Small changes in the climate system can be sufficiently understood by assuming linear relationships between variables. However, many climate processes are non-linear by nature, and conclusions based on linear models and processes may in these cases no longer be valid. Non-linearity is a prerequisite for the existence of thresholds in the climate system: small perturbations or changes in the forcing can trigger large reorganisations if thresholds are passed. The result is that atmospheric and oceanic circulations may change from one regime to another. This could possibly be manifested as rapid climate change.
There is no clear definition of "rapid climate change". In general, this notion is used to describe climate changes that are of significant magnitude (relative to the natural variability) and occur as a shift in the mean or variability from one level to another. In order to distinguish such changes from "extreme events", a certain persistence of the change is required. Among the classical cases are spontaneous transitions from one preferred mode to another or transitions triggered by slowly varying forcing. This occurs in non-linear systems which have multiple equilibria (Lorenz, 1993). Evidence for the possibility of such transitions can be found in palaeoclimatic records (see Chapter 2, Section 2.4; and Stocker, 2000), in observations of changes in large-scale circulation patterns from the instrumental record (see Section 18.104.22.168), and contemporary observations of regional weather patterns (e.g., Corti et al., 1999).
Here, we briefly summarise non-linear changes that have captured attention in the recent literature and that have been assessed in this chapter:
Reducing uncertainty in climate projections also requires a better understanding of these non-linear processes which give rise to thresholds that are present in the climate system. Observations, palaeoclimatic data, and models suggest that such thresholds exist and that transitions have occurred in the past. The occurrence of such transitions can clearly not be excluded in a climate that is changing. On the contrary, model simulations indicate that such transitions lie within the range of changes that are projected for the next few centuries if greenhouse gas concentrations continue to increase. A particular concern is the fact that some of these changes may even be irreversible due to the existence of multiple equilibrium states in the climate system.
Comprehensive climate models in conjunction with sustained observational systems, both in situ and remote, are the only tool to decide whether the evolving climate system is approaching such thresholds. Our knowledge about the processes, and feedback mechanisms determining them, must be significantly improved in order to extract early signs of such changes from model simulations and observations.
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