Folding pattern diversity of integral membrane proteins.

clear that total S02 is at least as important to climatic impact as eruptive volume, the traditional volcanological measure of eruption size and the only one that can be conveniently estimated for ancient events. Furthermore, these two measures are not always correlated, and several high-SO2 eruptions of modest size have put up significant stratospheric aerosols (5). These findings complicate attempts to evaluate climatic and other impacts of ancient eruptions. But all these complications and weaknesses in the record help to underscore the importance of alternative approaches from other disciplines to the building of a reliable chronology of global volcanism. The H2SO4 aerosols eventually settle to Earth, and pioneering work by Danish glaciologists in the 1970s (6) showed that the resulting acidity layers in deep ice cores from Greenland provide a volcanic chronology. American and French groups found the same evidence in Antarctica and, by correlating several layers and confirming the common composition of their (rare and tiny) volcanic glass fragments, showed that some eruptions have a truly global distribution of products (7). These results have continued, through painstaking work on ever more cores, and the report by Zielinski et al. discusses results from the newest and deepest Greenland core (1). The authors suggest more accurate dates for several large eruptions and provide many new dates (particularly before 0 B.C.) from unknown sources. The largest signal in the last 7000 years, also detected in Antarctic cores (7), was from an unknown source around 1258 A.D. However, four larger signals were found in the 7th millennium B.C., marking this as easily the most volcanically active part of postglacial time. The new results are exciting to all scientists interested in the volcanological record. The principal problem of this approach, however, is that aerosols move swiftly eastward around the globe but their latitudinal spread is relatively slow. This means that an eruption from high north latitudes (Iceland, Alaska and Kamchatka) leaves a relatively large volcanic deposit on Greenland, whereas a comparable one from low latitudes leaves a much smaller record, and one from the Southern Hemisphere may leave none at all. Until more cores are obtained from midand low-latitude sites (not famous for their stable glaciers), substantial uncertainty will surround the identification and calibration of eruptive sources. Added to this problem is the danger of misinterpreting the completeness of volcanism's recent historical record. Very large eruptions may well have been missed only a few hundred years ago in some parts of the world, so the matching of sulfate