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MILLER, GIFFORD  INSTAAR, University of Colorado, Boulder, CO 80309-0450.
Briner, Jason  INSTAAR, University of Colorado, Boulder, CO 80309-0450.
DeVogel, Steven  INSTAAR, University of Colorado, Boulder, CO 80309-0450.

Instrumental records show that the Earth has warmed ca. 0.7 °C over the past century, with the most dramatic increase in the decades since the 1960s. The Arctic has experienced a similar pattern, but the magnitude is greater, with average annual temperature increases of 2 to 5 °C across much of the Arctic since the1960s. Evidence of this warming includes decreases in arctic sea ice and snow cover, negative glacier mass balances, and increases in permafrost and ocean temperatures. The short time span of direct observations limits our ability to evaluate the roles of natural climate variability and greenhouse gas forcing in explaining these observations, but the pattern of change is consistent with GCM simulations of the consequences of increased greenhouse gases. Compelling questions that can be informed by the record of the past include “when the world was last as warm as present?”, “what is the range of natural climate variability relative to 20th century warming?” and “is present warmth unprecedented?”

The central plateau of northern Baffin Island Arctic Canada has long been considered one of the most sensitive areas for the Arctic in terms of snowline variations. Thin, cold-based ice caps dot the highest portions of the plateau at present. These ice caps have receded by 97% in area since their Little Ice Age maxima. The Tiger Ice Cap (7 km2) studied in the 1960s and early 1980s has now completely melted; multiple-year imagery of The Pleiades, a 17 km2 ice cap complex in 1958 shows that only 1 km2 remained in 2002, and complete disappearance is expected within 5 years. One of the largest ice cap complexes, Orion, diminished by 50% in the past 30 years; complete disappearance is projected by 2030.

We can begin to place constraints on the timing of growth and decay by radiocarbon dating vegetation that was entombed by the ice caps when they initially expanded. Polychritum moss collected beneath the receding margin of the Tiger Ice Cap in 1963 is 385±75 cal yr old, whereas moss beneath the 1981 ice margin is 510±10 cal yr old. Pronounced vegetation kill zones define the maximum extensions of the ice fields. For one of largest ice fields the maximum coverage was 3500 km2, whereas the current (2002) ice coverage is just 107 km2, a decrease in area of 97%.

Because the ice caps were always thin, cold-based, and lacking internal flow, their former lower limits define the maximum persistent snowline depression. In the case described above, the trimline lies at 500 m asl. The current snowline is above 1100 m asl. Using a lapse rate of 0.6 °C/100 m suggests a temperature increase since the late 1800s of >3.6 °C, or 5 to 7 times the global average. Even without additional global warming, all of the plateau ice caps are expected to disappear before 2050.

Figure 1. Present and past extent of plateau ice caps on a portion of northern Baffin Island

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