Climate forcing and feedbacks

The average state of the climate system is controlled by a combination of forcings external to the system (solar variability, astronomical effects, tectonic processes and volcanic eruptions), internal radiative forcings (atmospheric composition, cloud cover), anthropogenically induced changes (in atmospheric composition, surface land cover) and feedback effects (such as changes in atmospheric water vapour content or cloudiness caused by global temperature changes). It is useful to try and assess the magnitude of such effects, globally and regionally, and the time scales over which they operate.

1 External forcing

Solar variability. The sun is a variable star, and it is known that early in the earth's history (during the Archean three billion years ago) solar irradiance was about 80 per cent of the modern value. Paradoxically, however, the effect of this 'faint early sun' was offset, most likely, by a concentration of carbon dioxide that was perhaps 100 times higher than now, but also perhaps by the effects of a largely water-covered earth. The approximately 11-year solar cycle (and 22-year magnetic field cycle) is well known. Intervals when sunspot and solar flare activity were much reduced (especially the Maunder minimum of AD 1650-1700) may have caused cumulative effects leading to temperature decreases of about 1°C.

Tectonic processes. On geological time scales, there have been great changes in continental positions and sizes and in the configuration of ocean basins as a result of crustal processes (known as plate tec­tonics). These movements have also altered the size and location of mountain ranges and plateaus. As a result, the global circulation of the atmosphere and the pattern of ocean circulation and surface currents have also been modified. Changes in con­tinental location have contributed substantially to major ice age episodes (such as the Permo-Carboniferous glaciation of Gondwanaland) as well as to intervals with extensive arid (Permo-Triassic) or humid (coal deposits) environments during other geological periods. Over approximately the last few million years, the uplift of the Tibetan Plateau and the Himalayan ranges has caused the onset, or inten­sification, of desert conditions in western China and Central Asia.

Astronomical periodicities. The earth's orbit around the sun is subject to long-term variations. There are three principal effects on incoming solar radiation: the eccentricity (or stretch) of the orbit, with a period of approxi­mately 95,000 years and 410,000 years; the tilt of the earth's axis (an approximately 41,000-year period); and a wobble in the earth's axis of rota­tion, which causes changes in the timing of perihe­lion. This precessional effect has a period of about 21,000 years. Volcanic eruptions. Major explosive eruptions inject dust and sulphur dioxide aerosols into the stratos­phere, where they may circle the earth for several years causing brilliant sunsets. Equatorial eruption plumes spread into both hemi­spheres, whereas plumes from eruptions in mid to high latitudes are confined to that hemisphere. Records of such eruptions are preserved in the Antarctic and Greenland ice sheets for at least the last 150,000 years. Observational evidence from the last 100 years demonstrates that major eruptions cause a hemisphere/global cooling of 0.5-1.0°C in the year following the event.

Atmospheric composition. The greenhouse effect has been examined in Chapter 1, but it is appro­priate to recall that there is a large 'natural' green­house, as a result of the atmospheric composition, distinct from human-induced changes over the last few centuries. Glacial-interglacial changes in terres­trial vegetation and in the oceanic uptake of trace gases, as a result of changes in the thermohaline circulation of the global ocean, have caused major fluctuations in atmospheric carbon dioxide (±50 ppm) and methane (±150 ppb). Negative (positive) excursions are associated with cold (warm) intervals. The changes in greenhouse gases (C02 and CH4) and global temperatures are virtually coincident during both glacial and interglacial transitions, so that there is no clear causative agent. Both the long- term and rapid changes in atmospheric C02 seen in polar ice cores seem to result from the combined effects of ocean and land biological activity and ocean circulation shifts.

Rates of change. Obviously, changes in climate resulting from changes in the earth's geography through geological processes (e.g. position and size of ocean basins, continents and mountain ranges) are only perceptible on time scales of millions of years. Although geographical changes have had immense paleoclimatic significance, they are of less immediate concern to contemporary climatologists than the radiative forcing agents. Radiative forcing agents affect the supply and disposition of solar radiation. Solar radiation changes, like the non- radiative forcing agents, are external inputs into the atmosphere-earth-ocean-ice system but, unlike them, occur at a range of time scales from tens to hundreds of thousands and, probably, millions of years. Thus solar radiation is both a long-term and a short-term external forcing agent. Astronomical forcings give rise to global temperature fluctuations of ±2-5°C per 10,000 years. The timing of orbital forcing is also clearly represented in glacial-inter-glacial fluctuations with major glacial cycles span­ning about 100,000 years (or 100 Ka). However, the most striking fact to emerge from analysis of two recent deep ice cores in central Greenland is the great rapidity of large changes in atmospheric tempera­ture, precipitation and aerosol levels, presumably as a result of major readjustments of atmospheric cir­culation. The onset and termination of the Younger Dryas cold episode 12,900-11,600 bp (before pre­sent, with a switch from glacial to interglacial conditions and back again, apparently occurred within a five-year time interval for both transitions!

Упражнение 2.

Прочитайте следующие слова и определите их соответствия
в русском языке:

Combination, astronomical, volcanic, history, paradoxically, concentration, activity, minimum, position, configuration, episode, interval, period, rotation, aerosol, brilliant, demonstrate, result, negative, virtually.

Упражнение 3.

Найдите в тексте Climate forcing and feedbacks слова с суффиксами ~tion, ~ment, ~ty, ~ness, ~ance, ~ence, ~sion. Определите, какой частью речи они являются и что означают.

Упражнение 4.

В правой колонке найдите русские эквиваленты следующих английских словосочетаний:

Упражнение 5.

Заполните, где это возможно, таблицу, вставив недостающие части речи.

Упражнение 6.

Прочитайте следующие выражения:

AD 1650-1700; 410,000 years; ±50 ppm; CH4; 80%; 150 times.

Упражнение 7.

Подберите определение к каждому словосочетанию.

Упражнение 8.

Прочитайте текст и найдите в нем ответы на следующие вопросы:

1. Каковы внутренние климатообразующие факторы?

2. Каковы последствия роста глобальной температуры?

3. Какова роль облачного покрова с точки зрения механизма обратной связи?

(Контрольное время – 7 минут)