Geoscience Reference
In-Depth Information
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 percent during
the Maunder Minimum. It is suggested that
almost three-quarters of the variations in global
temperature between 1610 and 1800 were attrib-
utable 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 10-11 and 18.6-
year solar cycles. Assuming that the earth behaves
as a black body, a persistent anomaly of 1 percent
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 fluctua-
tions of about 0.1 percent would change the
mean global temperature by
°
C (based on
calculations of radiative equilibrium).
0.06
°
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 percent 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 3-4 percent more radiation than today
(
Figure 3.3B
). This same pattern will return about
10,000 years from now.
Figure 3.4
graphically illustrates the seasonal
variations of energy receipt with latitude. Actual
amounts of radiation received on a horizontal
°
Plate 3.1
A Hydrogen-Alpha image of the sun on
22 January 1990 from the spectroheliograph at the
Observatoire de Paris. The bright, active areas are
termed plages and the dark bands are filaments
related to magnetic fields (sunspots are apparent
only in visible light).
Source: Courtesy of the National Geophysical Data Center,
NOAA.
