environmental history is at odds with the humanities because it emphasises the division between "materialist, and cultural or constructivist explanations for human behaviour". Global sustainability Many of the themes of environmental history inevitably examine the circumstances that produced the environmental problems of the present day, a litany of themes that challenge global sustainability including: population, consumerism and materialism, climate change, waste disposal, deforestation and loss of wilderness, industrial agriculture, species extinction, depletion of natural resources, invasive organisms and urban development. The simple message of sustainable use of renewable resources is frequently repeated and early as 1864 George Perkins Marsh was pointing out that the changes we make in the environment may later reduce the environments usefulness to humans so any changes should be made with great care - what we would nowadays call enlightened self-interest. Richard Grove has pointed out that "States will act to prevent environmental degradation only when their economic interests are threatened". Advocacy It is not clear whether environmental history should promote a moral or political agenda. The strong emotions raised by environmentalism, conservation and sustainability can interfere with historical objectivity: polemical tracts and strong advocacy can compromise objectivity and professionalism. Engagement with the political process certainly has its academic perils although accuracy and commitment to the historical method is not necessarily threatened by environmental involvement: environmental historians have a reasonable expectation that their work will inform policy-makers. A recent historiographical shift has placed an increased emphasis on inequality as an element of environmental history. Imbalances of power in resources, industry, and politics have resulted in the burden of industrial pollution being shifted to less powerful populations in both the geographic and social spheres. A critical examination of the traditional environmentalist movement from this historical perspective notes the ways in which early advocates of environmentalism sought the aesthetic preservation of middle-class spaces and sheltered their own communities from the worst effects of air and water pollution, while neglecting the plight of the less privileged. Communities with less economic and sociopolitical power often lack the resources to get involved in environmental advocacy. Environmental history increasingly highlights the ways in which the middle-class environmental movement has fallen short and left behind entire communities. Interdisciplinary research now understands historic inequality as a lens through which to predict future social developments in the environmental sphere, particularly with regard to climate change. The United Nations Department of Economic and Social Affairs cautions