Emissions keep rising, states keep trading, and Earth will be 1.5°C warmer within five years. These are results of inadequate global governance. As globalisation has brought complexity to the international settings, creating overlapping webs of interactions: no International Organisation has properly responded. In this context, we are now situated in a trade regime complex with overlapping rules and norms, but what agency can be claimed? The thesis investigates the trade-environment nexus by addressing how legitimate climate leadership in a trade regime complex is pursued.   By challenging the conventionality of international leadership theory with a separation from hegemony and applying it outside multilateral negotiations, the thesis found it applicable for this new setting. Through mixed methods, statistics and textual analysis, the United States (US) Preferential Trade Agreements (PTA) were a surprisingly fitted case for this. To understand climate leadership performance and underlying objectives for such, the TREND database was adopted. Within, all US PTAs are shown and to what extent it has adapted 294 environmentally-related norms. Coded into means for leadership, the US legalistic approach of structurally directing others to mitigating measures seems to be a double-edged sword. Done right, it creates non-derogative measures. Done wrong, it creates a hegemon.