Page 200 - Earth's Climate Past and Future

176 PART III • Orbital-Scale Climate Change

Ice Cores Ice core site

Cores from today’s ice sheets span the last several Thicker
hundred thousand years and contain invaluable archives Fast ice flow annual
layers
of climatic signals that are not available from other flow
sources. Two of the most important records are those Slow ice Annual layers
thinned
of the greenhouse gases carbon dioxide (CO ) and
2
methane (CH ).
4
FIGURE 10-1 Ice coring The best place on an ice sheet to
10-1 Drilling and Dating Ice Cores
take ice cores is at the top of the ice dome because ice flows
Scientists searching for the oldest ice in an ice sheet slowly down into the ice sheet and old ice is preserved at the
drill down from the top of the highest ice domes bottom.
(Figure 10–1). They avoid the steeper edges because the
ice there flows relatively quickly toward the ice margins kilometers of ice. Drilling all the way through an ice
and melts. In contrast, the ice that accumulates on the sheet takes more than a single summer.
highest domes flows slowly down into the interior of Some ice cores can be dated by counting annually
the ice sheet, where it is stretched and thinned but not deposited layers (Chapter 2). Annual layering is recorded
melted. As a result, the oldest ice sits under the middle in several properties within the ice, including layers of
of an ice sheet. dust that may be visible to the eye. The dust is deposited
Because winter weather on ice sheets is inhospitable, at the end of cold, dry, windy winters. The count starts
drilling is done in the “summer” season (Figure 10–2). with the top layer, the year the coring operation began,
On Greenland a warm summer day may reach tempera- and it proceeds downward as far as annual layers remain
tures a few degrees below freezing (0°C), but on the detectable. Eventually the natural stretching of ice layers
Antarctic ice sheet temperatures rarely warm to –20°C by flow deeper in the ice sheet blurs the layers beyond
in summer. Drilling takes place in structures that provide recognition.
protection from the elements but keep the ice cores The layer-count method works best for ice sheets
frozen. Hundreds of ice cores, each a few meters in where snow is deposited rapidly. For the Greenland ice
length, are retrieved as drilling proceeds through several sheet, where ice accumulates at 0.5 m or more per year,

A B C

FIGURE 10-2 Ice coring operations (A, B) Ice drilling during cold “summer” conditions
retrieves sequences of ice cores thousands of meters thick. (C) Scientists may also examine upper
ice layers in pits dug into the ice. (Courtesy of Paul Mayewski, University of Maine.)