Geoscience Reference
In-Depth Information
Eccentricity
(A)
(B)
P
ERIODICIT
Y
c.
100,000 yrs
Minimum
0.06
Present
0.05
Maximum
0.04
0.03
a
b
SUN
EARTH
0.02
0
.
017
(a
2
- b
2
)
E =
0.01
a
0.00
500
400
300
200
100
Present
Years B.P. (thousands)
Obliquity
(C)
(D)
21.8° Min.
24.4° Max.
PERI
ODIC
ITY
c.
41-43,000 yrs
25°
24°
2
3
.5°
23°
Sun
Earth
22°
21°
500
400
300 200
Years B.P. (thousands)
100
Present
Precession
(E)
(F)
PERIODICITIES
c.
23
,00
0 yrs
10
c.
18,000 yrs
5
0
Sun
Earth
-5
-10
500
400
300 200
Years B.P. (thousands)
100
Present
Figure 13.3
Summary of astronomical (orbital) effects on solar irradiance and their relevant timescales
over the past 500,000 years. A and B: Eccentricity or orbital stretch; C and D: Obliquity or axial tilt; E and
F: Precession or axial path wobble.
Sources: Partly after Broecker and Van Donk 1970, and Henderson-Sellers and McGuffie 1984. B, D and F: from Review of
Geophysics and Space Physics8 (1970). Reproduced by kind permission of the American Geophysical Union.
fluctuations in solar irradiance (i.e., a depar-
ture from the solar constant; in terms of
radiation receipts globally averaged over the
top of the atmosphere, the effective value is
only 0.25W m
-2
) (see below and Chapter 3A.1).
Effects on ultraviolet radiation are proportion-
ally larger in terms of percent change. There
is also evidence for longer-term variations.
Intervals when sunspot and solar flare activity
were much reduced (especially the Maunder
Minimum of AD 1645-1715) may have been
associated with global temperature decreases of
about 0.5
twentieth century, when anthropogenic effects
became dominant. Turning to the distant past,
it is known that solar irradiance three billion
years ago (during the Archean) was about 80
percent of the modern value. Interestingly, 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,
and perhaps also by the effects of a largely
water-covered earth (meaning lots of water
vapor in the atmosphere).
•
Volcanic eruptions
. Major individual explosive
eruptions inject dust and sulfur gases (espe-
cially sulfur dioxide) into the stratosphere,
the latter forming sulfuric acid droplets.
C. Solar variability also seems to
have played a role in decadal-scale variations of
global temperature until the latter part of the
°
