8.7 Mechanisms Producing Thresholds and Abrupt Climate Change

8.7.1 Introduction

This discussion of thresholds and abrupt climate change is based on the definitions of ‘threshold’ and ‘abrupt’ proposed by Alley et al. (2002). The climate system tends to respond to changes in a gradual way until it crosses some threshold: thereafter any change that is defined as abrupt is one where the change in the response is much larger than the change in the forcing. The changes at the threshold are therefore abrupt relative to the changes that occur before or after the threshold and can lead to a transition to a new state. The spatial scales for these changes can range from global to local. In this definition, the magnitude of the forcing and response are important. In addition to the magnitude, the time scale being considered is also important. This section focuses mainly on decadal to centennial time scales.

Because of the somewhat subjective nature of the definitions of threshold and abrupt, there have been efforts to develop quantitative measures to identify these points in a time series of a given variable (e.g., Lanzante, 1996; Seidel and Lanzante, 2004; Tomé and Miranda, 2004). The most common way to identify thresholds and abrupt changes is by linearly de-trending the input time series and looking for large deviations from the trend line. More statistically rigorous methods are usually based on Bayesian statistics.

This section explores the potential causes and mechanisms for producing thresholds and abrupt climate change and addresses the issue of how well climate models can simulate these changes. The following discussion is split into two main areas: forcing changes that can result in abrupt changes and abrupt climate changes that result from large natural variability on long time scales. Formally, the latter abrupt changes do not fit the definition of thresholds and abrupt changes, because the forcing (at least radiative forcing – the external boundary condition) is not changing in time. However these changes have been discussed in the literature and popular press and are worthy of assessment here.