Holocene Climate Dynamics in High Latitude Europe and the North Atlantic
by
Keith Briffa
Climate Research Unit School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ (UK)
Coauthors: Korhola A., Snowball I., Additional contributions from Koç N. and Nesje A.

Compared to the glacial (stadial) and interglacial (interstadial) climatic cycles of the Quaternary period, the Holocene was considered a climatically stable epoch, against which the observation of post 19^th century AD warming has been considered exceptional. However, various climate proxies obtained from high latitude Europe indicated that significant climatic oscillations took place, which were superimposed upon a long-term cooling trend reflecting orbital forcing. Reconstructions of the amplitude, frequency and above all, the triggers, of these oscillations now form the focus of many multidisciplinary studies of terrestrial and marine environments.

The high-latitude region of the PEPIII transect, which extends from Greenland to the southern tip of Fennoscandia, is possibly unrivalled in its collection of high-temporal resolution geological archives and access to long meteorological observations and historical annals. The distinct seasonal variations in climate promote the formation of natural archives with inherent calendar year chronologies, such as ice-sheets, tree-rings, varves and speleothems. Greenland and Fennoscandia host mountain glaciers that are sensitive to changes in precipitation and temperature, and fluctuations in their mass balance can be reconstructed from studies of moraines and pro-glacial lake-sediments. Ecotones are found along steep climatic gradients, such as those on the slopes of the Scandinavian mountain range and the "limes Norrlandicus." The landscape of Fennoscandia is littered with lakes and peat bogs, and their sediments contain a rich diversity of physical, chemical and biological climate proxies.

Based on both relative and quantified proxies, a reasonably coherent story emerges if we attempt a regional synthesis of Holocene climate change in the PEPIII high latitude region. The early Holocene was a period of continued transition from glacial to interglacial. Orbital forcing (summer insolation) reached a maximum in the early Holocene and sea surface temperatures (SST's) may have reached their Holocene maximum. However, influxes of meltwater from the diminishing ice-sheets frequently interrupted the gradual intensification of North Atlantic thermohaline circulation (THC). Such interruptions caused the pre-Boreal Oscillation (PBO) and the most dramatic 300-400 year duration cold and dry event centred between c. 8200 and 7800 cal BP. According to varved lake-sediments, rapid warming within 75 years was associated with the end of this final vestige of the last ice age. THC intensity peaked for an extended period between c. 7800 and 5800 cal BP, which promoted a stable "thermal maximum" in Fennoscandia and the rapid retreat and disappearance of many mountain glaciers. Relatively invariable tree-ring widths and the structure of varved lake-sediments attest to this period of environmental stability, which allowed thermophilous plants to move northwards and upwards. Calibrated proxies suggest average summer temperatures up to 2°C higher than today. Several lines of evidence indicate that this period of stability was terminated by a sudden cooling at 5800 cal BP. Limnological proxies, such as aquatic chironomids and primary productivity suggest that this cooling was as effective as those initiated by earlier interruptions of the THC, and peatland expansion points to increased humidity with reduced seasonality. Here the story becomes more complex. Studies of tree-lines, forest composition, glacier activity, stable isotopes and catchment erosion point to a general, possibly stepwise, cooling of climate during the remainder of the Holocene, prior to the 20^th century AD. Major re-advances of glaciers, the reduction of tree lines and enhanced landscape erosion indicate a significant change between 4500 and 3700 cal BP to a considerably more variable climatic regime. It is also important to consider the significant impact of humans on the landscape during the middle and late Holocene, for whom climate change may have played a deterministic role.

Many archives from marine and terrestrial environments show evidence of the most recently recognised centennial scale oscillations in climate, such as the Little Ice Age (LIA) and the Medieval Warm Period (MWP, or Medieval Climate Anomaly - MCA). However, while the signals of such events can be correlated between different archives, it remains difficult to assign them definite ages. This difficulty most likely arises from the fact that different proxies reflect different components of the climate system, which simply shows its truly dynamic behaviour.

So, the evidences from various terrestrial and marine archives in high latitude Europe point to a long-term, post-glacial Holocene cooling trend, which reflects the dominant role of orbital forcing and summer insolation. However, a synthesis of palaeo-records shows that the long-term trend was possibly stepwise and punctuated by rapid climate shifts, predominantly as cooler periods. There is also no doubt that the northwards transfer of heat, via the Atlantic Ocean thermohaline circulation, has regulated climate in high latitude Europe on a wide range of timescales, from decades to millennia. However, the errors associated with the majority of geo-chronometers force us to admit that we have difficulties to temporally separate records of the triggers (causes) of rapid climate change from their effects on ecosystems. Statistical analyses of palaeo-data can be carried out on a wide range of well-dated archives in the high latitudes, and even true time-series analysis can be carried out on tree-ring and varve data sets. Such analyses will provide true tests of the hypothesis that repeated cooling events occurred with a recurrence interval of c. 1500 years through the late Pleistocene and the Holocene. Finally, synchronisation between ice-cores, high-resolution marine sediments and terrestrial archives via independent, accurate and precise dating techniques will lead to important advances in our understanding of the climate system in high latitude Europe, where ecosystems are already responding to post-19^th century AD warming.

Date received: May 9, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahi-82.