Thresholds and response to climate change forcing: The water sector

This paper explores the nature of critical thresholds in the natural and managed water environment, and examines the effect of thresholds on the shape of system response to change. The hydrological system is characterized by the presence of many critical thresholds, where processes change, and climate change will alter the frequency with which these thresholds are passed. The snow/rain threshold is particularly important, and examples in the paper show how the presence of this threshold can lead to very non-linear responses to incremental climate change. The water management system is also characterized by critical thresholds, but in most cases these are expressed in risk terms (a design standard typically has an associated risk of failure). In this context, climate change will alter the risk that design standards are exceeded, and the management threshold relates to the tolerable change in risk. In practice, this will be difficult to define, as changes in risk are difficult to determine because risk estimates are very uncertain. The paper concludes by outlining a risk-based procedure, using multiple scenarios, for estimating the risk of a threshold being crossed, and also indicates how defined critical thresholds can be used to implement an impact-oriented approach to climate change assessment. This approach focuses on the likelihood of encountering challenging circumstances.