Page 187 - Earth's Climate Past and Future

CHAPTER 9 • Insolation Control of Ice Sheets 163

When ice begins to accumulate on the land, the immedi- part of the rebound quickly lifts the bedrock and elimi-
ate (elastic) sagging of the bedrock depresses the land and nates part of the depression. But the (larger) viscous part
promotes ice sheet melting. But the slower (and larger) of the rebound leaves the ice sheet at lower elevations in
viscous bedrock response keeps the growing ice sheet at the depression it created and in warmer air that causes
higher elevations, where temperatures are colder, abla- further ice melting.
tion is slower, and the ice mass balance is more positive.
Overall, the delay in bedrock sinking provides a positive
feedback to the growing ice sheet. 9-4 Full Cycle of Ice Growth and Decay
Bedrock plays the same overall positive feedback role In this section, we explore the interactions of insola-
during times when the ice is melting (Figure 9–11 bot- tion, ice volume, and bedrock responses during a typi-
tom). The weight of a large ice sheet that exists for thou- cal cycle of ice sheet growth and decay (Figure 9–12).
sands of years creates a deep depression in the underlying Because of the long lags inherent in the responses, the
bedrock. As the ice begins to melt, the (smaller) elastic factors interact in an intricate way to create and destroy
ice sheets.
We start with an interglacial maximum with the cli-
mate point P located in the Arctic Ocean and with no
ice sheet present on the northern continent (Figure
Ice sheet Bedrock sinking 9–12A). As summer insolation begins to decrease from a
grows delayed for previous maximum, the equilibrium line shifts to the
thousands
Ice grows of years south and the climate point gradually moves onto the
faster land. Some snow survives summer ablation on the far
northern part of the continent, and a small ice sheet
begins to form (Figure 9–12B).
As the ice sheet slowly grows, it reaches higher,
Ice sheet colder elevations where accumulation dominates over
stays at higher, ablation (Figure 9–12C). The ice also advances south-
colder elevations
ward, partly because the equilibrium line is moving
south and partly because internal flow from the area of
More
positive ice accumulation in the north carries ice to the south.
ice mass The thickening ice sheet slowly begins to weigh down
balance the bedrock, but most of the bedrock depression lags
several thousand years behind ice accumulation. This
A Low summer insolation
delay in bedrock sagging helps to keep the surface of the
ice sheet at higher and colder elevations where accumu-
Ice sheet Bedrock rebound
melts lation exceeds ablation.
delayed for The highest rate of ice accumulation occurs when
thousands
Ice melts of years summer insolation reaches a minimum value and the
faster equilibrium line is displaced farthest south (Figure
9–12C). At this point the ice sheet has not yet reached
maximum size because of the lag of ice volume behind
the insolation driver. The rapid growth of new ice
Ice sheet continues to weigh down the bedrock even more, with
stays at lower,
warmer elevations a lag of thousands of years for each new increment
of ice.
More Summer insolation then begins to increase and shift
negative the equilibrium line slowly back to the north, but the
ice mass ice sheet continues to grow to its maximum size for sev-
balance eral thousand years (Figure 9–12D). Ice growth contin-
ues because insolation levels are still relatively low and
B High summer insolation
because most of the surface of the ice sheet lies above
FIGURE 9-11 Bedrock feedback to ice growth and melting the equilibrium line, protected from the slowly increas-
(A) Delayed bedrock sinking during ice accumulation and ing levels of ablation.
(B) delayed rebound during ice melting provide positive At some point, the combined effects of the ongoing
feedback to the growth and decay of ice sheets. northward shift of the equilibrium line along with the