Climate is a major factor in the habitat, food chains, competition, success and survival of species. Contemporary distributions and abundance of marine species and communities reflect adaptation to geologically recent climatic conditions and the impacts of human activities. Warming of the atmosphere and seawater has occurred in association with increasing levels of atmospheric carbon dioxide since the start of the twentieth century. Despite continuing scientific research and wider discussion of the relative roles of anthropogenic greenhouse gas increases and other influences on climate, climate change is occurring. The policy and legal issues have two core components: response to the effects of climate change, and addressing the human activities for which there is reasonable evidence of causation or exacerbation of climate change. For the purpose of this chapter, the focus will be on the response to the effects of climate change, rather than on the issue of anthropogenic causation and exacerbation.