HOLIVAR2006 Abstracts
Holocene climate research - progress, paradigms, and problems.
H.J.B. Birks
Department of Biology, University of Bergen, Allegaten 41, Bergen N-5007, Norway
Contact: H.J.B. Birks (John.Birks@bio.uib.no)
The reconstruction of natural climate variability in time and space over the last 11500 years of the Holocene has been a major research challenge for the last 125 years. There have been several paradigms about Holocene climate change closely linked to particular palaeoclimatic proxies and research approaches.
1. Early studies on peat-bog stratigraphy led to the paradigm of alternating warm and dry and cool and wet phases such as the Boreal, Atlantic, Sub-Boreal, and Sub-Atlantic, the so-called 'Blytt and Sernander scheme'.
2. The development of pollen analysis and the study of plant and animal macrofossils and of pollen indicators led to the first quantitative estimates of summer and winter temperature changes. The paradigm of a three-fold division of the Holocene into periods of increasing warmth, maximum warmth, and decreasing warmth prevailed until the 1960s.
3. The development and widespread application of radiocarbon-dating in the 1960s to 1980s permitted the temporal and spatial patterns of pollen-analytical data to be mapped. Such maps revealed complex and individualistic patterns of trees in the Holocene, suggesting that climate patterns may have been equally complex spatially and there may have been past climates with no modern analogues.
4. After the development of quantitative transfer functions in palaeoceanography, modern pollen–climate transfer functions were developed to derive quantitative estimates of past climate for comparisons with climate–model hindcasts (COHMAP).
5. With the development of palaeolimnology in the 1980s, a range of limnological proxies (biotic, isotopic, geochemical) have been studied. These have revealed major hydrological changes, particularly at low latitudes.
6. High-resolution studies of Greenland ice-cores in the 1990s and subsequent detailed investigations of high-latitude records have revealed the existence of several short-lived abrupt events in the Early Holocene, probably triggered by glacial meltwater spikes resulting from the melting of the North American Laurentide ice-sheet.
7. Gas bubbles preserved in ice-cores provide data on past atmospheric composition. They show that carbon dioxide levels increased about 8000 years ago and methane levels rose about 5000 years ago. Such changes do not occur in previous interglacials, and these Holocene increases have been interpreted as a result of prehistoric human activity, the 'Anthropocene' hypothesis.
8. Renewed research on Holocene glacial histories, particularly in Scandinavia, has shown that major glacial changes have occurred in the Holocene due primarily to changes in winter precipitation, suggesting the existence of periods of enhanced 'North Atlantic Oscillation' climate mode in the past.
9. An increasing number of fine-resolution studies using a range of proxies provides strong evidence for both solar and volcanic forcing and for climatic extremes, especially in the hydrological regime. Such extremes must have had major implications for human societies in different parts of the globe.
The current paradigm of Holocene climate history is of significant natural variability at annual, decadal, centennial, and millennial scales, especially in precipitation, and with considerable spatial variation. Global summaries are thus of limited relevance, whereas changes at different latitudes are more relevant and are of considerable significance environmentally, ecologically, and socially.
The major problems in Holocene climate research continue to be problems in absolute chronology and hence in reliable correlation between different proxies and between different geographical areas. Additional problems include evaluating the reliability of quantitative palaeoclimatic reconstructions and realistically estimating their uncertainties, understanding what aspects of the environment different proxies reflect and respond to, and interpreting observed changes in terms of different forcing functions.
John Birks is a Quaternary palaeoecologist with particular interests in pollen analysis, vegetational history, quantitative techniques, palaeolimnology, and biotic responses to environmental change. He graduated from the University of Cambridge in 1966 and completed his PhD there in 1969. After a post-doctoral year with Herb Wright in Minnesota, he was based in Cambridge until 1985 where he worked primarily on Holocene changes in Scotland and on developing quantitative techniques in pollen analysis. After moving to Bergen in 1985, he widened his interest to include palaeolimnology and quantitative palaeoenvironmental reconstructions. He has close links with the Environmental Change Research Centre at University College London. In recent years his research has focused on Holocene climate changes in Fennoscandia and in the development, evaluation, and application of quantitative transfer functions to infer past environmental conditions. He currently collaborates with colleagues in Bergen, London, Helsinki, Utrecht, Kingston, and Bern. In addition to palaeoecology, he retains a strong interest in modern plant ecology and floristics, in particular alpine areas around the world and in the response of alpine plants to recent environmental change.