Evidence for cooler European summers during periods of
changing meltwater flux to the North Atlantic

HEIRI et al / PNAS 26oct04

Oliver Heiri *,, Willy Tinner and André F. Lotter *

*Department of Palaeoecology, Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands; and Institute of Plant Sciences, University of Bern, Altenbergrain 21, CH-3013 Bern, Switzerland

Communicated by H. E. Wright, Jr., University of Minnesota, Minneapolis, MN, September 7, 2004 (received for review December 18, 2003)

We analyzed fossil chironomids (nonbiting midges) and pollen in two lake-sediment records to reconstruct and quantify Holocene summer-temperature fluctuations in the European Alps. Chironomid and pollen records indicate five centennial-scale cooling episodes during the early- and mid-Holocene. The strongest temperature declines of 1°C are inferred at 10,700–10,500 and 8,200–7,600 calibrated ¹⁴C years B.P., whereas other temperature fluctuations are of smaller amplitude. Two forcing mechanisms have been presented recently to explain centennial-scale climate variability in Europe during the early- and mid-Holocene, both involving changes in Atlantic thermohaline circulation. In the first mechanism, changes in meltwater flux from the North American continent to the North Atlantic are responsible for changes in the Atlantic thermohaline circulation, thereby affecting circum-Atlantic climate. In the second mechanism, solar variability is the cause of Holocene climatic fluctuations, possibly triggering changes in Atlantic thermohaline overturning. Within their dating uncertainty, the two major cooling periods in the European Alps are coeval with substantial changes in the routing of North American freshwater runoff to the North Atlantic, whereas quantitatively, our climatic reconstructions show a poor agreement with available records of past solar activity. Thus, our results suggest that, during the early- and mid-Holocene, freshwater-induced Atlantic circulation changes had stronger influence on Alpine summer temperatures than solar variability and that Holocene thermohaline circulation reductions have led to summer-temperature declines of up to 1°C in central Europe.

Author contributions: O.H., W.T., and A.F.L. designed research; O.H. and W.T. performed research; O.H., W.T., and A.F.L. analyzed data; and O.H., W.T., and A.F.L. wrote the paper. Abbreviations: THC, North Atlantic thermohaline circulation; cal. B.P., calibrated ¹⁴C years B.P.; IRD, ice-rafted debris.

To whom correspondence should be addressed. E-mail: o.heiri@bio.uu.nl.

source: http://www.pnas.org/cgi/content/abstract/101/43/15285?etoc 26oct04