Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 84

areas, but clouds add significantly to the global average albedo. The present day
global average albedo is estimated to be roughly 0.3. During epochs when there
are landmasses at high latitudes or at the poles, the potential for glaciation of
polar areas increases significantly since snow falling on land can accumulate.
This can increase the overall average albedo, leading to further cooling. Thus,
landmasses at high latitudes are widely believed to be conducive to colder climates.
Conversely, when most of the continents are at tropical or mid-latitudes, the accu-
mulation of snow and ice will be constrained and the albedo of the Earth will be
smaller. This will induce warming, and the lack of polar ice indicates that the
oceans will be higher. Thus, continental margins will be flooded and the area of
exposed continents will decrease (i.e., land area is converted to water area). This
will decrease the global albedo further, producing more warming. Hence the
occurrence of landmasses at polar or moderate latitudes promotes global cooling
or warming, respectively.
While landmasses in polar areas are ideal sites for ice sheet formation, the
total heat balance of the Earth is determined by how much solar energy gets
absorbed. Since the preponderance of solar energy input to the Earth is in the
tropics (where solar energy per unit area is a maximum, and land area per unit
latitude is also a maximum) absorption of solar energy in the tropics is of para-
mount importance. Since land has a much higher albedo than water, an unusual
preponderance of landmasses within middle to low latitudes will be conducive to
global glaciation. Indeed, such a continental distribution occurred some 600 mybp ,
and may have contributed to formation of a snowball Earth. This situation has
not been encountered at any time subsequent to that period. Any resultant
glaciation would further increase the Earth's albedo by lowering sea level, expos-
ing continental shelves. Additional continents in the tropics would also increase
the silicate weathering rate, thus reducing the atmospheric CO 2 concentration. It
was suggested that these combined effects might lead to the growth of large ice
caps, nucleated on islands or continents bordering the polar seas (Kirschvink,
2002).
Burrett (1982) carried out paleocontinental reconstructions for the period 570
to 200 mybp and made rough estimates of land distribution amongst deserts,
forests, etc. in order to estimate the albedo of the Earth. His goal was to test how
geographical placement of land and overall global albedo affected paleoclimates.
He did not find any obvious correlations of land placement and albedo with the
onset of glaciation and suggested that the issues are more complex. It seems likely
that merely having a landmass at one pole without major barriers to overall ocean
flow, may not lead to glaciation.
Another important factor in determining the global climate is the network of
pathways for ocean currents to transport heat. When the polar areas are openly
exposed to ocean currents from equatorial zones, heat is eciently transported to
polar areas, thus reducing glaciation, raising the oceans, and warming the planet.
When the polar areas are thermally isolated from equatorial zones, they are more
likely to freeze over, thus cooling the planet. The presence of a wide network of
mid-latitude landmasses can obstruct transport of heat to polar areas.