The astronomical theory and data compared - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 313

land (forest, desert, plains, etc.), but on average it is probably something like 0.35.
The albedo of the oceans is probably about 0.1. The distribution and character of
landmasses on the Earth are likely to have a profound effect on climate, affecting
greenhouse gas concentrations, ocean currents, and world average albedo. In
addition, landmasses in near-polar areas provide foundations for building ice
sheets. Over long time periods, continental drift changes the distribution of land-
masses, leading to variations in the Earth's climate. Mountain building provides
sites for glaciers to form and affects wind patterns.
The Earth is about 4.6 billion years old. During its history the Earth has
probably passed through a number of very marked climate changes ranging from
a snowball Earth in which the entire Earth was covered in a blanket of ice and
snow to a hothouse Earth when all glacial ice melted and the polar areas were
tropical.
Prior to about 34 mybp the Earth was much warmer than it is today. About
34 million years ago, the Earth entered a significant cooling trend and the East
Antarctic Ice Sheet (EAIS) began to form. The great ice sheets over Antarctica
gradually evolved over the ensuing millions of years. One theory is that this
occurred because of a series of movements in Earth's major tectonic plates.
Another theory is that the timing of ice sheet growth in Antarctica coincided with
sea-floor spreading, which pushed Antarctica away from Australia and South
America. The opening of these ocean gateways produced a strong circumpolar
current in the Southern Ocean that is thought to have thermally isolated the
Antarctic continent, cooling it to a level where an ice sheet could rapidly grow.
Yet another theory postulates a declining CO 2 concentration in the atmosphere as
the primary cause of Antarctic glaciation.
About 2.7 mybp , the Earth entered an enhanced cooling phase that has
continued unabated to the present day, although there have been significant fluc-
tuations about this long-term downward trend in temperature. One theory is that
this cooling trend was initiated by the gradual closing of the Isthmus of Panama,
which in turn affected ocean circulation in the North Atlantic.
Since then the Earth has undergone a large number of climate cycles ranging
from ice ages with large ice sheets in the Northern Hemisphere and a general
cooling of the entire Earth to interglacial periods that were comparable or warmer
than the present climate. As Figure 5.6 illustrates, these cycles have become
lengthier and of greater amplitude in the last million years.
During periods of glaciation great ice sheets formed in the NH. When great
ice sheets form they affect the overall climate of the Earth in a number of ways.
First, they increase the albedo by reflecting incident sunlight. Second, the presence
of large amounts of sea ice can affect ocean circulation that in turn may affect
heat transport to higher latitudes. Third, it is likely that cooler temperatures will
reduce the concentration of water vapor, thus reducing the greenhouse effect, and
have an (unknown) effect on cloudiness affecting the Earth's albedo. Fourth, as
the Earth cools a large reduction in CO 2 and CH 4 occurs in the atmosphere
producing a significant forcing that drives down worldwide temperatures. Hansen
and Sato (2011) estimated this effect to be about as great as the albedo effect.