The source of the energy injected into our atmosphere is the sun, which is continually shedding part of its mass by radiating waves of electromagnetic energy and high-energy particles into space. This constant emission represents all the energy available to the earth (except for a small amount emanating from the radioactive decay of earth minerals). The amount of energy received at the top of the atmosphere is affected by four factors: solar output, the sun-earth distance, the altitude of the sun, and day length.
1 Solar output
Solar energy originates from nuclear reactions within the sun's hot core (16 X 106K), and is transmitted to the sun's surface by radiation and hydrogen convection. Visible solar radiation (light) comes from a 'cool' (~6000 K) outer surface layer called the photosphere. Temperatures rise again in the outer chromosphere (10,000 K) and corona (106 K), which is continually expanding into space. The outflowing hot gases (plasma) from the sun, referred to as the solar wind (with a speed of 1.5 X 106 km hr-1), interact with the earth's magnetic field and upper atmosphere. The earth intercepts both the normal electromagnetic radiation and energetic particles emitted during solar flares.
The sun behaves virtually as a black body; i.e. it absorbs all energy received and in turn radiates energy
Main Atmospheric Absorption Bands n
2000 1000 500
Main Atmospheric Absorption Bands n
2000 1000 500
10 20 50
10 20 50
Figure 3.1 Spectral distribution of solar and terrestrial radiation, plotted logarithmically, together with the main atmospheric absorption bands. The cross-hatched areas in the infra-red spectrum indicate the 'atmospheric windows' where radiation escapes to space. The black-body radiation at 6000 K is that proportion of the flux which would be incident on the top of the atmosphere. The inset shows the same curves for incoming and outgoing radiation with the wavelength plotted arithmetically on an arbitrary vertical scale.
Source: Mostly after Sellers (1965).
at the maximum rate possible for a given temperature. The energy emitted at a particular wavelength by a perfect radiator of given temperature is described by a relationship due to Max Planck. The black-body curves in Figure 3.1 illustrate this relationship. The area under each curve gives the total energy emitted by a black body (F); its value is found by integration of Planck's equation, known as Stefan's Law:
where o = 5.67 X 10-8W m-2K-4(the Stefan-Boltzmann constant), i.e. the energy emitted is proportional to the fourth power of the absolute temperature of the body (T).
The total solar output to space, assuming a temperature of 5760 K for the sun, is 3.84 X 1026W, but only a tiny fraction of this is intercepted by the earth, because the energy received is inversely proportional to the square of the solar distance (150 million km). The energy received at the top of the atmosphere on a surface perpendicular to the solar beam for mean solar distance is termed the solar constant (see Note 1). Satellite measurements since 1980 indicate a value of about 1366 W m -2, with an absolute uncertainty of about ±2 W m -2. Figure 3.1 shows the wavelength range of solar (short-wave) radiation and the infra-red (longwave) radiation emitted by the earth and atmosphere. For solar radiation, about 7 per cent is ultraviolet (0.2-0.4 |m), 41 per cent visible light (0.4-0.7 |m) and 52 per cent near-infra-red (>0.7 |m); (1 |m = 1 micrometre = 10-6 m). The figure illustrates the black-body radiation curves for 6000 K at the top of the atmosphere (which slightly exceeds the observed extraterrestrial radiation), for 300 K, and for 263 K. The mean temperature of the earth's surface is about 288 K (15°C) and of the atmosphere about 250 K (-23°C). Gases do not behave as black bodies, and Figure 3.1 shows the absorption bands in the atmosphere, which cause its emission to be much less than that from an equivalent black body. The wavelength of maximum emission (X ) varies inversely with the absolute
temperature of the radiating body:
Thus solar radiation is very intense and is mainly shortwave between about 0.2 and 4.0 |m, with a maximum (per unit wavelength) at 0.5 |m because T ~ 6000 K. The much weaker terrestrial radiation with T ~ 280 K has a peak intensity at about 10 |m and a range from about 4 to 100 |m.
The solar constant undergoes small periodic variations of just over 1 Wm-2 related to sunspot activity. Sunspot number and positions change in a regular manner, known as sunspot cycles. Satellite measurements during the latest cycle show a small decrease in solar output as sunspot number approached its minimum, and a subsequent recovery. Sunspots are dark (i.e. cooler) areas visible on the sun's surface. Although sunspots are cool, bright areas of activity known as faculae (or plages), that have higher temperatures, surround them. The net effect is for solar output to vary in parallel with the number of sunspots. Thus the solar 'irradiance' decreases by about 1.1 Wm-2 from sunspot maximum to minimum. Sunspot cycles have wavelengths averaging 11 years (the Schwabe cycle, varying between 8 and 13 years), the 22-year (Hale) magnetic cycle, much less importantly 37.2 years (18.6 years - the luni-solar oscillation), and 88 years (Gleissberg). Figure 3.2 shows the estimated variation of sunspot activity since 1610. Between the thirteenth and eighteenth centuries sunspot activity was generally low, except during ad 1350-1400 and 1600-1645. Output within the ultraviolet part of the spectrum shows considerable variability, with up to twenty times more ultraviolet radiation emitted at certain wavelengths during a sunspot maximum than a minimum.
How to translate sunspot activity into solar radiation and terrestrial temperatures is a matter of some dispute. It has been suggested that the sun is more active when the sunspot cycle length is short, but this is disputed.
However, anomalies of temperature over northern hemisphere land areas do correlate inversely with cycle length between 1860 and 1985. Prolonged time-spans of sunspot minima (e.g. ad 1645-1715, the Maunder Minimum) and maxima (e.g. 1895-1940 and post-1970) produce measurable global cooling and warming, respectively. Solar radiation may have been reduced by 0.25 per cent during the Maunder Minimum. It is suggested that almost three-quarters of the variations in global temperature between 1610 and 1800 were attributable to fluctuations in solar radiation and during the twentieth century there is evidence for a modest contribution from solar forcing. Shorter term relationships are more difficult to support, but mean annual temperatures have been correlated with the combined 10 to 11 and 18.6-year solar cycles. Assuming that the earth behaves as a black body, a persistent anomaly of 1 per cent in the solar constant could change the effective mean temperature of the earth's surface by as much as 0.6°C. However, the observed fluctuations of about 0.1 per cent would change the mean global temperature by <0.06°C (based on calculations of radiative equilibrium).
2 Distance from the sun
The annually changing distance of the earth from the sun produces seasonal variations in solar energy received by the earth. Owing to the eccentricity of the earth's orbit around the sun, the receipt of solar energy on a surface normal to the beam is 7 per cent more on
3 January at the perihelion than on 4 July at the aphelion (Figure 3.3). In theory (that is, discounting the interposition of the atmosphere and the difference in degree of conductivity between large land and sea masses), this difference should produce an increase in the effective January world surface temperatures of about 4°C over those of July. It should also make northern winters warmer than those in the southern hemisphere, and southern summers warmer than those in the northern hemisphere. In practice, atmospheric heat circulation and the effects of continentality mask this global tendency, and the actual seasonal contrast between the hemispheres is reversed. Moreover, the northern summer half-year (21 March to 22 September) is five days longer than the austral summer (22 September to 21 March). This difference slowly changes; about 10,000 years ago the aphelion occurred in the northern hemisphere winter, and northern summers received 3 to max
Figure 3.2 Yearly sunspot numbers for the sun's visible surface for the period 1700 to 2000.
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