Лекция: Energy budgets
The key to these atmospheric variations must be linked to the heat balance of the earth-atmosphere system and this addresses us to the fundamental energy considerations.
The evidence for fluctuations greater than 0.1 per cent in the 'solar constant' is inconclusive, although significant variations apparently do occur in the emission of high-energy particles and ultraviolet radiation during brief solar flares. All solar activity follows the well-known cycle of approximately eleven years, which is usually measured with reference to the period between sunspot maximum and minimum, but numerous attempts to establish secure correlations between sunspot activity and terrestrial climates have produced mostly negative results. Nevertheless, a statistical relationship has been found between the occurrenceof drought in the western United States over the last 300 years and the approximately 22-year double(Hale) cycle of the reversal of the solar magnetic polarity. Drought areas are most extensive in the two to five years following aHalesunspot minimum (i.e. alternate eleven-year sunspot minima).
Changes in atmosphericcomposition may also have modifiedthe atmospheric heat budget. The presence of increased amounts of volcanicdust and sulphate aerosols in the stratosphere is one suggested cause of the 'Little IceAge'. Major eruptions can result in a surface cooling of perhaps 0.2°C for a few years after the event. Hence, frequent volcanic activity would be required for persistently cooler conditions. Conversely, it is suggested thatreduced volcanic activity after 1914 may have contributed in part to the early twentieth-century warming. New interest in this question has been aroused by eruptions of El Chichon (March 1982) and Mount Pinatabo (June 1991). It has been estimated that huge volcanic eruptions such as these, can, during a decade, produce a forcing effect onglobal temperature about one-third as great as that exerted by greenhouse gases — but in the opposite direction(i.e. to produce surface cooling). The role of low-level aerosols is also complex. These originate naturally, from wind-blown soil and silt for example, as well as from atmospheric pollution due to human activities (industry, domestic heating and modern transportation).
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