the impact of HDI on CO₂ emissions and renewable energy consumption, it can be seen that HDI and CO₂ emissions are positively correlated at the 1% and 5% significance levels, which supports the feedback hypothesis. This shows that CO₂ emissions are a strong and positive determinant of human development indicators, and on the contrary, CO₂ emissions can help increase HDI. This result was supported by studies undertaken in relation to 126 countries by Adekoya et al. (2021) (Adekoya et al., 2021). However, studies like Farhani et al. (2014) (Farhani and Shahbaz, 2014) and Chen et al. (2019) (Chen et al., 2019) have also disproved the claim of positive relationship between human development and CO₂ emissions. In addition, HDI has a negative impact on renewable energy consumption at the 5% significance level, and renewable energy consumption also has a negative impact on HDI, but it is not statistically significant. As expected, the increase in HDI levels has hindered the development of renewable energy in those countries. Our findings do not support the papers of Amer (2020) (Amer, 2020), Pîrlogea (2012) (Pîrlogea, 2012), and Ergun et al. (2019) (Ergun et al., 2019), who put forwarded the positive association between HDI and renewable energy consumption but agree with Ouedraogo (2013) (Ouedraogo, 2013). For example, in the newly industrialized countries (NIC’s), energy consumption sources are being considered as inputs in the production process given that these countries are considering them as part of the industrialization process. Based on this, we take the example of China, which has become the world’s largest energy consumer in the last decade or so, but the share of renewable energy is still relatively low, and it is not enough to support sustainable economic and environmental development. Therefore, in order to improve living standards and protect the global environment, energy needs to be used more efficiently and clean and reliable energy supplies need to be sought, and green growth must play a key role.

The estimated results of the PVAR model for lower-middle-income countries are shown in Table 10. First, in the lagging period, economic indicators (industrialization, FDI and trade openness), renewable energy consumption and CO₂ emissions are easily affected by themselves at the 10, 5, and 1% significance levels, but economic growth does not have an obvious effect on itself.

Secondly, from the impact of economic indicators on CO₂ emissions, it can be seen that economic growth has a positive impact on CO₂ emissions at the 1% significance level, which supports the feedback hypothesis. This shows that economic growth is the main reason for the increase in CO₂ emissions in lower-middle-income countries. At the same time, FDI promotes CO₂ emissions at the 1% significance level and supports the protection hypothesis. However, compared with economic growth, the correlation between FDI and CO₂ emissions is stronger. In addition, it is also found that industrialization has a positive impact on FDI, and there is a two-way causal relationship between trade openness, FDI and industrialization. This shows that FDI provides necessary resources for the industrialization and economic growth of lower-middle-income countries. However, in order to promote economic growth in lower-middle-income countries, it is necessary to reduce environmental standards to attract FDI, which will attract high-polluting industries and backward technologies in developed countries. As a result, CO₂ emissions in those countries have risen and environmental pollution has increased. That is, these countries need to improve their environmental standards in order to effectively attract FDI without negatively impacting on trade activities and industrialization development, thus increasing their technological requirements, for example, in highly polluting industries. Therefore, one may conclude that there is some evidence for the PHH in lower middle-income countries. These results were supported by studies undertaken in BRICS countries by He et al. (2020) (He et al., 2020) and in relation to 54 countries by Omri et al. (2014) (Omri et al., 2014).

Thirdly, from the impact of CO₂ emissions and economic indicators on renewable energy consumption, it can be seen that economic growth and FDI have a positive impact at the 10% and 5% significance levels, which supports the growth hypothesis. However, at the 1% significance level, CO₂ emissions have a negative impact on renewable energy consumption. This means that lower-middle-income countries have gradually realized that it is not advisable to attract FDI to promote their own economic growth by lowering environmental regulations. Therefore, the rational and effective use of renewable energy is the most effective way to reduce CO₂ emissions without affecting their own economic growth. More interestingly, it is also found that although renewable energy consumption has a negative impact on CO₂ emissions, it is not significant. This shows that despite the continuous growth of lower-middle-income economies in recent years, although the average consumption of renewable energy in their energy supply structure accounts for 5.6% of the total primary energy consumption, those countries have failed to have a favorable impact on environmental quality. Thus, lower middle-income countries need to advance their technology requirements in order to use energy efficiently without negatively affecting economic development (Amer, 2020). These results were similar to Charfeddine et al. (2019) (Charfeddine and Kahia, 2019) who found, for lower middle-income countries, that renewable energy consumption does not contribute to reductions in CO₂ emissions in the short run.

Fourthly, from the impact of economic indicators on HDI, it can be seen that economic growth and industrialization have a positive impact on HDI at the 1% and 10% significance levels, and positive growth has a positive impact on industrialization at the 1% significance level, which supports the growth hypothesis. This result is not surprising. For lower-middle-income countries, industrialization plays an important role in economic growth, job creation, productivity increase, and income generation. Therefore, the process of industrialization helps promote human development in lower middle-income countries. This result was supported by studies undertaken in relation to BRICS countries by Wang et al. (2020) (Wang et al., 2020) and in 90 countries over the period 1990–2014 by Tran et al. (2019) (Tran et al., 2019). However, FDI has a negative impact on HDI at the 1% significance level, which means that foreign direct investment reduces the level of human development in lower-middle-income countries. The most direct reason is that lower-middle-income countries have reduced environmental standards to attract FDI and provide key resources for their economic growth and industrialization, thereby ignoring environmental pollution. Therefore, FDI leads to environmental degradation, which seriously affects the health and well-being and life quality of people in those countries. Our findings do not support the papers of Wang et al. (2020) (Wang et al., 2020) and Tran et al. (2019) (Tran et al., 2019), who put forward the positive association between human development and foreign direct investment but agree with Khan et al. (2019) (Khan et al., 2019), Mustafa et al. (2017) (Mustafa et al., 2017), and Gorus et al. (2019) (Gorus and Aslan, 2019).

Finally, from the impact of HDI on CO₂ emissions, it can be seen that HDI has a positive impact on CO₂ emissions at a significance level of 10%, which supports the growth hypothesis. Although CO₂ emissions also have a positive impact on HDI, it is not significant. Obviously, this situation is not necessarily wrong, because this study only focuses on the meaning of the indicators. By analyzing this situation, it follows the point of view put forward by Pîrlogea (2012) (Pîrlogea, 2012), which believes that according to HDI, global CO₂ emissions are generally divided into CO₂ emissions generated by the economic development of developed countries with HDI higher than 0.8 and the CO₂ emissions developing countries with HDI lower than 0.8 need to develop. Obviously, in the process of rapid economic development, the CO₂ emission index of lower-middle-income countries has reached a relatively high level. But once it enters the category of developed countries, according to the strategy aimed at reducing emissions, the CO₂ emission level will begin to decline. Therefore, for lower-middle-income countries, the speed of economic development and human activities are the main reasons for the current increase in CO₂ emissions (Adekoya et al., 2021).

The estimated results of the PVAR model for low-income countries are shown in Table 11. First, in the lagging period, industrialization, FDI, and trade openness are susceptible to their own influence at the 1% and 10% significance levels, and the sign of the influence coefficient is positive. This means that more development in low-income countries is through FDI, industrialization and trade to bring more progress and success, thereby enhancing the overall economic strength and HDI of low-income countries. However, CO₂ emissions have a negative impact on themselves at the 1% significance level, with an impact coefficient of −0.245. This indicates that the consumption of renewable energy has little impact on the reduction of CO₂ emissions in low-income countries. This may be because most of the renewable energy consumed by economic development and human activities in low-income countries comes from traditional biomass, rather than clean modern renewable energy. Therefore, with the high demand for energy in low-income countries, since most of the energy supply comes from non-renewable resources, pollution levels are also increasing. These results were similar to Hasnisah, et al. (2019) (HasnisahAzlina et al., 2019) and Amer (2020) (Amer, 2020), who found that renewable energy consumption effect is insignificant in contributing to less pollution regarding the CO₂ emissions in selected 13 Asian countries.

Secondly, from the impact of renewable energy consumption on CO₂ emissions, it can be seen that renewable energy consumption has a negative impact on CO₂ emissions at the 10% significance level, with an impact coefficient of −0.197. However, CO₂ emissions have a positive impact on renewable energy consumption at the 1% significance level, with an impact coefficient of 0.149. This may be because that compared with relatively high-income countries, low-income countries have relatively low CO₂ emission coefficients, which makes renewable energy consumption more significant in reducing CO₂ emissions, but this situation will not last long. This result was similar to Bildirici et al. (2017) (Bildirici and Gökmenoğlu, 2017) and Ummalla et al. (2019) (Ummalla et al., 2019) who found that the marginal impact of CO2 emissions on economic growth is higher at the higher quantiles of income. Thus, in order to achieve the goal of high economic growth, those countries will rely more on fossil fuels, resulting in high carbon dioxide emissions that seriously affect climate change (Belaïd and Zrelli, 2019).

Thirdly, economic growth has a positive impact on HDI, but the impact is weak at the 10% significance level. This result shows that low-income countries are in the initial stage of economic development and economic transformation has not yet been completed. As the economy becomes more developed, the HDI of those countries will become higher, which also means that the economic growth of low-income countries will help improve the living standards of people, who can obtain better medical and educational services (Wang et al., 2020). However, this will inevitably lead to the unequal distribution of social income, social welfare and living resources in low-income countries, and will have a serious negative impact on the lives of the poor living in rural areas.

Finally, for low-income countries, HDI and industrialization are obviously the main determinants of CO₂ emission levels, but they are not statistically significant. Therefore, one may conclude that the EKC hypothesis and the PHH are invalid for low-income countries. This result was supported by studies undertaken in relation to MENA countries by Gorus et al. (2019) (Gorus and Aslan, 2019).

Figure 1 shows how CO2 emissions in high-income countries respond to standard deviation shock. For CO₂ emissions in high-income countries, the impact of renewable energy consumption, FDI, and trade openness on CO₂ emissions is not statistically significant in the previous PVAR estimates, then the interpretation of the impulse response function to one standard deviation shock on FDI should be considered carefully. The impulse response function shows that a standard deviation impact of economic growth and industrialization has a positive impact on CO₂ emissions, which will first rise and then fall, and reach the maximum value in the second year, with the impacts being 0.007 and 0.003, respectively. However, as time increases, the positive impact gradually decreases and approaches or equals zero after 5 years. However, one standard deviation impact of renewable energy consumption, FDI and trade openness has always had a negative impact on CO₂ emissions, and will be close to or equal to zero after 4 years. In addition, the standard deviation impact of HDI has a positive impact on CO₂ emissions from the first year to the second year, and then gradually decreases until the third year is negative and stabilizes, and its impact has always been maintained at −0.0007. Overall, the impulse response results are basically consistent with the PVAR model estimation results, but the opposite results have appeared in industrialization. The reason for this result may be that these countries have not fully completed the transition from high-energy-consumption and high-emission industries to low-carbon green industries. This result is supported by the studies of Mahmood et al. (2020) (Mahmood et al., 2020) in Saudi Arabia, which believes that there is an asymmetric linear relationship between industrialization and CO₂ emissions. As a country’s degree of industrialization increases, the possibility of using fossil fuel resources will increase, leading to many types of environmental degradation (Koengkan, 2019; Mongo et al., 2021). Thus, the increasing industrialization has larger environmental effect than decreasing industrialization.

Figure 2 shows the response of CO₂ emissions in upper-middle-income countries to standard deviation shock. Under a standard deviation shock, the impulse response of CO₂ emissions to HDI in upper-middle-income countries is basically consistent with the estimated results of the PVAR model. From the perspective of the response path of CO₂ emissions impulse emission, a standard deviation shock of economic growth, renewable energy consumption, HDI, industrialization and trade openness have a positive impact on CO₂ emissions, of which industrialization has a continuously increasing positive impact. This shows that economic growth, renewable energy consumption, HDI, industrialization, and trade openness have all contributed to the increase of CO₂ emissions in upper-middle-income countries. In the long run, the environment is deteriorating as the economy continues to develop, but CO₂ emissions will decrease after reaching a certain level (Kumari et al., 2021). However, this impact will not last long. With the continuous improvement of the industrialization level of these countries, CO₂ emissions begin to rise, making the environment deteriorate again. This finding is similar to existing energy literature such as Mahmood et al. (2020) (Mahmood et al., 2020) for the case of Saudi Arabia, and Li et al. (2019) (Li et al., 2019) for the case of China. In addition, the standard deviation impact of FDI has a negative impact on CO₂ emissions, and it stabilizes after 10 years, with an impact of −0.005. This shows that investors tend to abide by environmental regulations and international standards when entering these countries, so FDI inflows help the transfer of clean technology, thereby improving energy efficiency and reducing greenhouse gas emissions. This result is consistent with research of Zhang et al. (2016) (Zhang and Zhou, 2016) and Ghazouani (2021) (Ghazouani, 2021), which believes that these countries are technically more likely to obtain more environmentally friendly technologies from developed countries and conduct business in an environmentally friendly manner, supporting the pollution halo hypothesis.

Figure 3 shows the response of CO₂ emissions in lower-middle-income countries to standard deviation shock. It can be seen from Figure 3 that the impact path of economic indicators, renewable energy consumption and human development in lower-middle-income countries on CO₂ emissions is basically consistent with the previous PVAR model estimation results. As previously expected, HDI, FDI, industrialization and economic growth are the main reasons for the increase in CO₂ emissions in these countries. Compared with other influential factors, HDI has the greatest impact on CO₂ emissions, while the impact of industrialization fluctuates greatly. For example, for underdeveloped countries, FDI inflows promote the improvement of industrialization, provide necessary resources and advanced technology for the economic development of these countries, and play an important role in economic growth, job creation, income creation and productivity improvement. Therefore, we identify the positive effects of HDI, FDI, industrialization and economic growth on CO₂ emissions, which was consistent with the findings of Sinha et al. (2016) (Sinha and Sen, 2016) and Khan et al. (2019) (Khan et al., 2019). However, studies like Zaman et al. (2016) (Zaman et al., 2016) have also disproved the claim of positive relationship between human development and CO2 emissions in lower middle-income countries. In addition, it has also noticed that renewable energy consumption and trade openness have a continuous negative impact on CO₂ emissions, but the reduction of CO₂ emissions by renewable energy consumption is minimal, which is also in line with the current basic status quo of economic development in lower-middle-income countries. For example, most of Pakistan’s population mainly relies on agriculture, but the government’s excessive reliance on industrial development has led to a sharp deterioration in the environment, prompting trade openness to curb the increase in CO₂ emissions and also hindering human development, is in line with Wang et al. (2018) (Wang et al., 2018), Khan et al. (2021) (Khan et al., 2021) and Belaïd et al. (2019) (Belaïd and Zrelli, 2019). However, this study also confirms the inverted-U shaped relationship between trade openness and CO₂ emissions for the lower middle-income countries is in line with Shahbaz et al. (2017) (Shahbaz et al., 2017), this also shows that trade increase environmental degradation at initial stage but then it starts to improve environmental quality after a certain threshold level of trade openness, just as is also established by our study.

Figure 4 shows the response of CO₂ emissions in low-income countries to standard deviation shock. Firstly, it can be seen from Figure 4 that the standard deviation impact of economic growth and FDI has a positive impact on CO₂ emissions, and it reaches its maximum value in 2 years. The impacts are 0.010 and 0.024, respectively, but as time increases, the positive impact gradually decreases, and approaches or equals zero after 7 years. This shows that FDI and economic growth are the most direct factors affecting CO₂ emissions (Tran et al., 2019). Secondly, the standard deviation shock of HDI and trade openness has a negative impact on CO₂ emissions, and it reaches the maximum value in the second year, and its impact is −0.006 and −0.012. However, as time increases, the negative impact gradually decreases, and approaches or equals zero after 10 years. This is because the national income of most low-income countries mainly depends on agricultural production, which promotes trade openness to curb the increase in CO₂ emissions in the short term. But in the long run, with the inflow of FDI, low-income countries will be more likely to obtain the necessary resources and advanced technologies for life and production from developed countries, which will also promote trade openness and HDI to have a positive impact on CO₂ emissions (Bélaïd and Youssef, 2017; Sasmaz et al., 2020). Finally, under standard deviation shock, industrialization has a positive impact on CO₂ emissions, while renewable energy consumption has a negative impact, but the impact is minimal (Belaïd and Zrelli, 2019). As previously analyzed, CO₂ emissions have a positive effect on the development of renewable energy in low-income countries. This is because most low-income countries are in the primary stage of economic development and have not yet completed economic transformation, leading to a low level of industrialization and low efficiency of energy use in these countries (Tiba and Belaid, 2021). In general, there is an inverted U-shaped relationship between economic growth (or FDI) and CO₂ emissions in low-income countries (Ozcan, 2013), and a U-shaped relationship between trade openness (or HDI) and CO2 emissions.