The level of recognition of human impacts on climate, contained in the third assessment of the Intergovernmental Panel on Climate Change (IPCC 2001), surely represents a turning point in human history. Some impacts can now be factored into predictions of future states of the world's ecosystems, and, though some powerful countries may for the moment indicate otherwise, it is certainly more difficult now to ignore the concept of global human impacts on the environment and doubt their seriousness. But there are potential pit-falls in this progression; it worries me that attention to incremental albeit significant rises in sea level, for example, may divert serious concern away from the consequences of the ongoing intensive and extensive growth of human population, the sizeable global impact of which on the environment and human society can scarcely either be debated. Environmental science is challenging; it aspires to holism, but the resources and scientific tools are such that it can only very rarely be holistic at anything other than very small scale. And different disciplines can be disappointingly inarticulate in truly interdisciplinary work. Environmental science tilts at comprehensive global understanding, but the problems of extrapolating to that scale are for the most part insuperable. The scientists, typically reticent about erring beyond the disciplinary or geographical boundaries of their work, yet those most trained to deliver objective information, risk becoming an undervalued resource. The danger is that awkward strategic decisions will be made with much less certainty or at least consensus than they should.