Overview of the various models for ice ages in the recent past (3 MYBP to present) - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 237

Figure 8.8. Comparison of the lower-troposphere cloud cover anomaly with the cosmic ray
anomaly over the past two sunspot cycles (adapted from Agee et al., 2011).
model, the primary factor that controls large-scale variations in the Earth's climate is
the albedo of the Earth, which in turn is controlled to a considerable degree by the
amount of cloudiness. Cloudiness affects the Earth's albedo and, since average solar
input to the Earth is about 342W/m 2 , a change of only 1 or 2% in overall albedo can
produce an effective forcing of 3.4 to 6.8W/m 2 , which would have a significant
impact on the climate. Actually, solar input to the tropics is considerably higher,
and variability of cloudiness in the tropics could have a dramatic effect on net solar
input to the Earth.
Consider a cold glacial period. With the Earth colder than average, cloudiness
is assumed to be below normal because the vapor pressure of water is reduced. As
a result, solar penetration of the Earth increases above normal, instigating a
warming trend in the atmosphere. Gradually, this heat is transferred to the
oceans, but that may require many thousands of years. As the Earth warms up
and enters an interglacial period, the oceans warm up but do so slowly over a
considerable time lag. By the time that the Earth is well into the interglacial
period, the oceans have warmed up enough to significantly increase world
cloudiness by evaporation. This process reduces net solar input to the Earth,
instigating a cooling trend. Now the process reverses. High levels of cloudiness
cool the atmosphere quickly and the oceans follow slowly. By the time the Earth
enters a new glacial state, the oceans have not lost all their excess heat. As the
glacial state persists, the oceans eventually cool off, reducing cloudiness. Now, the
warming cycle begins all over again.
While the formation of large ice sheets is a slowly evolving process, the decay
of ice sheets can be accelerated by the formation of moulins that produce a liquid
layer below the ice sheet that enhances slippage and calving. Thus, the glaciation-
deglaciation cycle is asymmetric in time with a slow buildup of ice sheets and
comparatively rapid decay.