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CHAPTER 9 • Insolation Control of Ice Sheets 157

caused by lower solar radiation at that time of year Minimum
should help to accumulate larger amounts of snow and summer insolation
promote glaciation. But this seemingly reasonable idea N N
turned out to be wrong. One problem is that ice sheets June 21
grow at high latitudes where temperatures are always
cold in winter, even during intervals of relatively warm
climate like the one we live in today. In addition, the Aphelion
Sun at these latitudes always lies low in the winter sky, S S
regardless of ongoing orbital changes, and incoming Small tilt
solar radiation in winter is never strong. Winter is not A Northern hemisphere ice growth
the critical season.
The opposite idea—summer insolation control of ice
Maximum
sheets—was proposed by several scientists working in the summer insolation
late nineteenth and early twentieth centuries, including N N
Rudolf Spitaler (who was also the first to realize that June 21
summer insolation changes might drive monsoons),
Wladimir Köppen, and Alfred Wegener (who also pro-
posed the theory that continents drift). Their reasoning Perihelion
was simple: no matter how much snow falls during win-
ter, it can all be easily melted if the following summer is S Large tilt S
warm and ablation is rapid (see Figure 9–1). B Northern hemisphere ice decay
As a result, these scientists reasoned that low sum-
mer insolation is critical in producing summers cool FIGURE 9-2 Orbital changes and ice sheets (A) According
enough for snow and ice to persist from one winter to to the Milankovitch theory, ice sheets grow in the northern
the next. This idea gained popularity during the early hemisphere at times when summer insolation is low, because
and mid-twentieth century from work by the Serbian tilt is low and Earth lies in the aphelion position farthest from
astronomer Milutin Milankovitch, who first calculated the Sun. (B) Ice melts when summer insolation is high because
in a systematic way the impact of astronomical changes tilt is high and Earth lies in the perihelion position closest to
on insolation received on Earth at different latitudes the Sun. (Adapted from W. F. Ruddiman and A. McIntyre,
and in different seasons. This idea is now known as the “Oceanic Mechanisms for Amplification of the 23,000-Year
Milankovitch theory. Ice-Volume Cycle,” Science 212 [1981]: 617–27.)
Milankovitch proposed that ice growth in the
northern hemisphere occurs during times when sum-
mer insolation is reduced. Low summer insolation radiation penetrating to Earth’s surface is closely related
occurs when Earth’s orbital tilt is small and its poles to the amount arriving at the top of the atmosphere.
are pointed less directly at the sun (Figure 9–2A). Low
insolation also occurs when the northern summer sol- IN SUMMARY, the Milankovitch theory proposes that
stice occurs with Earth farthest from the Sun (in the when summer insolation is strong, more radiation is
aphelion or distant-pass position) and when the orbit is absorbed at Earth’s surface at high latitudes, making
highly eccentric (further increasing the Earth-Sun dis- the climate in those regions warmer. Warming
tance). Milankovitch reasoned that the most sensitive accelerates ablation, melts more snow and ice, and
latitude for low insolation values is 65°N, the latitude either prevents glaciation or shrinks existing ice
at which ice sheets first accumulate and last melt. He sheets (Figure 9–3 top). Conversely, when summer
also proposed that ice melts during the stronger sum- insolation is weak, less radiation is delivered to high
mer insolation resulting from the opposite orbital con- latitudes, and the reduction in radiation cools the
figuration (Figure 9–2B). regional climate. This cooling reduces the rate of
The amount of summer insolation arriving at the top summer ablation and allows snow to accumulate and
of Earth’s atmosphere at 65°N can vary by as much as ice sheets to grow (Figure 9–3 bottom).
±12% around the long-term mean value (Chapter 7).
We have no way of knowing how much of this incoming
solar radiation actually makes it through the atmosphere Modeling the Behavior of Ice Sheets
to Earth’s high-latitude ice sheets because of the com-
plicating effects of regional changes in atmospheric cir- To gain more insight into summer insolation control of
culation, clouds, and water vapor. Milankovitch noted ice sheets in the northern hemisphere, climate scientists
these complications but assumed that the amount of have developed numerical models based on an idealized