some environmental historians challenge this assumption, arguing that while traditional history is human history – the story of people and their institutions, "humans cannot place themselves outside the principles of nature". In this sense, they argue that environmental history is a version of human history within a larger context, one less dependent on anthropocentrism (even though anthropogenic change is at the center of its narrative). Dimensions J. Donald Hughes responded to the view that environmental history is "light on theory" or lacking theoretical structure by viewing the subject through the lens of three "dimensions": nature and culture, history and science, and scale. This advances beyond Worster's recognition of three broad clusters of issues to be addressed by environmental historians although both historians recognize that the emphasis of their categories might vary according to the particular study as, clearly, some studies will concentrate more on society and human affairs and others more on the environment. Themes Several themes are used to express these historical dimensions. A more traditional historical approach is to analyse the transformation of the globe's ecology through themes like the separation of man from nature during the neolithic revolution, imperialism and colonial expansion, exploration, agricultural change, the effects of the industrial and technological revolution, and urban expansion. More environmental topics include human impact through influences on forestry, fire, climate change, sustainability and so on. According to Paul Warde, “the increasingly sophisticated history of colonization and migration can take on an environmental aspect, tracing the pathways of ideas and species around the globe and indeed is bringing about an increased use of such analogies and ‘colonial’ understandings of processes within European history.” The importance of the colonial enterprise in Africa, the Caribbean and Indian Ocean has been detailed by Richard Grove. Much of the literature consists of case-studies targeted at the global, national and local levels. Scale Although environmental history can cover billions of years of history over the whole Earth, it can equally concern itself with local scales and brief time periods. Many environmental historians are occupied with local, regional and national histories. Some historians link their subject exclusively to the span of human history – "every time period in human history" while others include the period before human presence on Earth as a legitimate part of the discipline. Ian Simmons's Environmental History of Great Britain covers a period of about 10,000 years. There is a tendency to