Transfer functions and the quantitative reconstruction of past climate

A major paradigm shift in Quaternary paleoclimate research occurred in the early 1970s. John Imbrie (awarded the Vega Medal in 1999, 45 years after von Post) and the late Nilva Kipp (1925-1989) revolutionized marine paleoceanography by developing transfer functions to reconstruct quantitatively summer and winter sea-surface temperatures and salinity from fossil foraminiferal assemblages preserved in a deep-sea core covering several glacial and interglacial stages (Imbrie and Kipp 1971). The idea of transfer or calibration functions was also developed by Hal Fritts and colleagues for deriving quantitative estimates of past climate from tree rings in the western USA (Fritts et al. 1971; Fritts 1976) and by Tom Webb,

Reid Bryson, and associates for the quantitative reconstruction of past climate from fossil pollen assemblages in the Great Lakes region of North America (Webb and Bryson 1972).

The basic idea of transfer functions is very simple. If modern biologic assemblages (e.g. foraminifers in an ocean sediment core top) and modern environmental (e.g. climate) data are available for a wide range of sites and environmental conditions today, it is possible to model the relationship between the modern assemblages and the contemporary environment by some form of regression or calibration (inverse regression) procedure, to derive modern transfer functions. These functions summarize mathematically the relationships between modern biota and modern environment. If the assumption is made that the transfer functions are invariant in space and time, they can be applied to fossil assemblages to derive quantitative estimates of the past environment at the time the fossil assemblages were deposited.

Although there are now many numerical procedures for deriving, applying, and evaluating transfer functions (e.g. Birks 1995), the basic principles and assumptions presented by Imbrie and Kipp (1971) remain the same. Transfer functions are now widely used in Holocene climate research to derive quantitative estimates of several climatic variables (e.g. summer, winter, and annual temperatures, length of growing season, annual precipitation, ratio of actual to potential evapotranspiration) and climate-related variables (e.g. lake-water temperature and salinity, sea-surface temperatures and salinity, bog moisture) from a wide range of fossil assemblages (e.g. pollen, cladocera, chironomids, diatoms, foraminifers, radio-larians, dinoflagellate cysts, chrysophytes, coccolithophorids, testate amoebae). Several of the chapters in Mackay et al. (2003) give examples of the application of transfer functions in Holocene climate research.

Transfer functions played an important role in the CLIMAP Project Members (1976) (Climate: Mapping, Analysis, and Prediction) project that attempted to reconstruct the climate of the last ice-age Earth, primarily from paleoceanographic data. This project also involved the compilation and synthesis of a range of biologic and geologic paleoclimate data relating to 18 000 years ago and the first attempts at using climate models to simulate an 18 000-year climate (e.g. Williams et al. 1974; Gates 1976; Manabe and Hahn 1977).

Transfer functions and CLIMAP rapidly provided new paradigms for paleo-climate research, namely transfer functions for quantitative reconstructions of past climate and the use of climate models to simulate past climates for comparison with geologic and biologic "proxy-climate" evidence. This new paradigm led directly to COHMAP (Co-operative Holocene Mapping Project) and major developments in paleoclimate modeling.

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