Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 86

The authors believed that astronomical factors and other processes became signifi-
cant in driving glacial-interglacial events only after continental configurations,
gateways, and associated ocean gyres were established.
Gerhard and Harrison (2001) suggested:
''The primary driving force behind [long-term] climate cycling is tectonic,
specifically by controlling distribution of landmasses on the Earth's surface,
which in turn controls the geometry of ocean currents and thus the transfer of
heat around the Earth. When equatorial ocean currents exist, the Earth tends to
be in a greenhouse state. In contrast, when continents exist in positions that
impede or block significant equatorial currents, the Earth tends to be in the
icehouse condition. Transitions between the two states are slow but may be
punctuated by rapid shifts.''
The present distribution of land as a function of latitude is shown in Figure
2.39 . Two thirds of the land area occurs in the Northern Hemisphere. This
undoubtedly is the reason ice sheets form primarily in the NH during ice ages.
The presence of land is necessary to allow the accumulation of ice. In addition,
land responds more readily to seasonal changes than do the oceans. This might
suggest that the level of summer insolation would control the ability of ice fields
to expand or contract.
As M&M said:
''The fundamental reason for this is lack of convection. In the oceans, heat
can convect between depths as well as horizontally, but on land, heat must
Figure 2.39. Land area vs. latitude on the Earth. Two thirds of the landmass is north of the
equator (M&M, p. 189, by permission of Praxis Publishing).