Climate System Dynamics and Modelling

OUTLINE

This chapter analyses climate variations on time scales for one year to billion years, investigating the roles of both external forcing and the internal dynamics. Rather than an exhaustive description of the climate over the whole Earth's history, some key periods and mechanisms are selected. The goal is to illustrate the processes studied in the preceding chapters and to investigate how their interplay drove past climate changes.

Introduction

5.1.1 Forced and Internal Variability

We are all familiar with the large day-to-day variations in the weather in the absence of any major change in the energy coming from the Sun or any other external forcing. Similar fluctuations are observed at all the time scales and are a key element of climate dynamics. Hence, it is common to distinguish the forced variability in response to a forcing (see Chapter 4) from the unforced or internal variability which has its origin in the interactions between various elements of the system.

For forced variability, a cause of the observed changes can be found outside the climate system. This is not the case for internal variability. We can identify a chain of events with, for instance, a change in the wind stress which influences tropical temperatures and contributes to the development of El Niño conditions. However, we then should determine the cause of the modification in wind stress, with the full sequence rapidly blurred in the complexity of the system. We thus have no access to an ultimate cause of an event but only at best to proximate causes immediately contributing to the event.

The development of internal variability is sensitive to small perturbations. Consider, for instance, two situations which are very similar, with just a small difference in one variable which may appear originally negligible. This small difference still slightly perturbs the evolution of the system and, because of various mechanisms and amplifications, leads to states that are clearly different after some time in the two situations. This is often referred to the ‘chaotic nature’ of the climate (Lorenz 1963), implying that the uncertainty present at one moment, in particular, because of imprecision of observations, grows stronger with time.