In the Northern Hemisphere, peak summer insolation occurred about 9000 years ago, when the last of the large ice sheets melted. Since then the Northern Hemisphere summers have gradually seen less incoming solar radiation. The early Holocene was relatively warm, but was also subject to a number of distinct cooling events, suggesting that the global climate may be more sensitive to solar-induced changes than is usually thought, and that such changes may represent a contributing mechanism for sub-Milankovitch climate variability (e.g., Bjorck et al. 2001).
Evidence from the Nordic Seas near Svalbard documents the first half of the Holocene as the warmest period during the last 13,400 years (Koq et al. 1993), followed by cooling during the Late Holocene (Ko§ & Jansen 1994). For the Barents Sea south of Svalbard, the early Holocene warming trend deduced from 8180 records (Duplessy et al. 2001) apparently culminated between 9000 and 6500 bp, followed by rapid cooling (Ivanova et al. 2002).
In NE Greenland, 400 km west of Svalbard, Hjort (1997) documented that the main fjords were ice-free during early and mid-Holocene. Since 5700 bp, however, these fjords have mainly been ice-covered. Summarizing glaciolo-gical evidence, Kelly (1980) concluded that the Greenland Ice Sheet melted back to a position 60-120 km behind the present margin during the Early Holocene, and that a Late Holocene period of growth was initiated 3000-3500 bp, followed by major advances around 2000 bp. This development is also reflected by ice core isotope analysis from the ice sheet (Dansgaard et al. 1971, 1973). The formation of the 4208 km2 Hans Tausen Ice Cap in Pearyland, northernmost Greenland, after 3900 bp (Hammer et al. 2001), as well as updated Greenland isotope series (Johnsen et al. 2001), lend support to the notion of late Holocene climatic deterioration in Greenland. Reconstructed surface temperatures from the Greenland Ice Sheet (Dahl-Jensen et al. 1998) directly testify to a 3°C net cooling during the last 4000 years.
In northern Finland, based on studies of pollen assemblages, Seppa and Birks (2001) demonstrated a marked late Holocene summer temperature decline and that the last 2000 years have been the coolest since the early Holocene. Rosen et al. (2001), from diatoms, chironomids, pollen and near-infrared spectroscopy in northern Sweden, documented late Holocene lowering of the tree-limit, caused by decreasing summer temperature. Also Hammarlund et al. (2002) reported lowering of the tree line and cooling after 3000 bp from study sites in northern Sweden. Based on botanical and glaciological evidence Nesje and Kvamme (1991) concluded that early Holocene temperatures in southern Norway were about 3°C above modern temperatures and that temperatures since 5000 bp have shown a decreasing trend.
Thus, climatic cooling have characterized the land areas around the Nordic Seas since c. 4000 bp, probably reflecting the gradual reduction of solar summer insolation caused by orbital forcing. The Nordic Seas have at the same time experienced a transition to reduced Atlantic Water influence and lowered sea surface temperatures. The Holocene transgression of the Barents Sea may have been an important driver for such major changes in oceanographic patterns, by reducing the warm Atlantic Water inflow to the Arctic Ocean (Butt et al. 2000). Atlantic Water inflows to the Arctic Ocean may also have been affected by bathymetric changes elsewhere in the Arctic (i.e., Nares Strait and other eastern Canadian Arctic Channels, and the Bering Strait). Of these, the opening of Bering Strait c. 11,000 bp might be the single most important because it introduces relatively fresh Pacific waters into the Arctic Basin. In addition, the opening of the Bering Strait may have strengthened the meridional Atlantic Water circulation through the Fram Strait, presumably at the expense of Barents Sea inflow (Forman et al. 2000).
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