ing how glaciers have varied in the past has become cen- Glaciers in the tropics were rather small or even absent
tral to understanding the causes and possible future of in the early- to mid-Holocene, gradually re-advancing
contemporary glacier change. Historical reconstruction from about 4 000 years ago, probably as a result of in-
of glaciers in the Alps, Scandinavia, Alaska, the Cana- creasing humidity
18
. The moraines (accumulations of
dian Rockies, Patagonia, the Tropics of South America, unsorted, unstratified mixtures of clay, silt, sand, gravel,
Tibet, the Arctic and Antarctica shows that the fluctua- and boulders deposited by the glaciers) that were formed
tions in the state of the glaciers are largely consistent during the so-called Little Ice Age (from the early 14th
with the reconstruction of climatic and environmental to the mid 19th centuries) mark a Holocene maximum
changes provided by other indicators, such as ice-cores, extent of glaciers in many regions of the world, although
tree-line shifts, pollen records and lake sediments
14
. the time period for this maximum varies among the dif-
ferent regions.
General warming during the transition from the Late
Glacial period (between the Last Glacial Maximum and There is evidence that mountain glaciers had retreated
about 10 000 years ago) to the early Holocene (about 10 during various periods of the Holocene in many regions
000 to 6000 years ago) led to a drastic general glacier of both hemispheres at least as much as they had in the
retreat with intermittent periods of re-advances. About 1980s–1990s
14
. However, caution must be exercised when
11 000 to 10 000 years ago, this pronounced warming re- using glacier extent as an indicator of climate; the glacier
duced the glaciers in most mountain areas to sizes com- surfaces of the European Alps today, for example, are still
parable with conditions at the end of the 20th century
15
. far larger than expected given the climatic conditions of
In northern Europe and western North America, which the past decade, and are thus not in equilibrium
19
.
were still influenced by the remnants of the great ice
sheets, this process was delayed until about 6000 to 4000 Since the end of the Little Ice Age
years ago. Several early-Holocene re-advances, especially
those in the North Atlantic and North Pacific as well as There has been a general retreat of glaciers worldwide
possibly in the Alps, cluster around an event about 8000 since their Holocene maximum extent towards the end
years ago, and were likely triggered by changes in the of the Little Ice Age, between the 17th and the second
ocean thermohaline circulation and subsequent cooling half of the 19th century, with intermittent periods of gla-
resulting from the outbursts of Lake Agassiz
14
. cier re-advance in certain regions. Direct measurements
of glacier fluctuations started in the late 19th century
On a timescale of hundreds of years there were periods (see box on worldwide glacier monitoring) with annual
of synchronous glacier advance around the world – peak- observations of glacier front variations
20
. These observa-
ing in the late Holocene in the Northern Hemisphere, tions and the positions of the Little Ice Age moraines are
and in the early Holocene in the Southern Hemisphere
16
. used to measure the extent of glacier retreat. Total retreat
The difference in the amount of sunlight that reaches over this time period of glacier termini (the ends of the
the Earth’s surface in the two hemispheres
17
, accounts glaciers) is commonly measured in kilometres for larger
for these differences in long-term glacier evolution
16
. glaciers and in hundreds of metres for smaller ones
21
.
120 GLOBAL OUTLOOK FOR ICE AND SNOW