July mean temperature and annual precipitation trends during the Holocene in the Fennoscandian tree-line area: pollen-based climate reconstructions

Abstract

July mean temperature and annual precipipation during the last 9900 cal. yr BP were reconstructed from pollen assemblages preserved in a sediment core from northern Finland. Quantitative reconstructions were performed using a modern pollen-climate calibration model based on weighted-averaging partial least squares regression. The predictive ability of the model was evaluated against modern meteorological data using leave-one-out cross-validation. The prediction error for July mean temperature is c. 1.0°C and for annual precipitation 340 mm. The July mean temperatures during the earliest Holocene were low, c. 11.0°C, and annual precipitation was high, c. 600-800 mm. Between 8200 and 6700 cal. yr BP July mean temperatures reached their maxima, 12.5-13.0°C, which are c. 1.4-1.7°C higher than at present. At the same time precipitation decreased. During the late Holocene, July mean temperatures declined and the last 2000 years have been the coolest since the early Holocene. Precipitation has slightly increased. The spatial coherence between our results and of several other climate reconstructions from northern Europe indicates that the Holocene climate was strongly influenced by North Atlantic oceanic and atmospheric circulation patterns. We propose that the distinctly oceanic climate of the early Holocene was due to enhanced westerly (latitudinal) airflow which was replaced at c. 8200 cal. yr BP by a more meridional flow pattern and by the development of predominantly anticyclonic summer conditions.

Climate Change During the Holocene (Past 12,000 Years)

“…The temperature estimates based on macrofossils (for example, tree-line position) indicate a rapid and stable warming after 10,000 cal year BP across the entire northern Scandinavia (Seppä and Birks 2001). Climate reconstructions for central Sweden using pollen spectra from Lake Gilltjärnen sediments show a rapid increase in summer temperatures from 10.0 to 12.0°C between 10,700 and 9000 cal year BP.…”

“…In central and southern Norway, the snow line (equilibrium line altitude) lowered by 200 m (Nesje and Dahl 2001;Nesje 2009). The altitude of the upper boundary of arboreal vegetation also lowered (Seppä and Birks 2001;Seppä et al 2007Seppä et al , 2008.…”

“…According to the increasing Pinus sylvestris pollen percentages and by the decrease of lycopod and fern spore values, the mean annual sum of precipitation must have rapidly fallen to below 500 mm year −1 ca. 6650-5700 cal year BP (Seppä and Birks 2001), due to still unknown causes.…”

“…Davis et al (2003) reconstructed quantitative characteristics of the climate for the past 12,000 years in western Europe based on analyses of pollen data from more than 500 sections. In northern Scandinavia, summer air temperatures close to the modern levels have been inferred from pollen data (Seppä and Birks 2001) and independent estimates based on chironomid and diatom assemblages (Rosén et al 2001).…”

Did a bi-polar multi-level oceanic oscillation cause the Little Ice Age and other high latitude climate extremes?

“…In a seminal paper, Bond et al [2] reported variations of ice-rafted sand and sediment in the northeastern North Atlantic that imply 1500yr cycles in the strengths of the Drift, the SAC flow, and NADW formation with corresponding climatic effects. However, the reality of the 1500yr higher latitude climate cycle has not been well confirmed in the Fennoscandian climate proxy record of Seppä and Birks [3], possibly due to overprinting of the proxies by random regional variations. The cause of a related climatic anomaly, the Little Ice Age, has also been vigorously debated.…”