Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 246

6.1.9 Global climates: A summary

outgoing radiation, yet looking back through recorded
history and into the young and then older geological
record, it is obvious that significant and sometimes major
changes have occurred in both regional and global climate.
We first enquire as to whether the global amount, surface
distribution, and greenhouse entrapment of incoming
solar energy have remained constant through geological
time. Given the great importance of the oceans in the cli-
mate machine, we must also enquire as to how known
changes in continent-ocean distributions may have
affected global and regional climate.
There is little direct evidence that the solar constant
changes in response to short-term solar activities like
sunspot cycles but there is circumstantial evidence that
over periods of several hundred years the decreased activ-
ity of sunspots may be reflected in lower solar energy out-
put since such periods are associated with severe global
cooling (e.g. Maunder sunspot minimum and the Little
Ice Age of c .350 years BP ). Variations in the orbital path of
Earth around the Sun and in Earth's own rotation induce
longer-term (10 4 -10 5 yr) changes in the relative solar
energy flux to particular parts of the planetary surface. We
stress the term “relative” because small changes in orbital
parameters lead to no net increase or decrease in solar radi-
ation received: the changes simply tend to apportion the
radiation at different times of the solar cycle in particular
hemispheres. It is this cyclical preferred apportionment
that is thought to lead to longer-term climate change and
the accumulation or melting of great ice-sheets. These
physical changes in the seasonal distribution of incoming
energy cannot of course be measured, but, in one of the
great scientific breakthroughs of the twentieth century,
their indirect climatic effects have been carefully ascer-
tained by sophisticated geochemical studies of Quaternary
marine fossils.
Following the lead of the nineteenth-century amateur
scientist James Croll, Milankovitch in the 1920s and
1930s calculated how variations in the three orbital param-
eters (Fig. 6.12) - eccentricity, wobble, and tilt - would
lead to different amounts of radiation being received at
different latitudes. The key was found in the variation of
radiation received at temperate latitudes during summer.
During winter we know that the polar and high latitudes
are cold enough to form snow and ice; it is the survival of
these seasonal features that will determine whether the ice-
sheets can expand below the Arctic or Antarctic circles.
Thus any orbitally induced changes that encourage sum-
mer cooling by decreasing received radiation should lead
to climate change sufficient to trigger an Ice Age.
The first orbital mechanism is based on the fact
that Earth's rotation around the Sun is elliptical and not

1 Equatorial latitudes are dominated by high insolation, low
pressures, light to variable winds (these are the latitudes of the
“Doldrums”), high mean annual temperatures, low daily
temperature changes, high water vapor saturation pressures,
and copious convective precipitation, particularly in summer
months.
2 Trade wind latitudes extend 15-35

N and S of the
equatorial low-pressure belt. Over land, air masses are dry
and skies clear over the hot deserts, with high radiative
heat transfer and consequent high daily temperature varia-
tions from hot to cold. Over the oceans, evaporation rates
into the initially unsaturated advecting air masses are high.
Hurricanes and typhoons result from instabilities set up
during convective oceanic heating (Section 6.2). The large
and high landmass of southern Asia (also east Africa,
Southeast Asia, northern Australia, and northern Mexico)
develops its own low-pressure system and high latitude jet
stream in summer that attracts and reverses the normal
northeast trade winds, thus initiating torrential summer
monsoonal precipitation.
3 The subtropical high-pressure belt at about latitudes
35-40 N and S is characterized by descending unsaturated

air masses and generally light winds, resulting in
Mediterranean climates with hot dry summers and cool
wetter winters.
4 The mid-latitude to temperate-latitude maritime low-
pressure zones are dominated by the movement of frontal
systems that form at the polar-subtropical transition. These
sweep warmer saturated air north and cooler unsaturated
air south as wave-like intrusions. Plentiful precipitation
results in the cool to cold spring, winter, and autumn sea-
sons. Oceanic currents like the warm north-flowing Gulf
Stream lead to highly important contributions to air tem-
peratures by latent heat released in the rainstorms associ-
ated with frontal systems blowing over them. Continental
areas at these latitudes (40-60 N and S) suffer much larger

temperature extremes and generally lower precipitation.
5 The polar anticyclone presides over a stable regime of
cold to very cold descending dry air masses with very high
albedos over snow- and ice fields under cloudless or perva-
sive “thin” cloudy skies in summer giving high radiative
heat losses to the atmosphere.

6.1.10 Milankovich mechanisms for long-term climate

change

We accept the notion of a mean climate for particular
regions and a net global balance between incoming and