Geoscience Reference
In-Depth Information
approximately 1.5 K cooler than today from about 1560 to1850 due to large
amounts of volcanic activity, with significant stratospheric injection of aero-
sols, and reduced solar activity (
chapter 4)
. Cooling was not sufficient for ice
sheets to grow, but more sea ice formed in the North Atlantic and mountain
glaciers in Europe advanced. Long winters shortened the growing season by up
to 2 months. The resulting malnutrition accompanied by wet summers favored
the spread of disease such as the bubonic plague, which killed more than one-
third of Europeans.
Changes in the solar constant on millennial time scales force the
glacial/
interglacial oscillation
, that is, the variation of global climate between colder
and warmer climate states that is characteristic of the last 2.6 million years (the
Quaternary
geologic period). These variations in insolation are attributed to
regular, predictable changes in the earth's orbital parameters according to the
“astronomical theory of climate change,” or the
Milankovich theory
in honor
of the Serbian mathematician who developed this idea. The earth's orbit about
the sun is perturbed by the gravitational pull of other planets, especially Jupiter
and Saturn.
The following three attributes of the earth's orbit around the sun change
with time:
• The eccentricity,
e
, of the earth's orbit changes with a 100,000-year
periodicity, from a nearly circular orbit (
e
0.005) to a more elongated orbit
(
e
0.061). Today's value is
e
0.012, a relatively small value.
• The tilt between the earth's axis of rotation and the plane of its orbit varies
with a period of 41,000 years. Today it is 23.5°, but it ranges between 21.8°
and 24.4°.
• The precession of the equinox changes the season during which the earth
is closest to the sun (perihelion) over a 23,000-year cycle. Today, perihelion
occurs during Northern Hemisphere winter, while 11,500 years ago
perihelion occurred during Northern Hemisphere summer. In today's orbital
configuration, the earth system intercepts 6% more solar energy in January
than in July. When the earth's orbit is at its maximum eccentricity, this
difference is as large as 30%.
These changes in the amount and distribution of solar energy fueling the
earth system influence climate. For example, low values of the tilt favor cooler
summers in the Northern Hemisphere. Because most of the land mass has been
located in the Northern Hemisphere throughout the Quaternary, low tilt is as-
sociated with ice age climates—continental ice sheets must survive the summer
months if glaciers are to grow.
The periodicities with which the orbital parameters change (23,000, 41,000,
and 100,000 years) are clearly found in the geologic record, but the relation-
ships between the orbital parameters and climate characteristics are not simple
or synchronous. For example, the peak of the last ice age was about 21,000
years ago when the orbital parameters were similar to those of today.
On even longer time scales, climate change forcing factors include the distri-
bution of the continents, orographic uplift, the sun's evolution (
chapter 4
has
an example), and the evolution of the atmosphere's chemical composition (sec-
tion 2.5). The difference between the present-day globally averaged surface air
