Introduction

The present characteristics of the Earth system, and in particular of the climate system, in which human societies are embedded, are a product of a long-term evolution: a current snapshot in a six-billion-year movie that is still running. After colonisation of the continents by plants and animals, the Earth has developed a climate system – composed of atmosphere, ocean, cryosphere, land surfaces, and the marine and terrestrial biosphere – which very effectively, through complex interactions, has produced a rather stable equilibrium state, around which climate variations evolve.

Until 250 years ago, the interference of man was small, and climate variations were a product of natural processes and interactions alone. Since the beginning of industrialisation, the composition of the atmosphere, especially the concentrations of greenhouse gases like carbon dioxide and methane, have significantly increased. In addition, the character of the land surface has been largely modified through human activities. Part of the observed global warming during the past 100 years is attributed to these anthropogenic impacts.

One of the most important characteristics of the climate system is its variability, which extends on timescales ranging from days to millions of years. Short-term variations of atmospheric variables – such as air temperature, pressure, humidity, the three wind components, precipitation, and cloud cover – on the order of days denote the weather. Climate variations are associated with long-term (months and longer) changes of the atmosphere, which mostly originate from interactions with the slow components of the climate system.