| Authors (year) | Sample (Period) | Environmental variables | Econometric specification | Estimation technique (s) | Results |
| Selected studies that confirm the EKC | |||||
| Pata [18] | Turkey (1974-2014) | CO2 emissions | Quadratic | ARDL, FMOLS and CCR | ECK is valid with turning points between 13,523 and 14,077 USD |
| Hanif [19] | 34 SSA countries (1995-2015) | CO2 emissions | Quadratic | System GMM | EKC is confirmed |
| Ahmad et al. [20] | India (1971-2014) | Aggregated and disaggregated CO2 | Quadratic | ARDL bounds testing and causality test | Confirmed for aggregated and disaggregate CO2 indicators |
| Heidari et al. [21] | Five ASEAN countries: Indonesia, Malaysia, Philippines, Singapore and Thailand | Carbon dioxide |
| PSTR | EKC is supported for the panel. |
| Selected studies that reject the EKC | |||||
| Zoundi [22] | 25 SSA countries (1980-2012) | CO2 emissions | Quadratic | Dynamic OLS, Syst GMM, DFE, MG, PMG | Monotonic increasing relationship. |
| Abid [23] | 23 SSA African countries (199-2010) | CO2 emissions | Quadratic | Static (Pooled OLS, RE, FE) and dynamic (diff GMM, syst GMM and LSDVC) panel techniques | Monotonic increasing relationship. |
| Allard et al. [24] | 74 countries (1994-2012) | CO2 emissions | Cubic | Quantile regression | N-shaped relationship. |
| Selected studies that provided mixed results | |||||
| Keho [25] | 59 countries from different regions and periods: SSA and MENA (1976-2011) America, Asia and Europe (1971-2011) | CO2 emissions | Quadratic | Quantile regression | EKC is confirmed for SSA, American and European countries at all quantiles. EKC is valid for Asian and MENA countries only at lower levels of CO2 emissions. |
| Alam et al. [26] | Brazil, China, India and Indonesia (1970-2012) | CO2 emissions | Linear and quadratic | ARDL bounds testing | EKC is confirmed only in Brazil, China and Indonesia and rejected for India. |