OUTLINE

The major types of perturbations influencing the climate system are briefly reviewed in this chapter. The main focus is on the notions of forcing and feedback which provide a widely used framework to interpret the response of the system to changes in external conditions. The standard physical feedbacks are first presented before describing interactions implying jointly the energy-balance, hydrological and biogeochemical cycles.

Climate Forcing and Climate Response

4.1.1 Notion of Radiative Forcing

The climate system is influenced by different types of perturbations: changes in the amount of incoming solar radiation, in the composition of the atmosphere, and so on. To compare the magnitude of these perturbations and to evaluate their effect on the climate, it is convenient to analyse their impact on the radiative balance of the Earth. In this framework, a positive radiative forcing corresponds to more energy input to the system (or less output).

The climate changes in response to perturbations ultimately reach a new equilibrium (see Section 4.1.3). To have a clearer view of the dynamics of the system, it is important to separate as objectively as possible the forcing from this response. Consequently, the radiative forcing should be evaluated whilst maintaining the state variables of the system at their values before the perturbation is applied. In the case of an instantaneous forcing, this can be interpreted as the immediate changes in radiative budget of the Earth imposed by the perturbation before any adjustment of the climate. For some agents, such as the solar forcing (see Section 4.1.2.4), evaluation of the radiative forcing is relatively direct, as this can be done by measuring the variations in incoming solar radiation at the top of the atmosphere. For other agents, such as greenhouse gases, estimates are less straightforward as they should be based on computation of the impact of the changes in composition on the radiative transfer in the whole atmosphere.

The instantaneous forcing is not necessarily representative of the perturbation of the heat budget of the troposphere (which is generally the part of the Earth that is of interest for climate) on the time scale needed for its adjustment. In particular, the stratosphere is nearly in equilibrium with the perturbation after a few months, whilst the surface takes several decades at least to achieve equilibrium (see Section 4.1.5).

OUTLINE

This chapter describes how estimates of future climate changes are derived and the main results which have been obtained up to now. Particular attention is paid to interpretation and the limitations of the forecasts at different time scales and on the most robust mechanisms which explain the changes. This allows underlining the links with previous chapters.

Scenarios

6.1.1 The Purpose of the Scenarios and Scenario Development

As discussed in Chapter 5 the changes in external forcings have driven major past climate variations. In order to “predict” the climate of the twenty-first century and beyond, it thus is necessary to estimate future changes in the forcings. This is achieved by the development of scenarios for the emission of greenhouse gases, aerosols, various pollutants in the atmosphere, land use and so on. These scenarios depend on many uncertain elements (as discussed later), and some of the uncertainties in the estimates of future climate changes are related to these factors (see Figure 6.10). This is the reason why, in the scientific literature, the term ‘climate projection’ is generally preferred for estimates of the changes during the twenty-first century and beyond to the term ‘climate prediction’ (see also Section 6.2.2). Climate projection emphasises the fact that the results depend on the scenarios chosen and the hypotheses employed in these scenarios. The scenarios are also used for analysing impact, adaptation and vulnerability, thus providing a consistent approach for socio-economic and climatic issues.

Various types of scenarios have been proposed in recent years and decades. In the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC), the climate projections were based on the Special Report on Emission Scenarios (SRES) scenarios (see Section 6.1.2), which cover the whole of the twenty-first century (Nakicenovic and Swart 2000). These scenarios were derived in a sequential form (Figure 6.1). Firstly, the main driving forces influencing the emissions from demographic, social and economic development have to be identified.

OUTLINE

This first chapter describes the main components of the climate system as well as some processes that will be necessary to understand the mechanisms analysed in the chapters that follow. Complementary information is available in the Glossary for readers not familiar with some of the notions introduced here.

Introduction

Climate is traditionally defined as a description in terms of the mean and variability of relevant atmospheric variables such as temperature, precipitation and wind. Climate thus can be viewed as a synthesis or aggregate of weather. This implies that portrayal of the climate in a particular region must contain an analysis of mean conditions, of the seasonal cycle and of the probability of extremes such as severe frost, storms and so on. In accordance with the standard of the World Meteorological Organisation (WMO), thirty years is the classic period for performing the statistics used to define climate. This is well adapted for studying recent decades because it requires a reasonable amount of data along with a good sample of the different types of weather that can occur in a particular area. However, when analysing the more distant past, such as the last glacial maximum around 21,000 years ago, climatologists are often interested in variables which are characteristic of longer time intervals. As a consequence, the thirty-year period proposed by the WMO should be considered more as a practical indicator than as a norm that must be followed in all cases. This definition of climate as representative of conditions over several decades should not, of course, obscure the fact that climate can change rapidly. Nevertheless, a substantial time interval is needed to observe a difference in climate. In general, the smaller the difference between two periods, the longer is the time required to confidently identify any climate changes between those periods.

We also must take into account the fact that the state of the atmosphere used in the preceding definition of climate is influenced by numerous processes involving not only the atmosphere but also the oceans, sea ice, vegetation and so on. Climate is therefore now defined with increasing frequency in the wider sense of a description of the climate system.

1–5 Invocation of the Muse. Like Book 3, Book 4 begins with a 5-verse invocation of a Muse, and we will naturally infer that this is the same Muse who was invoked in Book 3, namely Erato: she ‘herself’ is now to take over the tale, whereas in Book 3 she was asked to ‘stand beside’ the poet and tell him the story; 3.1–5 clearly introduce the whole second half of the poem, not just Book 3, and this too suggests continuity here. Erato remains an appropriate Muse to tell of the sufferings of Medea (cf. δυσίμερον in 4), just as in Book 3 she had been asked to tell how Jason's success depended upon Medea's erôs, but the fact that the Muse's name is not repeated lessens the special emphasis upon erôs, and assimilates the invoked Muse more to the traditional Muse of epic poetry (1–2n.). Acosta-Hughes 2010: 43–7 argues that the anonymity points to the poet's generic uncertainty as to whether a ‘lyric’ or an ‘epic’ voice is now to predominate, and Payne 2013: 305–6 associates the poet's abandonment of his narrative to the Muse with the fact that Medea's departure ‘maps exactly onto the moment at which the poet must surrender his fictional Medea to the bigger story to which she belongs as a character of myth’.

The poet asks the Muse herself to take over because he cannot decide which motive for Medea's flight to privilege (4–5n., Hunter 1987: 134–9). He thus puts the poet's dependence upon the Muse to a new use: like a historian, the poet is presented with traditional ‘facts’ which are incontrovertible but which require interpretation, and here he can only turn to the Muse for help. At the opening of Od., by contrast, Homer had had no doubt at all what caused the death of the suitors; Ap. is now a much less confident narrator than the poet of the proem to Book 1; see, e.g., Feeney 1991: 90–1. Such puzzles of motivation were, however, not restricted to historians.

Our short journey in climate science is close to its end. It has shown the diversity of the domain and has provided a sample of the large number of processes potentially responsible for climate variations on a wide range of time and spatial scales. This diversity is exciting because it allows us to create the links with pre-existing knowledge, learned in other textbooks and lectures, whilst stimulating our curiosity about many additional applications and topics not covered here. It is also frustrating because the content of an introductory textbook should be as self-sufficient as possible. Ideally, new points had to be demonstrated or justified, relying on standard skills in mathematics and physics, for instance, with a few references for additional lectures. The main mechanisms included in the preceding chapters are all explained qualitatively, often using schematics, and many of them quantitatively too. Nevertheless, determining the stability criteria of a numerical scheme or estimating precisely the magnitude of a feedback requires techniques which cannot all be described in detail here. Only the conclusions of existing studies can be given, with some general information on the method(s) applied. I hope that this will encourage many readers to go deeper into the topics that appear the most promising for them to investigate by themselves how precisely those conclusions are reached and the strong scientific arguments behind them.

Many of the cited references are recent, showing the rapid development of this knowledge. This situation also underlines the fact that uncertainties are still present on the subject addressed, justifying the current intense scientific activity. These uncertainties have been mentioned several times in this text, but it is important to recall here that many results are also well established. We have not insisted on the historical development of the field, but climatology is based on laws applied with success in mechanics, astronomy, thermodynamics, electromagnetism, chemistry, geology and numerical analysis over decades at least. Studies focussing on important topics such as the impact of changes in greenhouse gases concentrations or the astronomical theory of paleoclimates have their roots in the beginning of the twentieth century or even before [see, e.g., Arrhenius (1896) and the references therein and the discussions in Berger (1988)].