The key question underlying the controversies addressed in this chapter is to what extent the global biogeochemical cycles and the climate system are being disturbed by anthropogenic processes. The CYCLES submodel is used to explore the influence of various model routes on future projections of global environmental change. These routes are characterised by specific assumptions about the key processes within the global carbon (C) and nitrogen (N) cycle and the climate system. This creates the possibility to assess various emission scenarios, paving the way for the more integrated experiments described in Chapters 17 and 18. It is reiterated that the current state of scientific knowledge with respect to global C and N cycles and climate change is still beset with major uncertainties.

Introduction

Although scientific knowledge of global biogeochemical cycles is increasing rapidly, there are still major gaps. Subjective interpretation of these gaps results in different assessments of the rate, magnitude and impacts of human-induced changes in global cycles. The climate debate exemplifies the kind of intellectual battle that can take place, given uncertainties about the global biogeochemical cycles of the basic elements carbon (C), nitrogen (N), phosphorus (P) and sulphur (S), and the climate system. In general, the controversies within the scientific community on global biogeochemical cycles focus on the relationships among the physical, biological and chemical processes comprising the complex dynamics of the global biogeochemical cycling as well as the role of the various feedbacks.