Geoscience Reference
In-Depth Information
Table 13.1
Orbital forcings and characteristics
Element
Index range
Present value
Average periodicity
Obliquity of Ecliptic (´)
22-24.5°
23.4°
41 ka
(Tilt of axis of rotation)
Effects equal in both hemispheres,
effect intensifies poleward (for caloric seasons)
Low ´
High ´
Weak seasonality,
Strong seasonality,
steep poleward
more summer
radiation gradient
radiation at poles,
weaker radiation
gradient
Precession of Equinox (v)
0.05 to -0.05
0.0164
19, 23 ka
(Wobble of axis of rotation)
Changing earth-sun distance alters
seasonal cycle structure; complex effect,
modulated by eccentricity of orbit
Eccentricity of Orbit (e)
0.005 to 0.0607
0.0167
410, 95 ka
Gives 0.02% variation in annual incoming
radiation; modifies amplitude of precession
cycle changing seasonal duration and
intensity; effects opposite in each
hemisphere; greatest in low latitudes
Equatorial eruption plumes spread into both
hemispheres, whereas plumes from eruptions
in mid-to high latitudes are confined to that
hemisphere. Observational evidence from the
past 100 years demonstrates that major erup-
tions can be associated with global averaged
cooling of several tenths of a degree C in the
year following the event and much larger
changes on a regional to hemispheric basis.
The cooling is primarily from the sulfuric acid
droplets which reflect solar radiation. Dust also
causes surface cooling by absorbing solar
radiation in the stratosphere, but compared to
the sulfuric acid these effects are short-lived
(weeks to months) Stratospheric aerosols may
also cause brilliant sunsets (see
Figure 2.12
).
The most recent major volcanic eruption with
significant climate impacts was Mt. Pinatubo
in 1991.
•
Human-induced changes in atmospheric compo-
sition and land cover
. The effect of greenhouse
gases such as carbon dioxide and methane on
the radiation budget has already been
introduced (see also Chapter 2). The observed
buildup of these gases since the dawn of the
industrial age represents a positive forcing.
Human activities have also led to a buildup of
tropospheric aerosols, which induce a partly
compensating cooling. Changes in land use
and land cover have also led to a small increase
in surface albedo that promotes cooling.
While the common feature of all of these
forcings is that they influence aspects of the earth's
radiation budget, they are obviously distinguished
in large part by the timescales at which they
operate. In terms of inducing global temperature
change over the past 100 years, as well as changes
