Timescales and feedbacks

The climate system is complex not just because within it a large number of processes and exchanges occur, but because different components act on widely differing timescales (Fig. 1.22) and interact in highly non-linear ways. The atmosphere, particularly the lower troposphere, has a large diurnal cycle. It reacts to changes in the basic heating source very rapidly. Within a few days transport and mixing can occur over continental scales, and within a few weeks, globally. In contrast, the biosphere's basic cycle is annual, as it responds to the

Fig. 1.22. Schematic of the different processes which affect the climate system, and their timescales.

Solar Plate variation tectonics

Orbital variations

Thermohaline ocean circulation

Changes Solar ENSO Seasonal cycle volcanism cycle

Air-sea interaction

100 million 10 m 1m 100,000 10,000 1,000 100 10

Years change from winter to summer, or, in the tropics, wet season to dry. A diurnal cycle exists, but its significance is much less than in the atmosphere. It represents transitions from activity to dormancy rather than the annual cycle of growth and decay.

The other parts of the climate system act on much longer timescales. The ocean has a weak diurnal signal in its near-surface temperature, but this extends to only a few metres. Even the annual cycle in temperature only penetrates beyond 100 m in polar and mid-latitude regions. Large-scale mixing within the ocean occurs over months to years. Local eddies, the weather systems of the ocean, may exist for over a year; flow about the North Atlantic sub-tropical gyre takes two to three decades. The deep circulation imposes the absolute scale for over-turning and ventilation of the ocean. It may take several hundred, perhaps a thousand, years for some of the water that sank in the Greenland or Labrador Seas to eventually upwell back to the surface in the Pacific Ocean.

The cryosphere changes on two timescales. There is advance and retreat of sea-ice margins in response to the annual cycle in solar radiation. Glaciers and ice sheets take much longer to alter substantially. Large-scale advances and retreats of the major ice caps occur over about a hundred thousand years, with smaller scale perturbations on scales of thousands of years (but see ยง6.2.2). Alpine glaciers tend to change on intermediate scales to these, altering their length by kilometres in tens of years.

The longest timescales occur within the geosphere. Diurnal and seasonal temperature signals only penetrate a few metres, at most, into the soil, although the response at the surface is large and strongly coupled to the incident radiation because of the low specific heat of the top-soil. Underground water movement occurs on these timescales to depths of hundreds of metres. The basic exchange of elements in the various cycles, however, occurs over hundreds of millions of years. Continents move at millimetres per year, creating new landforms in tens of millions of years.

These enormous differences in timescale are very important for the climate system. The atmosphere reacts quickly to change, but the other components act as buffers, restraining change. This buffering can delay unwanted alteration, but it also slows transitions to less extreme states.

There are also feedbacks within the system. We have seen how some components interact; for example, the biosphere and the atmosphere. However, these interactions, or changes within a component, may create conditions favourable, or unfavourable, to such processes continuing. This accentuation, or damping, of processes is known as positive, or negative, feedback. An illustration of positive feedback you have probably experienced is the magnification of noise that can occur in a broadcasting system when too much sound from loudspeakers enters the microphone. This can create a build-up of sound quickly as the

Fig. 1.23. Schematic of some of the many processes involved in the coupling of clouds to climate.

Fig. 1.23. Schematic of some of the many processes involved in the coupling of clouds to climate.

signal continually feeds from the speakers to the microphone in ever-increasing volume, until the microphone is redirected to break the sound conveyor belt.

An example in the atmosphere of such a positive feedback is contained within the hydrological cycle. As water is evaporated the water vapour adds to the greenhouse effect, warming the atmosphere. This warming promotes further evaporation from the land or sea, and the cycle continues, producing greater and greater warmth. Eventually, as in most positive feedback processes, another process reacts to this amplification by dampening, and finally reversing, the initial mechanism. In the cycle just described negative feedback mechanisms are linked to the growth of clouds. Clouds will form when enough water vapour has accumulated in the atmosphere. These are powerful reflectors of solar radiation, thus preventing much primary solar fuel from entering the positive feedback process. The water vapour/cloud interactions are even more extensive than the simple example just discussed. They are extremely complex, and not fully understood. Fig. 1.23 conveys an impression of this complexity.

Another area in which feedbacks are strong is in the links with the biosphere. It is only just becoming clear how extensive these are, in both a positive and negative sense. Chapter 4 will examine some of these newly discovered interactions.

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