Recent years have seen an appreciable growth in the level of understanding of the dangers facing the international environment and an extensive range of environmental problems is now the subject of serious international concern. These include atmospheric pollution, marine pollution, global warming and ozone depletion, the dangers of nuclear and other extra-hazardous substances and threatened wildlife species. Such problems have an international dimension in two obvious respects. First, pollution generated from within a particular state often has a serious impact upon other countries. The prime example would be acid rain, whereby chemicals emitted from factories rise in the atmosphere and react with water and sunlight to form acids. These are carried in the wind and fall eventually to earth in the rain, often thousands of miles away from the initial polluting event. Secondly, it is now apparent that environmental problems cannot be resolved by states acting individually. Accordingly, co-operation between the polluting and the polluted state is necessitated. However, the issue becomes more complicated in those cases where it is quite impossible to determine from which country a particular form of environmental pollution has emanated. This would be the case, for example, with ozone depletion. In other words, the international nature of pollution, both with regard to its creation and the damage caused, is now accepted as requiring an international response. The initial conceptual problem posed for international law lies in the state-oriented nature of the discipline. Traditionally, a state would only be responsible in the international legal sense for damage caused where it could be clearly demonstrated that this resulted from its own unlawful activity. This has proved to be an inadequate framework for dealing with environmental issues for a variety of reasons, ranging from difficulties of proof to liability for lawful activities and the particular question of responsibility of non-state offenders. Accordingly, the international community has slowly been moving away from the classic state responsibility approach to damage caused towards a regime of international co-operation. A broad range of international participants are concerned with developments in this field. States, of course, as the dominant subjects of the international legal system are deeply involved, as are an increasing number of international organisations, whether at the global, regional or bilateral level. The United Nations General Assembly has adopted a number of resolutions concerning the environment, and the UN Environment Programme was established after the Stockholm Conference of 1972.