The Last Glacial Maximum was the last period in the Earth's climate history
during the last glacial period when ice sheets were at their greatest extension.
Growth of the ice sheets reached their maximum positions 26,500 years ago.
Deglaciation commenced in the Northern Hemisphere approximately 19,000 years
ago, and in Antarctica approximately 14,500 years ago which is consistent
with evidence that this was the primary source for an abrupt rise in the sea
level 14,500 years ago. At this time, vast ice sheets covered much of North
America, northern Europe and Asia. These ice sheets profoundly affected Earth's
climate, causing drought, desertification, and a dramatic drop in
sea levels. It was followed by the Late Glacial Maximum.
Glacial climate The formation of an ice sheet or ice cap
requires both prolonged cold and precipitation. Hence, despite having
temperatures similar to those of glaciated areas in North America and
Europe, East Asia remained unglaciated except at higher elevations. This
difference was because the ice sheets in Europe produced extensive anticyclones
above them. These anticyclones generated air masses that were so dry on reaching
Siberia and Manchuria that precipitation sufficient for the formation of glaciers
could never occur. The relative warmth of the Pacific Ocean due to the shutting
down of the Oyashio Current and the presence of large 'east-west' mountain
ranges were secondary factors preventing continental glaciation in Asia.
All over the world, climates at the Last Glacial Maximum were cooler and almost
everywhere drier. In extreme cases, such as South Australia and the Sahel,
rainfall could be diminished by up to 90% from present, with florae diminished
to almost the same degree as in glaciated areas of Europe and North
America. Even in less affected regions, rainforest cover was greatly diminished,
especially in West Africa where a few refugia were surrounded by tropical
grasslands. The Amazon rainforest was split into two large blocks by extensive
savanna, and the tropical rainforests of Southeast Asia probably were similarly
affected, with deciduous forests expanding in their place except on the
east and west extremities of the Sundaland shelf. Only in Central America
and the Chocó region of Colombia did tropical rainforests remain
substantially intact – probably due to the extraordinarily heavy rainfall of
these regions. Most of the world's deserts expanded.
Exceptions were in what is now the western United States, where changes in
the jet stream brought heavy rain to areas that are now desert and large
pluvial lakes formed, the best known being Lake Bonneville in Utah. This also
occurred in Afghanistan and Iran, where a major lake formed in the Dasht-e
Kavir. In Australia, shifting sand dunes covered half the continent, whilst the
Chaco and Pampas in South America became similarly dry. Present-day subtropical
regions also lost most of their forest cover, notably in eastern Australia, the
Atlantic Forest of Brazil, and southern China, where open woodland became
dominant due to drier conditions. In northern China – unglaciated despite its
cold climate – a mixture of grassland and tundra prevailed, and even here, the
northern limit of tree growth was at least 20° farther south than today.
In the period before the Last Glacial Maximum, many areas that became
completely barren desert were wetter than they are today, notably in southern
Australia, where Aboriginal occupation is believed to coincide with a wet
period between 40,000 and 60,000 years Before Present.
World impact During the Last Glacial Maximum, much of
the world was cold, dry, and inhospitable, with frequent storms and a
dust-laden atmosphere. The dustiness of the LGM atmosphere is a prominent
feature in ice cores; dust levels were as much as 20 to 25 times greater than
at present. This was probably due to a number of factors: reduced vegetation,
stronger global winds, and less precipitation to clear dust from the
atmosphere. The massive sheets of ice locked away water, lowering the sea
level, exposing continental shelves, joining land masses together, and
creating extensive coastal plains. = Europe =
Northern Europe was largely covered by ice, the southern boundary of the ice
sheets passing through Germany and Poland. This ice extended northward to
cover Svalbard and Franz Josef Land and northeastward to occupy the Barents Sea,
the Kara Sea and Novaya Zemlya, ending at the Taymyr Peninsula.
Permafrost covered Europe south of the ice sheet down to present-day Szeged in
Southern Hungary. Ice covered the whole of Iceland and almost all of the British
Isles but southern England. Britain was no more than a peninsula of Europe, its
north capped in ice, and its south a polar desert.
= Asia = There were ice sheets in modern Tibet as
well as in Baltistan and Ladakh. In Southeast Asia, many smaller mountain
glaciers formed, and permafrost covered Asia as far south as Beijing. Because of
lowered sea levels, many of today's islands were joined to the continents:
the Indonesian islands as far east as Borneo and Bali were connected to the
Asian continent in a landmass called Sundaland. Palawan was also part of
Sundaland, while the rest of the Philippine Islands formed one large
island separated from the continent only by the Sibutu Passage and the Mindoro
Strait. = Africa and the Middle East =
In Africa and the Middle East, many smaller mountain glaciers formed, and
the Sahara, Gobi, and other sandy deserts were greatly expanded in extent.
The Persian Gulf averages about 35 metres in depth and the seabed between
Abu Dhabi and Qatar is even shallower, being mostly less than 15 metres deep.
For thousands of years the Ur-Shatt provided fresh water to the Gulf, as it
flowed through the Strait of Hormuz into the Gulf of Oman.
Bathymetric data suggests there were two palaeo-basins in the Persian Gulf. The
central basin may have approached an area of 20,000 km², comparable at its
fullest extent to lakes such as Lake Malawi in Africa. Between 12,000 and
9000 years ago much of the Gulf floor would have remained exposed, only being
flooded by the sea after 8,000 years ago.
= Australasia = Australia, New Guinea, and Tasmania were
one land mass called Sahulland. Between the East Asian mass of Sundaland and
Sahulland, Wallacea remained islands, though the number and width of water
gaps between the two continents were considerably smaller.
= North America = In North America, the ice covered
essentially all of Canada and extended roughly to the Missouri and Ohio Rivers,
and eastward to Manhattan. In addition to the large Cordilleran Ice Sheet in
Canada and Montana, alpine glaciers advanced and ice caps covered much of
the Rocky Mountains further south. Latitudinal gradients were so sharp that
permafrost did not reach far south of the ice sheets except at high
elevations. LGM glaciers forced early human populations who had originally
migrated from northeast Siberia into refugia, reshaping their genetic
variation through mutation and drift. This phenomenon established the older
haplogroups found among Native Americans, whereas post-LGM migrations
are responsible for northern North American haplogroups.
On the Island of Hawaii geologists have long recognized deposits formed by
glaciers on Mauna Kea during recent ice ages. The latest work indicates that
deposits of three glacial episodes since 150,000 to 200,000 years ago are
preserved on the volcano. Glacial moraines on the volcano formed about
70,000 years ago and from about 40,000 to 13,000 years ago. If glacial deposits
were formed on Mauna Loa, they have long since been buried by younger lava flows.
= South America = In the Southern Hemisphere, the
Patagonian Ice Sheet covered the whole southern third of Chile and adjacent
areas of Argentina. On the western side of the Andes the ice sheet reached sea
level as far north as in the 41 degrees south at Chacao Channel. The western
coast of Patagonia was largely glaciated but some authors have pointed out the
possible existence of ice-free refugia for some plant species. On the eastern
side of the Andes glacier lobes occupied the depressions of Seno Skyring, Seno
Otway, Inútil Bay and Beagle Channel. On the Straits of Magellan, ice reached as
far as Segunda Angostura. See also
Notes Further reading
Developments in Quaternary Science Series
Gillespie, Alan R.; Porter, Stephen C.; Atwater, Brian F.. The Quaternary Period
in the United States. Elsevier. ISBN 978-0-444-51471-4.
Ehlers, Jürgen; Gibbard, Philip L.. Quaternary Glaciations Extent and
Chronology. 1. Europe. Elsevier. ISBN 978-0-444-51462-2.
Ehlers, Jürgen; Gibbard, Philip L.. Quaternary Glaciations: Extent and
Chronology. 2. North America. Elsevier. ISBN 978-0-444-51592-6.
Ehlers, Jürgen; Gibbard, Philip L.. Quaternary Glaciations: Extent and
Chronology. 3. South America, Asia, Africa, Australia, Antarctica. Elsevier.
ISBN 978-0-444-51593-3. Šibrava, V.; Bowen, D.Q; Richmond, G.M.,
ed.. "Quaternary Glaciations in the Northern Hemisphere". Quaternary Science
Reviews 5: 1–514. Bibcode:1986QSRv....5....1S.
doi:10.1016/0277-3791(86)90167-8. External links
Adams, J.M.. "Global land environments since the last interglacial". Atlas of
Palaeovegetation: Preliminary land ecosystem maps of the world since the
Last Glacial Maximum. Oak Ridge National Laboratory, TN.
"Map and GIS database of glacial landforms and features related to the
last British Ice Sheet". BRITICE. Department of Geology, University of
Sheffield. 2004. Dyke, A.S.; Moore, A.; Robertson, L..
"Deglaciation of North America". Geological Survey of Canada Open File,
1574. with full map series.) Manley, W.; Kuaffman, D. "Alaska
PaleoGlacier Atlas: A Geospatial Compilation of Pleistocene Glacier
Extents". INSTAAR. University of Colorado.
Paleoclimate Modelling Intercomparison Project PMIP Web Site and 'Publications
: Last Glacial Maximum. Paleoclimate Modelling Intercomparison
Project Phase II PMIP2 Home page and PMIP 2 Publications.
Osipov, Eduard Y.; Khlystov, Oleg M. "Glaciers and meltwater flux to Lake
Baikal during the Last Glacial Maximum".