HOLOCENE WARMTH IN THE ARCTIC, A NEW LITERATURE-BASED SYNTHESIS

During the early to middle Holocene “thermal maximum”, much of the Arctic experienced warmer-than-present temperatures. Although the major control on Holocene warmth was increased summer insolation caused by changes in the Earth's orbital geometry, the timing and intensity of the thermal maximum exhibits considerable variability. These differences result from oceanic, atmospheric, and terrestrial processes that work in concert with physiography to modulate the effects of broader-scale forcing. The same nonlinear processes that influenced the variability of past warmings are important to understanding future trends. Paleoenvironmental Arctic Sciences (PARCS) researchers are synthesizing their data on the state of the Arctic when it experienced warmer conditions during the Holocene. The PARCS working group on Holocene warmth has compiled a preliminary database of published records of Holocene paleoenvironmental change in the Arctic. The database includes data from more than 81 previously published studies of lake and marine sediments, peat, and glacier ice with paleoclimatic inferences from a variety of proxies (pollen, macrofossils, chironomids, diatoms, geochemistry, oxygen isotopes, etc.). We identified the initiation and termination of the thermal maximum as it was either explicitly stated by the author, or interpreted from the author's data. All 14C ages were calibrated using a third-order polynomial fit to the CALIB data, which yields ages within about ±150 years of the calibrated age. Preliminarily, we have identified the following major features of the data: 1. Nearly all sites where warming took place prior to 12 ka are in eastern Beringia, 2. The most recent (post 6 ka) warming took place at sites around residual glacial ice, 3. Two main pulses of warming occurred across the Arctic: one at 11-10 and one at 8-7 ka, 4. Few sites show warming that was initiated between 8.5 and 9.0 ka, 5. There was no systematic latitudinal pattern to the warming, 6. The Holocene warm interval was terminated by 3 ka at nearly all sites, 7. Sites that warmed earlier tended to cool earlier, 8. Most sites where temperature estimates are quantitative register warming of 1-2° C, 9. Of the few sites where P-E is reported, most (75%) register higher effective moisture.