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Chapter 1 Scientific Basis of Global Climate Change

III. Changes in Earth’s Orbital Parameters

Changes in Earth’s orbital parameters, including eccentricity, obliquity, and precession,
constitute key natural processes shaping long-term climate patterns. These cyclical parame-
ter variations profoundly influence global climate evolution by modulating solar radiation re-
ceived by Earth, forming what are known as “Milankovitch Cycles.” These periodic changes
not only reveal the long-term fluctuation patterns of Earth’s climate but also provideimport-
ant scientific basis for understanding the alternation between glacial periods andinterglacial
periods.
The ellipticity of Earth’s orbit, known as eccentricity, is one of the crucial parameters
affecting the amount of solar radiation Earth receives. Earth’s orbital path around the Sun is
not a perfect circle but slightly elliptical, with its eccentricity fluctuating between 0 (circular)
and 0.06 (highly elliptical) over a cycle of approximately 100,000 years. These changes in
eccentricity are primarily caused by gravitational interactions with other planets in the solar
system. When eccentricity is higher, the distance difference between Earth’s perihelion (clos-
est to the Sun) and aphelion (farthest from the Sun) becomes more pronounced, resulting
in a substantial increase in solar radiation received at perihelion and a significant decrease
at aphelion. This disparity profoundly impacts global climate. For instance, during periods
of higher eccentricity, increased solar radiation in the Northern Hemisphere summer (often
coinciding with perihelion) may trigger the melting of ice sheets, leading to interglacial pe-
riods. Conversely, during periods of lower eccentricity, the relatively uniform distribution of
solar radiation across the globe may contribute to the formation of ice ages.
The tilt of Earth’s rotational axis, known as axial tilt or obliquity, is another crucial
parameter influencing climate. The axial tilt varies between 22.1° and 24.5° with a cycle of
approximately 41,000 years. Changes in axial tilt directly affect the distribution of solar radi-
ation across different latitudes. When the axial tilt is greater, the seasonal migration range of
the Sun’s direct rays between northern and southern latitudes expands, leading to increased
solar radiation in high-latitude regions during summer and decreased radiation during winter.
This variation facilitates ice sheet melting, potentially triggering interglacial periods. Con-
versely, when the axial tilt is smaller, the migration range of the Sun’s direct rays contracts,
resulting in more uniform solar radiation distribution in high-latitude regions that may con-
tribute to glacial formation. For instance, during the Quaternary glacial periods, reduced axi-
al tilt was considered one of the key drivers of ice sheet expansion.
Earth’s precession, also called axial precession, refers to the gradual rotational wobble
of Earth’s axis, analogous to the oscillation of a spinning top. This precessional motionThis
motion has a cycle of approximately 26,000 years, primarily caused by the gravitational in-
teractions of the Moon and Sun. Precession causes Earth’s perihelion and aphelion to shift
gradually along the orbital path, thereby altering the amount of solar radiation received
during different seasons. For instance, when Northern Hemisphere summer coincides with
perihelion, the increased solar radiation received may trigger ice sheet melting and intergla-

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