A revised version of this working paper has been published as: Biermann, Frank. 2012. Greening the United Nations Charter: World Politics in the Anthropocene. Environment: Science and Policy for Sustainable Development. May / June 2012. A constitutional turn is needed to bring the UN system in line with the urgent needs of planetary stewardship and earth system governance in the 21st century. Yet how this could be organized in practice remains a challenge and subject to political and scholarly debate. This paper contributes to this debate by outlining four reforms of the UN system that would advance global decision-making by addressing major shortcomings in the current system: Lack of integration of economic and environmental policies in the UN system; institutional fragmentation and weakness of the environmental pillar of sustainable development; lack of high-level regulatory competence and oversight regarding areas beyond national jurisdiction; and insufficient integration of scientific insights into political decision-making. The reforms proposed would together create an Earth Alliance in the UN system, consisting of a high-level UN Sustainable Development Council, a World Environment Organization, a UN Trusteeship Council for Areas beyond National Jurisdiction, and an UN Global Environmental Assessment Commission.