Im westlichen Kanada gaben die schmelzenden Gletscher jetzt Baumstümpfe, Humusboden, Pflanzenreste, Karibu-Exkremente und menschliche Artefakte frei, die ein Alter von einigen tausend Jahren haben. Dies zeigt, dass die Gletscher während des Mittelholozäns zur Zeit des holozänen Klimaoptimums dort bereits einmal kürzer waren als heute. Abstract von Koch et al. 2014:
Alpine glaciers and permanent ice and snow patches in western Canada approach their smallest sizes since the mid-Holocene, consistent with global trends
Most alpine glaciers in the Northern Hemisphere reached their maximum extents of the Holocene between ad 1600 and 1850. Since the late 1800s, however, glaciers have thinned and retreated, mainly because of atmospheric warming. Glacier retreat in western Canada and other regions is exposing subfossil tree stumps, soils and plant detritus that, until recently, were beneath tens to hundreds of metres of ice. In addition, human artefacts and caribou dung are emerging from permanent snow patches many thousands of years after they were entombed. Dating of these materials indicates that many of these glaciers and snow patches are smaller today than at any time in the past several thousand years. This evidence, in turn, suggests that glacier recession in the 20th century is unprecedented during the past several millennia and that glaciers in western Canada have reached minimum extents only 150–300 years after they achieved their maximum Holocene extents.
Wie ein Jojo expandierten und schrumpften die Gletscher während der letzten 10.000 Jahre, wie eine Studie aus Britsh Columbia von Mood & Smith 2015a zeigt:
Latest Pleistocene and Holocene behaviour of Franklin Glacier, Mt. Waddington area, British Columbia Coast Mountains, Canada
Franklin Glacier is an 18-km-long valley glacier that originates in a broad icefield below the west face of Mt. Waddington in the central British Columbia Coast Mountains, Canada. Radiocarbon-dated wood samples from the proximal faces of lateral moraines flanking Franklin Glacier show that the glacier expanded at least nine times since 13,000 cal. yr BP. A probable Younger Dryas advance of Franklin Glacier at 12,910–12,690 cal. yr BP followed the late glacial retreat and down wasting of the Cordilleran Ice Sheet from ca. 16,000 to 12,900 cal. yr BP. During the succeeding early Holocene warm period, Franklin Glacier appears to have retreated significantly, leaving no record of glacial expansion until the mid-Holocene when it repeatedly advanced at 6360–6280, 5470–5280 and 4770–4580 cal. yr BP. Down wasting of the glacier surface after ca. 4770–4580 cal. yr BP was followed by intervals of expansion at 4260–4080, 3210–3020 and 2620–2380 cal. yr BP. Following ice expansion at ca. 2620–2380 cal. yr BP into trees over 224 years in age, there is no record of the glacier activity until 1570–1480 cal. yr BP when Franklin Glacier thickened and advanced into young subalpine fir trees. During the ‘Little Ice Age’, advances at 800–680, 610–560 and 570–510 cal. yr BP preceded a mid-19th to early 20th century advance that saw Franklin Glacier attain its maximum Holocene extent. The dendroglaciological record at Franklin Glacier is among the most comprehensive recovered from the British Columbia Coast Mountains and showcases the complexity of mid- to late Holocene glacier expansion in the region.
Passend dazu ein weiterer Artikel von Mood & Smith 2015b:
Holocene glacier activity in the British Columbia Coast Mountains, Canada
The Coast Mountains flank the Pacific Ocean in western British Columbia, Canada. Subdivided into the southern Pacific Ranges, central Kitimat Ranges and northern Boundary Ranges, the majority of large glaciers and icefields are located in the Boundary and Pacific ranges. Prior descriptions of the Holocene glacial history of this region indicate the Holocene was characterized by repeated episodes of ice expansion and retreat. Recent site-specific investigations augment our understanding of the regional character and duration of these events. In this paper, previously reported and new radiocarbon evidence is integrated to provide an updated regional assessment. The earliest evidence of glacier expansion in the Coast Mountains comes from the Boundary Ranges at 8.9 and 7.8 ka and in the Pacific Ranges at 8.5–8.2 ka, with the latter advance corresponding to an interval of rapid, global climate deterioration. Although generally warm and dry climates from 7.3 to 5.3 ka likely limited the size of glaciers in the region, there is radiocarbon evidence for advances over the interval from 7.3 to 6.0 and at 5.4–5.3 ka in the Pacific Ranges. Following these advances, glaciers in the Pacific Ranges expanded down valley at 4.8–4.6, 4.4–4.0, 3.5–2.6, 1.4–1.2, and 0.8–0.4 ka, while glaciers in Boundary Ranges were advancing at 4.1–4.0, 3.7–3.4, 3.1–2.8, 2.3, 1.7–1.1, and 0.8–0.4 ka. After 0.4 ka, it appears that most glaciers in the Coast Mountains continued to expand to attain their maximum Holocene extents by the early 18th to late 19th centuries. This enhanced record of Holocene glacier activity highlights the temporal synchrony in the Coast Mountains. Individual expansion events in the mid-to late Holocene broadly correspond to intervals of regional glacier activity reported in the Canadian Rocky Mountains, in Alaska, and on high-elevation volcanic peaks in Washington State.
Auch Kanada hat Probleme mit Klimaleugnern. Ein ganz besonders großer Leugner ist der Hubbard Glacier im Yukon, der sich trotz Klimawandels weiter ausdehnt. Hat er das Memo vielleicht nicht erhalten? CBC News von 2015:
Hubbard Glacier defies climate change, continues advancing
A Yukon glacier is slowly advancing towards an Alaskan river, setting the stage for an awesome collision of natural forces. If the glacier that originates near Mount Logan in the Yukon continues advancing at its current rate, it could block access to a fiord in Alaska and „strongly impact“ the nearby ecosystem. New research from the University of Kansas suggests the Hubbard Glacier could permanently dam the entrance to Russell Fiord, on the Gulf of Alaska, within 25 years.