本文主要藉由歐盟環境政策之法律基礎演變，探求歐盟環境政策權限擴張的歷程，並勾勒歐盟如何藉由內部權限的取得，以支撐其在區域乃至於國際環境政治議題逐漸取得全球行為者之角色。就歐盟做為一個全球氣候變化政策之行為者而言，其成功將區域經濟整合組織納入氣候變化綱要公約之締約主體，使得共同體得以簽署該公約；歐盟並藉由經貿議題與氣候變化議題之連結，成功說服俄羅斯批准京都議定書。歐盟適時把握美國退出國際環境政治之契機，取得在氣候變化議題之領導者地位。然而，由於氣候變化議題屬歐盟與會員國之共享權限，對內與對外均可能面臨挑戰。因而，在歐盟與會員國立場之一致與各部門政策之一貫，則顯得格外重要。就此目的之達成，真誠合作原則具有高度之價值。

This article examines the legal basis of the EU’s competence in environment affairs with a view to ascertaining how the expansion of internal competence supports EU external action in international environmental policies. With respect to climate change, firstly, the EU was successful in including a “regional economic integration organisation” in the UNFCCC, which provides a legal basis for the EU to become a Contracting Party to it; and, secondly, in assuming a leadership role in global climate change politics when the US retreated from environmental politics, and successfully persuading the Russian Federation to ratify the Kyoto Protocol with trade and environment linkages. However, given that environmental affairs are a shared competence between the EU and its Members States, as the EU seeks to lead the fight against global climate change it may confront internal challenges as well as external challenges. Therefore, the consistency of the positions of the EU and its Members States, and coherence among different policies objectives, are key to EU success as a leading actor in the fight against global climate change. Thus, the principle of sincere cooperation plays a critical role in ensuring the consistency, coherence, and unity of EU international representation.