Tracking long-term changes in climate using algal indicators in lake sediments

Interest in climate change research has taken on new relevance with the rea lization that human activities, such as the accelerated release of the so-c alled greenhouse gases, may be altering the thermal properties of our atmos phere. Important social, economic, and scientific questions include the fol lowing. Is climate changing? If so, can these changes be related to human a ctivities? Are episodes of extreme weather, such as droughts or hurricanes, increasing in frequency? Long-term meteorological data, on broad spatial a nd temporal scales, are needed to answer these questions. Unfortunately, su ch data were never gathered; therefore, indirect proxy methods must be used to infer past climatic trends. A relatively untapped source of paleoclimat e data is based on hindcasting past climatic trends using the environmental optima and:tolerances of algae (especially diatoms) preserved in lake sedi ment profiles. Paleophycologists have used two approaches. Although still c ontroversial, attempts have been made to directly infer climatic variables, such as temperature, from past algal assemblages. The main assumption with these types of analyses is that species composition is either directly rel ated to temperature or that algal assemblages are related to some variable linearly related to temperature. The second more commonly used approach is to infer a limnological variable (e.g. water chemistry, lake ice cover, etc .) that is related to climate. Although paleolimnological approaches are br oadly similar across climatic regions, the environmental gradients that pal eophycologists track can be very different. For example, climatic inference s in polar regions have focused on past lake ice conditions, whereas in lak es near arctic treeline ecotones, paleophycologists have developed methods to infer past lakewater-dissolved organic carbon, because this variable has been Linked to the density of coniferous trees in a drainage basin. In clo sed-basin lakes in arid and semiarid regions, past lakewater salinity, whic h can be robustly reconstructed from fossil algal assemblages, is closely t ied to the balance of evaporation and precipitation (i.e. drought frequency ). Some recent examples of paleophycolgical work include the documentation of striking environmental changes in high arctic environments in the 19th c entury believed to be related to climate warming. Meanwhile, diatom-based r econstructions of salinity (e.g. the Great Plains of North America and Afri ca) have revealed prolonged periods of droughts over the last few millennia that have greatly exceeded those recorded daring recent times. Marked clim atic variability that is outside the range captured by the instrumental rec ord has a strong bearing on sustainability of human societies. Only with a long term perspective can we understand natural climatic variability and th e potential influences of human activities on climate and thereby increase our ability to understand future climate.