Geoscience Reference
In-Depth Information
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 (λ
max
) varies inversely with the absolute
temperature of the radiating body:
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 relation-
ships 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
2897
λ
max
= --—— 10
-6
m (Wien's Law)
T
Thus solar radiation is very intense and is mainly short-
wave between about 0.2 and 4.0 µm, with a maximum
(per unit wavelength) at 0.5 µm because
T
~ 6000 K.
The much weaker terrestrial radiation with
T
280 K
has a peak intensity at about 10 µm and a range from
about 4 to 100 µm.
The solar constant undergoes small periodic vari-
ations of just over 1 Wm
-2
related to sunspot activity.
Sunspot number and positions change in a regular
manner, known as sunspot cycles. Satellite measure-
ments 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.
≈
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 inter-
position 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
