Holocene Climate Variability in the Northern North Atlantic Region: A Review of Terrestrial and Marine Evidence

Abstract

The Holocene epoch, which followed the last major pulse of glacialion (the Younger Dryas) at the end of the last glaciation, encompasses a period before there was any substantial anthropogenic forcing of climate. A synthesis of climatic development during the Holocene (ca. 11,500 cal. yr BP to the present) is based on pollen-based quantitative temperature reconstructions, tree-line variations, chironomids, tree-ring records, spelcothem data, glacier variations, and marine records (stable isotopes, species abundance, lithological changes) from the Nordic Seas. The Holocene has been regarded as a period of relatively stable climate, but recent evidence suggests there have been several significant millennial-scale cli- mate fluctuations (larger than the post mid-19th century warming trend) through- out the Holocene. A general climate warming in the first part of the Holocene was punctuated by a few, abrupt climate reversals, centred at 10,000, 9,700, and 8,200 cal. yr BP. The data suggest there was a period of relatively warm conditions in the first half of the Holocene, in many areas warmer than in the 20th century, after which temperatures generally declined. The temperature decline was punctuated by centennial-scale warmer and colder periods with the most recent cold episode (-AD 1550-1925), including the "Little Ice Age", being one of the coldest of the entire Holocene. The kind of data presented here can be used for detecting mech- anisms and forcing factors behind the reconstructed climate variations and to study leads and lags in the Earth's climate system. A comparison between the cli- mate-forcing factors and the Holocene records presented here suggests that the early Holocene thermal maximum observed in most records from the Nordic Seas region was caused by increased summer solar insolation to the northern Hemisphere. The suborbital millennial to decennial climate variability observed in the Holocene climate reconstructions was most probably a combined effect of solar activity changes, periods of increased explosive volcanism, and internal feedback mechanisms in the Earth's climate system. Internal modes of the climate system variability (for example, the North Atlantic Oscillation) were most likely responsible for some of the observed climate variability in winter. In addition to external climate-forcing factors, mcltwaler from the retreating northern hemi- sphere ice sheets and abrupt, catastrophic lake drainage from ice-marginal lakes during the termination of the last Ice Age had a significant, regional climate impact. In the near future, it will be possible to combine high-resolution palaco- climatic reconstructions with fully coupled ocean-atmosphere climate models in order to test the complex interactions between the different forcing factors to help explain the primary forcing factors and the accompanying feedbacks behind the observed Holocene climate development in the North Atlantic region.