Page 316 - Earth's Climate Past and Future

292 PART V • Historical and Future Climate Change

snowfields apparently killed the older lichen sometime annual signals (Figure 16–3A), but ice flow may degrade
during the Little Ice Age and kept them from growing the resolution deeper in the ice.
back until late in the nineteenth century. Scattered Retrieving ice cores from mountains is a formidable
radiocarbon dating confirms that the lichen formed task. Heavy equipment (including solar-powered ice
until just before the start of the Little Ice Age. drills) must be hauled up to subfreezing mountain sum-
It makes sense that permanent snowfields would mits (Figure 16–3B). Mountain ice caps are between 100
have begun to expand across this region. Baffin Island is and 200 m thick, and most expeditions drill and sample
one of three locations in which the last remnants of the the entire thickness of ice at several locations on each ice
great glacial ice sheets melted near 6000 years ago (see cap (Figure 16–3C and D). Lack of oxygen at these alti-
Figure 13–2). Climate model simulations also suggest tudes quickly causes exhaustion and other problems. The
that it is one of the regions in which ice sheets probably ice cores that are extracted must be lugged down to lower
began each new advance during the last 3 million years. elevations and kept from melting in the warmer air.
In this context, the growth of snowfields during the Lit- Only a few mountain glaciers have been cored. By far
tle Ice Age was a very small step toward a real ice age. the most extensive efforts have been those by the intrepid
The Little Ice Age is often used as the “type exam- glacial geologist Lonnie Thompson. His expeditions have
ple” of a cooler Earth from the recent past, along with retrieved ice cores at elevations of 5670 m (about 18,500
the implicit assumption that the cooling that caused it ft) above sea level from the Quelccaya ice cap in the Peru-
was hemispheric or even global in extent. Yet evidence vian Andes. By counting annual layers and matching vol-
in Chapter 14 showed that millennial-scale climatic canic ash layers with historically documented eruptions,
changes during the last 8000 years have been highly he found that these records extend back to 1500 years ago.
variable from region to region and that no single pat- Cores taken in the 1980s show annual-scale changes
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tern appears to have persisted on larger spatial scales. in δ O values and dust concentrations that can be aver-
From this point of view, the cooling in Iceland and aged over decadal intervals (Figure 16–4A). The varia-
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nearby areas of the North Atlantic Ocean might just as tions in δ O values reflect the same processes as those
easily have been a local phenomenon restricted to just affecting continent-sized ice sheets: changes in source
this one region. area, transport paths, and amount of water vapor car-
The next section examines several methods that ried to the glacier, and particularly changes in the tem-
provide climate records spanning the last 1000 years. perature at which the snow condenses above the ice.
The purpose of this exploration is to see what these Higher dust concentrations indicate some combination
methods indicate about the regional extent of the Little of drier source areas and stronger winds.
Ice Age cooling. By examining climate records covering Early cores from the Quelccaya glacier record regis-
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major portions of the last millennium, we can also begin tered a shift toward more positive δ O values and less
to address the issue of whether or not the last 100 years dust near 1900, implying that a change toward some
of climate change fit into natural longer-term trends. combination of warmer temperatures, weaker winds,
and different source areas occurred at that time (see
Proxy Records of Historical Climate Figure 16–4A). This δ O record also resembles the
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Little Ice Age pattern shown in Figure 16–1, with more
Before the nineteenth century, people rarely kept records positive (warmer?) values from 1000 to 1400 and then
of temperature or precipitation. As a result, climate sci- more negative (cooler?) values after 1500. In contrast,
entists have to rely mainly on proxy records in archives the dust record does not show the expected match
like those used for earlier intervals of Earth’s history, such before 1600; if anything, the lowest dust concentra-
as lake sediments and ice cores. In addition, sources of tions occur within the early part of the Little Ice Age
proxy records such as tree rings and corals can also be (1400–1600), with higher concentrations during the
used for this interval. These various archives provide lim- medieval warm period (1000–1300).
ited (but steadily improving) coverage of climate changes In a return expedition to Quelccaya in 1993,
across Earth’s surface during the last few centuries. Thompson encountered something totally unexpected.
During the 1970s and 1980s, previous coring expedi-
16-1 Ice Cores from Mountain Glaciers
tions at the top of the ice cap (Figure 16–4B) had found
Like the huge ice sheets on Antarctica and Greenland, annual layering extending from the most recent ice
glaciers in mountain valleys and small ice caps covering all the way down to the deepest layers deposited 1500
mountain summits are also excellent climate archives years ago. In the new cores taken in 1993, however,
(Figure 16–3). Some mountain ice dates back many meltwater percolating down from the surface had begun
thousands of years into the last glaciation, while other to destroy the annual layers (Figure 16–4C). This dra-
glaciers span only a few hundred years of climate matic finding means that a tropical ice cap that had been
history. Layers deposited at the surface usually contain continuously recording intact annual layering for 1500