Abstracts
SEARCH Open Science Meeting
October 27, 2003
Seattle, Washington, USA
Holocene Thermal Maximum in the Western Arctic
Darrell Kaufman1, PARCS Holocene Thermal Maximum PARCS2
1Geology and Environmental Sciences, Northern Arizona University, Department of Geology, Flagstaff, AZ, 86011-4099, USA, Phone 928-523-7192, Fax 928-523-9220, Darrell.Kaufman@nau.edu
2USA
One overall goal of NSF-PARCS (Paleoenvironmental Arctic Science) research is to contribute to the understanding of the nature and consequences of warmth in the Arctic and its impact on the global climate system. An immediate objective is to describe the state of the Arctic when it shifted toward, and experienced, warmer conditions during the Holocene (the present interglacial period). During the early to middle Holocene, much of the Arctic experienced warmer-than-present (20th century) temperatures, but the warming occurred at different times and to varying degrees in different places. The pattern of this variability can be examined to understand how climate in the Arctic responded to radiative forcing driven by changes in insolation and other factors. By characterizing the pattern of early Holocene warming, we can hypothesize possible mechanisms that underlie the heterogeneity of the observed response to forcing. Such mechanisms reflect the particular geography of the Arctic and its feedback processes that might influence the pattern and magnitude of potential future changes. The spatial pattern of the HTM can, for example, be compared with the observed pattern of recent warming, and with the characteristic signatures of modes of variability known from the instrumental record.
As the first step in addressing this objective, the PARCS working group on the Holocene thermal maximum has compiled a database of published and unpublished records of Holocene paleoenvironmental change in the Arctic. The spatio-temporal pattern of peak Holocene warmth (Holocene thermal maximum, HTM) was traced over 140 sites across the western hemisphere of the Arctic (0 to 180°W; north of ~60°N). Paleoclimate inferences based on data from a variety of sources (lake and marine sediment, peat, and glacier ice) and proxies (pollen, macrofossils, chironomids, diatoms, geochemistry, oxygen isotopes, etc.) provide clear evidence for warmer-than-present conditions at 120 of these sites. At the 16 terrestrial sites where quantitative estimates have been obtained, local HTM temperatures (primarily summer estimates) were on average 1.6 ± 0.8°C higher than present (approximate average of the 20th century), but the warming was time-transgressive across the western Arctic.
As the precession-driven summer insolation anomaly peaked 12-10 ka (thousands of calendar years ago), warming was concentrated in northwest North America, while cool conditions lingered in the northeast. Alaska and northwest Canada experienced the HTM between ca. 11 and 9 ka, about 4000 yr prior to the HTM in northeast Canada. The delayed warming in Quebec and Labrador was linked to the residual Laurentide Ice Sheet, which chilled the region through its impact on surface energy balance and ocean circulation. The lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change. The spatial asymmetry of warming during the HTM resembles the pattern of warming observed in the Arctic over the last several decades.
Although the two warmings are described at different temporal scales, and the HTM was additionally affected by the residual Laurentide ice, the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing. Unlike the HTM, however, future warming will not be counterbalanced by the cooling effect of a residual North American ice sheet.
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