end of the Cretaceous period (65 Myr BP) is
thought to have had that effect and initiated the
extinction of the dinosaurs.
It is also possible that the output of solar
radiation from the sun varies through time, possibly in a cyclic manner. Some good evidence
derived from sunspot activity shows a pattern of
11- and 22-year cycles.
What has become more certain in recent
years, however, is that the amount of radiation
received at the Earth’s surface through time has
varied as a consequence of the Earth’s everchanging position relevant to the sun. This is
called the Milankovitch cycle after its discoverer.
The basic idea is that there are three ways in
which the Earth’s position varies (Fig. 10.4).
First, the Earth’s orbit around the sun is not a
perfect circle but an ellipse (a). This orbital
eccentricity results in approximately 3.5 % variation in the total amount of solar radiation
received. Second, the tilt of the Earth’s axis of
rotation varies (b). And third, there is a mechanism which is based on the fact that the time of
the year at which the Earth is nearest the sun
varies (c). At times, the northern hemisphere is
closest to the sun in winter; other times it is
closest to the sun in summer. The reason for
this is that the Earth wobbles like a slowing top
and swivels its axis around.
The climatic effect of these cycles is a variation in the degree of contrast between summer
and winter temperatures. When the contrast
between seasons is comparatively slight, summer
temperatures are not high enough for the previous winter’s snow and ice to melt. Snow and ice
accumulate, building up huge continental ice
sheets in temperate latitudes. During another
phase of the cycle when there are high summer
temperatures, ice melts before the onset of the
succeeding winter: the result is the advent of an
interglacial period, such as the one now
approaching (Fig. 10.5).
10.2 Use of the Ko ¨ ppen Climate
Classification to Detect Climate
Change
The Intergovernmental Panel on Climate Change
science report (IPCC 2007) represents the consensus view on greenhouse-induced climatic
changes expressed by the overwhelming majority of atmospheric scientists throughout the
world. The reported equilibrium changes for
Fig. 10.3 Suggested
variability of the Earth’s
climatic zones over the last
few hundred thousand
years. From Fairbridge
1963
10.2 Use of the Ko ¨ ppen Climate Classification to Detect Climate Change
97
thought to have had that effect and initiated the
extinction of the dinosaurs.
It is also possible that the output of solar
radiation from the sun varies through time, possibly in a cyclic manner. Some good evidence
derived from sunspot activity shows a pattern of
11- and 22-year cycles.
What has become more certain in recent
years, however, is that the amount of radiation
received at the Earth’s surface through time has
varied as a consequence of the Earth’s everchanging position relevant to the sun. This is
called the Milankovitch cycle after its discoverer.
The basic idea is that there are three ways in
which the Earth’s position varies (Fig. 10.4).
First, the Earth’s orbit around the sun is not a
perfect circle but an ellipse (a). This orbital
eccentricity results in approximately 3.5 % variation in the total amount of solar radiation
received. Second, the tilt of the Earth’s axis of
rotation varies (b). And third, there is a mechanism which is based on the fact that the time of
the year at which the Earth is nearest the sun
varies (c). At times, the northern hemisphere is
closest to the sun in winter; other times it is
closest to the sun in summer. The reason for
this is that the Earth wobbles like a slowing top
and swivels its axis around.
The climatic effect of these cycles is a variation in the degree of contrast between summer
and winter temperatures. When the contrast
between seasons is comparatively slight, summer
temperatures are not high enough for the previous winter’s snow and ice to melt. Snow and ice
accumulate, building up huge continental ice
sheets in temperate latitudes. During another
phase of the cycle when there are high summer
temperatures, ice melts before the onset of the
succeeding winter: the result is the advent of an
interglacial period, such as the one now
approaching (Fig. 10.5).
10.2 Use of the Ko ¨ ppen Climate
Classification to Detect Climate
Change
The Intergovernmental Panel on Climate Change
science report (IPCC 2007) represents the consensus view on greenhouse-induced climatic
changes expressed by the overwhelming majority of atmospheric scientists throughout the
world. The reported equilibrium changes for
Fig. 10.3 Suggested
variability of the Earth’s
climatic zones over the last
few hundred thousand
years. From Fairbridge
1963
10.2 Use of the Ko ¨ ppen Climate Classification to Detect Climate Change
97