A recent paper in the Journal of Environmental Economics and Management points out that time effects are not uniquely identified in reduced-form models, such as the environmental Kuznets curve. This Research Report proposes a solution that assumes the time effect is common to each pair of most similar countries. The between estimator makes no a priori assumption about the nature of the time-effects and is likely to provide consistent estimates of long-run relationships in real-world data situations. This research applies several common panel data estimators to the data set for carbon and sulfur emissions in the OECD collected by Vollebergh et al. and the global sulfur dataset compiled by Stern and Common The between estimates of the sulfur-income elasticity are 0.732 in the OECD and 1.157 in the global data set. The estimated carbon-income elasticity is 1.612.