HOLIVAR2006 Abstracts
Droughts and floods: proxy records of rapid hydrological change during the Holocene.
Dirk Verschuren and Dan Charman
Ghent University, Ledeganckstraat 35, Ghent, B-9000, Belgium
Contact: Dirk Verschuren (dirk.verschuren@rug.ac.be)
Improving our understanding of regional climate and hydrological variability at decade-to-century time scales challenges both data analysis and climate modelling communities because external forcing (solar variability, volcanic eruptions) is relatively modest and expressed through both direct and cumulative effects, and because both delayed responses of individual system components (e.g., oceans, the cryosphere, continental aquifers) to this forcing and internally-generated climate variability become more prominent. Consequently, uncertainty exists as to what extent decadal and century-scale hydrological variation can simply be treated as the cumulative effect of inter-annual variation and extreme events. A satisfactory answer to this question is critical to evaluate the relationship between climate variability (rainfall and temperature-controlled evaporation) and water-resource availability at the landscape scale, which has most directly impacted on agricultural societies of the past, present, and future.
Here we search palaeorecords of Late Holocene (last ~4000 years) hydrological change in continental Europe, Africa, and surrounding oceans for evidence of mechanistic links between north-temperate and tropical hydrological variability at decade-to-century time scales. Major water-table changes in north-west Europe reconstructed from British and Dutch peatlands are synchronous with lake-level changes in mid-latitude central Europe, with wetter and cooler conditions associated with enhanced polar circulation and cold periods in the North Atlantic. The relative magnitude of individual events is regionally diverse. Some pronounced episodes of high lake level and aquifers (at ~3400, 2750, 1250 and 700 cal. yr BP) corresponding to periods of reduced solar activity support a causative link, but the anomalies are not proportional and some hydrological events have no equivalent in the solar-forcing record. Records from Iceland to tropical Africa reveal a pronounced shift to wetter conditions ~1250 AD, terminating a warm and/or dry Medieval Optimum with the onset of the most recent North Atlantic cool event. Such events cause anomalous Atlantic trade wind patterns, which affect cross-equatorial moisture transport and the mean position of the Intertropical Convergence Zone (ITCZ). In western tropical Africa, at least some north Atlantic cool episodes shifted the ITCZ sufficiently southward to cause drought in northern tropical and subtropical regions but high rainfall in the southern subtropics. In parts of eastern equatorial Africa, hydrological variability at this time scale has been inversely related to solar activity over the last ~1800 years at least, and thus largely synchronous with moisture-balance variation in north-temperate Europe. In central equatorial Africa, drought extremes during recent millennia appear to have occurred at ~700-year intervals both during minima and maxima of the north Atlantic cycle, presumably because at either time the zone of surface convergence and rainfall was displaced northward or southward away from its mean position near the equator.
Dirk Verschuren has a LicSc in Zoology from Ghent University (Belgium) and a PhD in Ecology from the University of Minnesota (USA). Following postdoctoral appointments at Ghent, the Large Lakes Observatory (Duluth, USA), and the GeoForschungsZentrum Potsdam (Germany) he returned to Ghent to take up a research professor position. His research interests include African climate history at both long and short time scales, and the long-term dynamics of African lake and terrestrial ecosystems under the dual influence of natural processes and human impact, which he studies using multiple proxy indicators extracted from lake-sediment records. Current projects focus on regional gradients in African climate change within the last 1000 years, the separation of temperature and hydrological variability during post-glacial climate history on the Equator, and late-Holocene climate environmental history in the central Sahara desert. Dirk Verschuren is SC member of the ESF programmes ‘Holocene climate variability’ (HOLIVAR) and EuroCLIMATE.