Page 180 - Earth's Climate Past and Future

156 PART III • Orbital-Scale Climate Change

Milankovitch Theory: Orbital Control of
4
Ice Sheets

Continental ice sheets exist in regions where the overall
rate of snow and ice accumulation across the entire 3
mass of ice equals or exceeds the overall rate of ice loss Ablation
or ablation (companion Web site, pp. 27–30). Snow
accumulates as ice at high latitudes and altitudes where Meters of ice per year 2
temperatures are cold enough to permit frozen pre-
cipitation and to prevent melting in summer. For
continent-sized ice sheets that reach sea level, tempera- 1
tures sufficiently cold to sustain ice occur today only at
high latitudes. Accumulation
Rates of ice accumulation and ablation vary with tem- 0
perature, but the two relationships differ in a critical way. A -30 -20 -10 0 10
Ice (snow) accumulates at mean annual temperatures Surface temperature (˚C)
below 10°C, but rates of accumulation remain below 0.5 1
meter per year regardless of temperature (Figure 9–1A).
At higher temperatures, ice accumulation is limited by Equilibrium line
the fact that more of the precipitation falls as rain. At Ice sheet grows
extremely low temperatures, all the precipitation is snow, 0
but frigid air carries so little water vapor that rates of ice
accumulation are low.
In contrast, ablation of ice accelerates rapidly when Ice mass balance (m/yr)
temperatures warm. Melting begins at mean annual Ice sheet
temperatures above –10°C, equivalent to summer tem- -1 melts
peratures above 0°C, and can reach rates equivalent to
several meters of ice per year, much larger than maxi-
mum rates of accumulation.
Ice ablation can occur as a result of incoming solar -2 -30 -20 -10 0 10
radiation, by uptake of sensible or latent heat delivered Surface temperature (˚C)
by warm air masses (and rain), and by shedding of ice- B
bergs to the ocean or to lakes (called calving). Calving FIGURE 9-1 Temperature and ice mass balance
differs from other ablation processes because icebergs Temperature is the main factor that determines whether ice
leave the main ice mass and move elsewhere to melt, sheets are in a regime of net ablation (negative mass balance)
often in a warmer environment than the one in or near or accumulation (positive mass balance). Ablation increases
the ice sheet. sharply at higher temperatures. (Modified from J. Oerlemans,
The net balance between accumulation and ablation “The Role of Ice Sheets in the Pleistocene Climate,” Norsk Geologisk
over an entire ice sheet is called the ice mass balance Tidsskrift 71 [1991]: 155–61.)
(Figure 9–1B). The mass balance at very cold tempera-
tures (below –20°C) is positive but small because so
little snow falls. The mass balance at mean annual tem- areas of ablation, but the total mass of the ice sheet
peratures near –15° to –10°C is more positive because remains unchanged. Here, our focus is not on ice sheets
snow accumulation rates are more rapid but ablation is that remain in equilibrium. We seek to understand what
not strong. The mass balance turns sharply negative at makes ice sheets grow and shrink.
temperatures above –10°C because ablation accelerates Beginning with the Belgian mathematician Joseph
and overwhelms accumulation. The boundary between Adhemar in the 1840s, scientists suspected that orbitally
positive and negative mass balance is called the equilib- driven changes in solar insolation might be linked in
rium line. some way to the growth and melting of continent-sized
If net accumulation and ablation are in balance over ice sheets. Because orbital changes alter the amount of
an entire ice sheet, the ice sheet is said to be in a condi- insolation received on Earth in all seasons (Chapter 7),
tion of stable equilibrium. Net accumulation high on the scientists faced an important question: Which season is
ice sheet is exactly balanced by ablation at lower eleva- critical in controlling the size of ice sheets?
tions, and no net change in total ice volume occurs. Ice Winter would seem to be the obvious choice,
flows within the ice sheet from areas of accumulation to because snow falls mainly in winter. Colder winters