A central question behind Article 2 of the UN Framework Convention on Climate Change is: What levels of climate change should (as a minimum) be avoided in order to avert significant threat to natural ecosystems, food production and economic development? This paper defines what is implied by a 'dangerous' climate change in terms of (a) thresholds of weather or climate events and (b) critical levels of climate change. It describes a method of identifying these thresholds and critical levels, and illustrates how these can assist the UNFCCC process. Copyright (C) 1996 Elsevier Science Ltd.