*4.1. Data Issues*

This study utilizes the balanced panel data of fifteen member states of NATO (See Table A1 in the appendix for the list of the countries in the sample), spanning the period 1965–2018. The lack of available data for the new member states that joined the alliance in the late 1990s and early 2000s forced us to limit our empirical analysis to older member states of the alliance. We incorporated the annual balanced panel data for carbon dioxide emissions (CO2), the defense burden in the percentage of military expenditure to GDP ratio (ME), and primary energy use (EN). We collected the data for the defense burden from the World Development Indicators (WDI) database of the World Bank [54], whereas the data for carbon dioxide emissions (kilotons) and primary energy use (kg of oil equivalent per capita) were compiled from the Statistical Review of World Energy database of British Petroleum (BP) [55]. Empirical analysis was carried out using GAUSS 10 and Stata 16 software. Table 1 presents the definitions and data sources of the variables.

**Table 1.** List of variables and data sources.


Note: \*\* dependent variable.

Table 2 displays descriptive statistics. The standard deviation for CO2 and EN are relatively higher due to larger differences between maximum and minimum values. The minimum values for CO2, ME, and EN were observed in Luxembourg, while the maximum values for these variables by far were observed in the USA. It should also be noted that the variables were converted into natural logarithmic form to reduce the likelihood of skewness in the original data set in order to obtain more reliable statistical results.

**Table 2.** Descriptive statistics.

