OUTLINE

Models are essential tools in climatology to understand the processes responsible for observed changes and to make predictions. After defining what is traditionally meant by climate modelling, this chapter presents the different types of models. Then the chapter briefly reviews the main characteristics of the components of those models. Finally, a discussion is provided of the methodology applied to test the performance of the models and to interpret their results in conjunction with observations.

Introduction

3.1.1 What Is a Climate Model?

In general terms, a ‘mathematical model’ of the climate system is defined as a mathematical representation of the system based on physical, biological and chemical principles or laws. The equations derived from these laws are so complex that they must be solved numerically, leading to the formulation of a numerical or computer model (Figure 3.1). Such a model is used to perform simulations corresponding to realistic past or future conditions and idealised experiments. This is the reason why the term ‘climate simulator’ has been proposed to make a distinction between various types of models, such as physical, statistical or conceptual models (e.g., Crucifix 2012a). Nevertheless, we will keep here the classical denomination of climate model, which includes the following model equations (mathematical model) and the way these are solved (numerical model). This approach is largely practised in the field, keeping in mind that the word ‘model’ can have different meanings in different communities.

Numerical resolution of the equations provides an approximation of the true solution, which is, in practise, discrete in space and time. This can be interpreted as if the numerical models give estimates of climate variables averaged over regions, whose size depends on model spatial resolution, and over specific time intervals, which are related to the time step. For instance, some models provide globally or zonally averaged values, whilst others have a numerical grid, whose spatial resolution could be less than 100 km (Figure 3.2).

The climate has a significant impact on life on Earth as well as on human activities. Temperature and precipitation strongly constrain the type of vegetation that can grow in a particular region. The design and location of houses depend on summer and winter temperatures and also on the probability of flooding. One single, late frost or a heavy hail storm could ruin an entire crop. Since the beginning of humanity, people thus have had to cope with climate and, if possible, to adapt to it. As a consequence, the various human civilisations have observed and tried to understand climate variations. They first provided mythological or religious explanations, often relying on weather lore to obtain forecasts. In parallel, climate has evolved as a science, elaborating more and more sophisticated representations of the observed phenomena. Such a description of climate now involves a very broad range of expertise corresponding to different domains of the sciences, including physics, chemistry, biology and geology.

A comprehensive analysis of all the components of the climate system (i.e., atmosphere, ocean, ice sheets, land surfaces, etc.) and of all the interactions between them is beyond the scope of any course or book. Here I provide only a relatively brief overview of the processes that rule the behaviour of the individual components. More detailed descriptions are provided in meteorology, oceanography and glaciology textbooks, for instance, with some suggestions for reading given in the reference section. The focus of this book is on the interactions between the different elements of the climate system and on the main feedbacks that govern climate variability on all the time scales. On this basis, the first goal of this book is to analyse the dominant causes of past climate changes and to critically discuss the projections of climate change over the next centuries or millennia.

Because of the complexity of the system, many analyses devoted to a quantitative estimate of climate change or climate variability rely on the use of comprehensive three-dimensional numerical models. Simple models are also widely applied to underline clearly the fundamental properties of the climate.

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.

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)].

OUTLINE

Climate is characterised by large exchanges of energy and various chemical species between its components. Modifications of those exchanges are a major source of climate change. For energy, water and carbon, this chapter describes first the budget of the various reservoirs on a global scale before focussing on the spatial distribution of some key variables.

The Earth's Energy Budget

2.1.1 The Radiative Balance at the Top of the Atmosphere: A Global View

Nearly all the energy entering the climate system comes from the Sun in the form of electromagnetic radiation. Additional sources are present, such as geothermal heating, for instance, but their contribution is so small that their influence can safely be neglected. At the top of the Earth's atmosphere, a surface at the mean Earth–Sun distance perpendicular to the solar rays receives about 1360 W m−2 (Kopp and Lean 2011) (see also Figure 4.6). This is called the ‘total solar irradiance’ (TSI) or the ‘solar constant’S0. A bit less than half this energy comes in the form of radiation in the visible part of the electromagnetic spectrum, the remaining part occurring mainly in the near-infrared region, with a smaller contribution from the ultraviolet region of the spectrum (Figure 2.1).

On average, the total amount of incoming solar energy per unit of time outside the Earth's atmosphere (Figure 2.2) is the TSI times the cross-sectional surface (i.e., the surface that intercepts the solar rays, which corresponds to a surface equal to πR2, where R is the Earth's radius of 6,371 km). For simplicity and because it is a reasonable approximation, we will neglect the thickness of the atmosphere compared to the Earth's radius in our computations of distances or surfaces. Some of the incoming energy is reflected straight back to space by the atmosphere, in particular, by the clouds, and the Earth's surface. This fraction of the energy that is reflected is called the ‘albedo of the Earth’ or ‘planetary albedo’ αp. In present-day conditions, it has a value of about 0.3.

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.