The growing global interest in mitigating climate change implies an increased importance of sustainable development to achieve greenhouse gas emission reductions. The paper analyses the impact of key economic and environmental factors, such as the share of renewable energy, gross domestic product (GDP), fossil fuel consumption, final energy consumption in households and industry, and forest area, on greenhouse gas (GHG) emissions in the European Union (consisting of 27 members) and Poland for comparison—for the period from 1990 to 2023. The study fills a gap in identifying the cross-sectoral determinants of greenhouse gas emissions in the EU, focusing specifically on Poland and the whole EU region since the beginning of the European Union. The research involved the implementation of statistical analyses, dynamic analyses, correlation analyses, and regression analyses. The results showed an increase in the share of renewable energy, GDP, and forest area, which was negatively correlated with the volume of GHG emissions. In contrast, final fossil fuel consumption and final energy consumption in industry and households (to a slightly lesser extent) were also significantly but positively correlated. It is worth noting that the strength of calculated relationships differed for the EU and Poland. The study revealed trends and correlations that affect GHG and are relevant to policy implications for EU climate goals. Considering the various determinants of GHG emissions and Poland’s unique situation (high dependence on coal and a large share of heavy industry), conclusions were formulated for Poland’s and the EU’s climate policies in the context of the European Green Deal.