25.04.2001 | Meeting reportWorkshop ĢReconstructing Late Holocene Climateģ
Charlottesville, Virginia, U.S.A., 17 - 19 April 2001
Sponsored by different organizations (PAGES/CLIVAR, U.S. NSF, NOAA, GKSS Geesthacht, Netherlands NRP, University of Bern), the workshop was convened by Mike Mann, *yvind Nordli, Nanne Weber, Hans von Storch and Heinz Wanner. It was attended by 44 invited participants who, in different sessions with lectures and longer discussion blocks, tried to outline future goals, procedures and possible joint projects in the area of paleoclimate reconstruction and modeling.
The first session on Reconstruction of European Climate from Historical and Proxy Data was mainly centered on NAO reconstruction and dynamics, and the stronger consideration of documentary data in climate reconstruction (lectures by R. Brāzdil, Brno and M. Barriendos, Barcelona). Ed Cook (Lamont Doherty Earth Observatory) demonstrated that the Luterbacher index is actually the best long term data base for NAO studies. Jürg Luterbacher (University of Bern) presented a new multi-proxy based reconstruction of a 500 years European temperature time series with monthly and seasonal resolution, and Heinz Wanner (University of Bern), for the same time period and area, discussed the dynamics of the preferred climate modes in winter. The U.S. colleagues were quite amazed by the rich and broad availability of significant documentary data in the European (and Asian) area.
The session on Multidecadal Droughts in Western North America was mainly centered on the question of how to define droughts and how far these droughts connect to El Niņo - La Niņa events. Malcolm Hughes (University of Arizona) and Connie Woodhouse (NOAA, Boulder), by using historical drought indices, showed that a strong correlation not only exists between El Niņo/La Niņa events and U.S. drought. They also speculated about a possible influence on the Atlantic-European area.
In the session on Records of ENSO and Tropical Variability, Julie Cole (University of Arizona) and Konrad Hughen (Woods Hole Oceanogr. Institute) pointed to the large temporal changes in the ENSO variability, e.g. also in the cooling period between 3300 and 2800 BP. They also expressed the strong request for a better coordination with Atlantic-European projects dealing with exchange mechanisms between the Pacific and the Atlantic basin, including thermohaline ocean circulation.
The sessions on Large-scale Climate Reconstruction and on Forced Climate Variability in Past Centuries were split into general overviews by Judith Lean (Naval Res. Lab. Washington) and Ray Bradley (University of Massachusetts), and more specific talks dealing with open questions. Keith Briffa (CRU Norwich) reported about the actually unexplicable drop of dendrodata after 1960 which is in contradiction with the increasing global temperatures (influence of the environmental pollution?). Alexey Kaplan (Lamont Doherty Earth Observatory) and Scott Rutherford (University of Virginia) discussed new successful approaches to get to an optimal long term reconstruction of spatially sparse climate data. Thomas Crowley (Texas A&M University) and Jonathan Overpeck (University of Arizona) discussed the problem of the sensitivity of the climate system. By using his simple radiative-convective model, Crowley showed that a possible sensitivity change leads to a divergence between observed global temperatures between 1830 and 1880 (?). Overpecks model studies show that the sensitivity of the global climate system might be rather in the 3 to 4 °C range than below.
The final session on Model/Paleodata Comparison was introduced by an interesting talk of Hans von Storch (GKSS Geesthacht) on data assimilation techniques. He voted for a better inclusion of all rich data sources in longer term model integrations. Several authors demonstrated longer term model based reconstructions of sea surface temperature, sea level, tree-ring growth and glacier dynamics. They emphasized that precise estimations of long term sea levels and ice mass balances of glaciers are still lacking.
Future challenges in Reconstructing Late Holocene Climate will include:
- the more precise spatiotemporal reconstruction of global state variables (including not only air temperature, but also SST, air pressure and precipitation);
- the improvement and comparison of related reconstruction, interpolation and assimilation techniques;
- the closer collaboration between U.S and European programmes, as well as between experimentalists and modelers in the central time windows of the last 500 and 1000 years.
Heinz Wanner, University of Bern