Climate transitions on long timescales

Abstract

The climate has changed through the history of the Earth as evidenced in the geological records. Today we might be experiencing a climate change of the same magnitude as the transition into an ice age caused by very rapid burning and emission to the atmosphere of a substantial part of the fossilised carbon. Whether this leads to a gradual warming or if we will experience a transition into a different climatic state is presently unknown. The present day state-of-the-art numerical climate models are capable of producing fair representations of the current climate and are as such trusted to also predict the climate changes due to increasing atmospheric concentrations of greenhouse gases. However, the models are not presently capable of reproducing the rapid transitions from one climatic state, such as a glacial climate, into another, such as the present climate. The reason for this is unknown. The transitions are inherently ‘non-linear’ and thus not accessible through linear response theory. The term ‘non-linear’ is in this context defined as the phenomenon that the response of the system to a change in the forcing of the system is not linearly proportional to the forcing. This would happen if a threshold is reached such that the state of the system becomes unstable and the system bifurcates into a different state. There are strong indications in the geological records of this kind of behaviour for the climate. These dynamics can be understood in the context of fairly simple models of the climate.

Keywords:

• climate change
• climate models