1.1 Introduction

Climate is traditionally defined as the description in terms of the mean and variability of relevant atmospheric variables such as temperature, precipitation and wind. Climate can thus be viewed as a synthesis or aggregate of weather. This implies that the portrayal of the climate in a particular region must contain an analysis of mean conditions, of the seasonal cycle, of the probability of extremes such as severe frost and storms, etc. Following the World Meteorological Organisation (WMO), 30 years is the classical period for performing the statistics used to define climate. This is well adapted for studying recent decades since it requires a reasonable amount of data while still providing a good sample of the different types of weather that can occur in a particular area. However, when analyzing the most distant past, such as the last glacial maximum around 20 000 years ago, climatologists are often interested in variables characteristic of longer time intervals. As a consequence, the 30-year period proposed by the WMO should be considered more as indicative rather than a norm that should be followed in all cases. This definition of the climate as representative of conditions over several decades should, of course, not mask the fact that climate can change rapidly. Nevertheless, a substantial time interval is needed to observe a difference in climate between any two periods. In general, the less the difference between the two periods, the longer is the time needed to be able to identify with confidence any changes in the climate between them

**Figure 1.1:** Schematic view of the components of the climate system and of their potential changes. FAQ1.2, Figure 1 from IPCC (2007) using a modified legend, published in: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, copyright IPCC 2007. Reproduced with permission.

We must also take into account the fact that the state of the atmosphere, used in the definition of the climate given above is influenced by numerous processes involving not only the atmosphere but also the ocean, the sea ice, the vegetation, etc. Climate is thus now more and more frequently defined in a wider sense as the statistical description of the climate system. This includes the analysis of the behaviour of its five major components: the atmosphere (the gaseous envelope surrounding the Earth), the hydrosphere (liquid water, i.e. ocean, lakes, underground water, etc), the cryosphere (solid water, i.e. sea ice, glaciers, ice sheets, etc), the land surface and the biosphere (all the living organisms), and of the interactions between them (IPCC 2007, Fig 1.1). We will use this wider definition when we use the word climate. The following sections of this first chapter will provide some general information about those components. Note that the climate system itself is often considered as part of the broader Earth System, which includes all the parts of the Earth and not only the elements that are directly or indirectly related to the temperature or precipitation.