Internally and externally induced climate variability
Climate variations, both in the mean state and in other statistics such as, for example, the occurrence of extreme events, may result from radiative forcing, but also from internal interactions between components of the climate system. A distinction can therefore be made between externally and internally induced natural climate variability and change.

When variations in the external forcing occur, the response time of the various components of the climate system is very different. With regard to the atmosphere, the response time of the troposphere is relatively short, from days to weeks, whereas the stratosphere comes into equilibrium on a time-scale of typically a few months. Due to their large heat capacity, the oceans have a much longer response time, typically decades but up to centuries or millennia. The response time of the strongly coupled surface-troposphere system is therefore slow compared with that of the stratosphere, and is mainly determined by the oceans. The biosphere may respond fast, e.g. to droughts, but also very slowly to imposed changes. Therefore the system may respond to variations in external forcing on a wide range of space- and time-scales. The impact of solar variations on the climate provides an example of such externally induced climate variations.

But even without changes in external forcing, the climate may vary naturally, because, in a system of components with very different response times and non-linear interactions, the components are never in equilibrium and are constantly varying. An example of such internal climate variation is the El Niño-Southern Oscillation (ENSO), resulting from the interaction between atmosphere and ocean in the tropical Pacific.

Feedbacks and non-linearities
The response of the climate to the internal variability of the climate system and to external forcings is further complicated by feedbacks and non-linear responses of the components. A process is called a feedback when the result of the process affects its origin thereby intensifying (positive feedback) or reducing (negative feedback) the original effect. An important example of a positive feedback is the water vapour feedback in which the amount of water vapour in the atmosphere increases as the Earth warms. This increase in turn may amplify the warming because water vapour is a strong greenhouse gas. A strong and very basic negative feedback is radiative damping: an increase in temperature strongly increases the amount of emitted infrared radiation. This limits and controls the original temperature increase.

A distinction is made between physical feedbacks involving physical climate processes, and biogeochemical feedbacks often involving coupled biological, geological and chemical processes. An example of a physical feedback is the complicated interaction between clouds and the radiative balance. Chapter 7 provides an overview and assessment of the present knowledge of such feedbacks. An important example of a biogeochemical feedback is the interaction between the atmospheric CO₂ concentration and the carbon uptake by the land surface and the oceans. Understanding this feedback is essential for an understanding of the carbon cycle. This is discussed and assessed in detail in Chapter 3.

Many processes and interactions in the climate system are non-linear. That means that there is no simple proportional relation between cause and effect. A complex, non-linear system may display what is technically called chaotic behaviour. This means that the behaviour of the system is critically dependent on very small changes of the initial conditions. This does not imply, however, that the behaviour of non-linear chaotic systems is entirely unpredictable, contrary to what is meant by “chaotic” in colloquial language. It has, however, consequences for the nature of its variability and the predictability of its variations. The daily weather is a good example. The evolution of weather systems responsible for the daily weather is governed by such non-linear chaotic dynamics. This does not preclude successful weather prediction, but its predictability is limited to a period of at most two weeks. Similarly, although the climate system is highly non-linear, the quasi-linear response of many models to present and predicted levels of external radiative forcing suggests that the large-scale aspects of human-induced climate change may be predictable, although as discussed in Section 1.3.2 below, unpredictable behaviour of non-linear systems can never be ruled out. The predictability of the climate system is discussed in Chapter 7.

Global and hemispheric variability
Climate varies naturally on all time-scales. During the last million years or so, glacial periods and interglacials have alternated as a result of variations in the Earth’s orbital parameters. Based on Antarctic ice cores, more detailed information is available now about the four full glacial cycles during the last 500,000 years. In recent years it was discovered that during the last glacial period large and very rapid temperature variations took place over large parts of the globe, in particular in the higher latitudes of the Northern Hemisphere. These abrupt events saw temperature changes of many degrees within a human lifetime. In contrast, the last 10,000 years appear to have been relatively more stable, though locally quite large changes have occurred.

Recent analyses suggest that the Northern Hemisphere climate of the past 1,000 years was characterised by an irregular but steady cooling, followed by a strong warming during the 20th century. Temperatures were relatively warm during the 11th to 13th centuries and relatively cool during the 16th to 19th centuries. These periods coincide with what are traditionally known as the medieval Climate Optimum and the Little Ice Age, although these anomalies appear to have been most distinct only in and around the North Atlantic region. Based on these analyses, the warmth of the late 20th century appears to have been unprecedented during the millennium. A comprehensive review and assessment of observed global and hemispheric variability may be found in Chapter 2.

The scarce data from the Southern Hemisphere suggest temperature changes in past centuries markedly different from those in the Northern Hemisphere, the only obvious similarity being the strong warming during the 20th century.

Regional patterns of climate variability
Regional or local climate is generally much more variable than climate on a hemispheric or global scale because regional or local variations in one region are compensated for by opposite variations elsewhere. Indeed a closer inspection of the spatial structure of climate variability, in particular on seasonal and longer time-scales, shows that it occurs predominantly in preferred large-scale and geographically anchored spatial patterns. Such patterns result from interactions between the atmospheric circulation and the land and ocean surfaces. Though geographically anchored, their amplitude can change in time as, for example, the heat exchange with the underlying ocean changes.

A well-known example is the quasi-periodically varying ENSO phenomenon, caused by atmosphere-ocean interaction in the tropical Pacific. The resulting El Niño and La Niña events have a worldwide impact on weather and climate.

Another example is the North Atlantic Oscillation (NAO), which has a strong influence on the climate of Europe and part of Asia. This pattern consists of opposing variations of barometric pressure near Iceland and near the Azores. On average, a westerly current, between the Icelandic low pressure area and the Azores high-pressure area, carries cyclones with their associated frontal systems towards Europe. However the pressure difference between Iceland and the Azores fluctuates on time-scales of days to decades, and can be reversed at times. The variability of NAO has considerable influence on the regional climate variability in Europe, in particular in wintertime. Chapter 7 discusses the internal processes involved in NAO variability.

Similarly, although data are scarcer, leading modes of variability have been identified over the Southern Hemisphere. Examples are a North-South dipole structure over the Southern Pacific, whose variability is strongly related to ENSO variability, and the Antarctic Oscillation, a zonal pressure fluctuation between middle and high latitudes of the Southern Hemisphere. A detailed account of regional climate variability may be found in Chapter 2.

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