Earth as a Planet: Atmosphere and Oceans 171
cause nonperiodic changes. Over long timescales, plate tec-
tonics and mantle convection significantly alter the moment
of inertia and hence the length of day.
An important and persistent torque that acts on Earth is
the gravitational pull of the Moon and the Sun on the solid
planet’s tidal bulge, which, because of friction, does not line
up exactly with the combined instantaneous tidal stresses.
This torque results in a steady lengthening of the day at the
rate of about 1.4 ms per century and a steady outward drift
of the Moon at the rate of 3.7±0.2 cm/year, as confirmed
by lunar laser ranging. On the top of this steady torque,
it has been suggested that observed 5 ms variations that
have timescales of decades are caused by stronger, irregu-
lar torques from motions in Earth’s liquid core. Calculations
suggest that viscous coupling between the liquid core and
the solid mantle is weak, but that electromagnetic and topo-
graphic coupling can explain the observations. Mountains
on the core–mantle boundary with heights around 0.5 km
are sufficient to produce the coupling and are consistent
with seismic tomography studies, but not much is known
about the detailed topography of the core–mantle bound-
ary. Detailed model calculations take into account the time
variation of Earth’s external magnetic field, which is ex-
trapolated downward to the core–mantle boundary. New
improvements to the determination of the magnetic field
at the surface are enhancing the accuracy of the downward
extrapolations.
Earth’s atmosphere causes the strongest torques of all.
The global atmosphere rotates faster than the solid planet
by about 10 ms−^1 on average. Changes in the global circula-
tion cause changes in the pressure forces that act on moun-
tain ranges and changes in the frictional forces between the
wind and the surface. Fluctuations on the order of 1 ms in
the length of day, and movements of the pole by several
meters, are caused by these meteorological effects, which
occur over seasonal and interannual timescales. General cir-
culation models (GCMs) of the atmosphere routinely cal-
culate the global atmospheric angular momentum, which
allows the meteorological and nonmeteorological compo-
nents of the length of day to be separated. All the variations
in the length of day over weekly and daily timescales can
be attributed to exchanges of angular momentum between
Earth’s atmosphere and the solid planet, and this is likely
to hold for timescales of several months as well. Episodic
reconfigurations of the coupled atmosphere–ocean system,
such as theEl Nino-Southern Oscillation ̃ (ENSO), cause
detectable variations in the length of day, as do changes in
the stratospheric jet streams.
2. Vertical Structure of the Atmosphere
Earth may differ in many ways from the other planets, but
not in the basic structure of its atmosphere (Fig. 1). Plan-
etary exploration has revealed that essentially every atmo-
FIGURE 1 Representative temperature structure for the Earth
(thick solid line) as compared with those of several other planets,
including Jupiter (dash-dot), Saturn (dashed), Uranus (dotted),
and Neptune (solid). For Earth, the altitude scale runs from the
surface to about 130 km altitude. Atmospheres have high
pressure at the bottom and low pressure at the top, so pressure is
a proxy for altitude. Starting at the bottom of Earth’s atmosphere
and moving up, the troposphere, stratosphere, mesosphere, and
thermosphere correspond to layers where temperature
decreases, increases, decreases, and then increases with height,
respectively. The top of Earth’s troposphere, stratosphere, and
mesosphere are at altitudes of about 10–15 km, 50 km, and 100
km, respectively. Note that other planets also generally have
tropospheres and thermospheres, although the details of the
intermediate layers (the stratosphere and mesosphere) differ
from planet to planet.sphere starts at the bottom with atroposphere, where
temperature decreases with height at a nearly constant rate
up to a level called the tropopause, and then has astrato-
sphere, where temperature usually increases with height
or, in the case of Venus and Mars, decreases much less
quickly than in the troposphere. It is interesting to note
that atmospheres are warm both at their bottoms and their
tops, but do not get arbitrarily cold in their interiors. For
example, on Jupiter and Saturn there is significant methane
gas throughout their atmospheres, but nowhere does it get
cold enough for methane clouds to form, whereas in the
much colder atmospheres of Uranus and Neptune, methane
clouds do form. Details vary in the middle-atmosphere
regions from one planet to another, where photochem-
istry is important, but each atmosphere is topped off by
a high-temperature, low-density thermosphere that is sen-
sitive to solar activity and an exobase, the official top of