The Fate of Greenland: Lessons from Abrupt Climate Change

5. A Wobbly North Atlantic Conveyor?

5 A WOBBLY NORTH ATLANTIC CONVEYOR? The Little Ice Age In the 1930s Francois Matthes, working for the United States Geological Survey , studied very young moraines deposited by small glaciers in the Sierra NevadaofCalifornia ,andduringaninterviewwithajournalistintroducedtheterm Little Ice Age to describe the time interval during which these moraines were deposited. The term has endured ever since, although its usage has varied. The modern application of the term generally applies to a relatively cold interval over the last few centuries, marked by expansion of mountain glaciers. Hubert Lamb, the great climate historian, defined the Little Ice Age as the period when “not only in Europe but in most parts of the world the extent of snow and ice on land and sea seems to have attained a maximum as great as, or in most cases, greater than, at any time since the last major ice age.” The period that Lamb referred to was approximately 1550 to 1850. George Denton’s first encounter with the Little Ice Age occurred in the early 1960s in the northern reaches of the St. Elias Mountains of southwestern Yukon Territory. These spectacular mountains comprise the highest peaks in Canada, including Mt. Logan at 5,959 meters, and harbor the largest ice field in North America. Long outlet glaciers drain this ice field in star-like fashion. 123 A WOBBLY NORTH ATLANTIC CONVEYOR? On the south, many of these outlets calve into the Gulf of Alaska. But in the north, these outlet glaciers end on land in glacially carved valleys. One of these outlets is the beautiful Kaskawulsh Glacier, which terminates at the head of the valley occupied by the braided outwash stream of Slims River. Denton landed in a bush plane with his field party on the Slims outwash plain near the glacier terminus, where they established a field camp with Dahl sheep, grizzly bears, moose, and caribou to keep them company. They mapped the recent moraines deposited a kilometer of so in front of the terminus and found spruce logs embedded in the outermost fresh-appearing ridge, including the remains of one tree that was still rooted but had been tipped over and sheared by advancing ice. Radiocarbon dates of these trees showed that the glacier advance that created these moraines had occurred within the last several centuries. In the last few years, Alberto Reyes and Brian Luckman revisited the moraines in front of Kaskawulsh Glacier and, from a comparison with a regional treering data base, placed the glacier kill dates of these trees in the mid-eighteenth century, thus obtaining a more precise age for the outermost moraine of the Little Ice Age. But to return to their earlier study of the 1960s, Denton and his field group examined the valley floor just outboard of the young Kaskawulsh moraines in areas not covered by Slims outwash. Here they found a thick layer of windborne loess deposited during the retreat of Kaskawulsh Glacier from Slims valley at the end of the last ice age, and another layer from the recent glacier resurgence in the second half of the ongoing interglaciation. Grass buried at the base of one of the exposures of loess yielded a radiocarbon date of 11,200 calendar years ago, which was significant because the section exposed only windblown loess, with the conspicuous absence of any glacial deposits that would indicate an overriding of the site by Kaskawulsh Glacier. The fact that this loess deposit nestled right up next to the moraine with the radiocarbondated wood meant that, within the last few centuries, Kaskawulsh Glacier attained its maximum extension of the last 11,200 years Thus, in the case of Kaskawulsh Glacier, the Little Ice Age advance of the last few centuries represented the greatest glacial advance since the close of the last ice age, just as Lamb envisioned. This situation immediately raised the interesting question of what the Little Ice Age—the coldest time of this Figure 5.1 Long outlet glaciers drain Alaska’s ice fields, many flowing into the Gulf of Alaska. (Photo by Gary Comer) 125 A WOBBLY NORTH ATLANTIC CONVEYOR? interglaciation—is trying to tell us about the progression of climate over the ongoing interglaciation. The Little Ice Age in Europe Denton’s next encounter with the Little Ice Age occurred in the late 1960s, when, papersandmapsinhand,hevisitedtheclassicglaciersoftheSwissAlpsandthose of southwest Norway. Here he saw the evidence of fluctuations of mountain glaciers during the Little Ice Age amply...