millennia, partly because the vegetation that produces pollen lags in its response to climatic forcing.
The calibration approach used for all of these organisms is based on modern training sets rather than matching them against past temperature changes in the instrumental record. A minimum of about 30 sites are analyzed along a climatic gradient for their contemporary species mix (e.g., diatoms in surface muds) and for a range of environmental measurements (pH, salinity, water, air temperature, etc.). These modern training sets are then used to calibrate past species assemblages preserved in sediment cores using regression-based multivariate statistical techniques (Birks 1998). Using this approach, freshwater diatoms from Alpine lakes were found to track lake-water pH, which in turn has followed 20th century temperatures (Koinig et al. 1998).
Speleothems are cave deposits such as stalagmites. They record changes in the external climate via a range of different proxies, including luminescence intensity, growth rate, and elemental and isotope chemistry (McDermott et al. 1999, Lauritzen 2003). Speleothem calcium carbonate registers the changing isotopic composition of cave groundwater along with cave temperature. Calcium carbonate deposits grow radially as well as upward in a stalagmite, and records of annual isotopic changes such as δ18O can be evaluated chronologically by accurate dating techniques (e.g., uranium-thorium ratio) and then converted to a climate signal. Such records represent a com-