Maximum parasitism rates and successful biological control.

where Tt and Ct are the temperature and accumulation rate, respectively, at time t. Even small changes in precipitation lead to a difference of several degrees Celsius between Tm and Tweighted' At Summit, Greenland, 8180 more closely tracks Tweighted than Tm. A potential problem with attributing 8180 shifts to changes in moisture source distribution is that the mechanisms required (for example, orographic steering by the Laurentide ice sheet) do not occur as rapidly as some observed CO anomalies. On the other hand, plausible rapid changes in North Atlantic sea surface conditions (such as extent of sea ice) could cause significant changes in the subannual distribution of precipitation events. Many of the artifacts that would be introduced into the ice core record because of changes in the location of moisture source regions would also result from changes in precipitation seasonality. For example, an increase in deuterium excess values (d), which could be interpreted as indicative of a change in source region sea-surface temperature or humidity, would also result from an increase in the ratio of summer as opposed to winter accumulation, because d exhibits a marked seasonal cycle with a late summer peak. At present, anomalies in Greenland ice core records should not be interpreted solely in terms of source region variations. On the other hand, the emphasis by Charles et al. on the need for a multidimensional perspective in the interpretation of ice cores is important. The combined use of b180, 5D, and ionic species in the new Summit, Greenland, cores, should make it possible to answer empirically some of the questions raised by GCM experiments as to the interpretation of C180 records in terms of temperature. Eric J. Steig Pieter M. Grootes Minze Stuiver Quaternary Isotope Laboratory, AK-60, University of Washington, Seattle, WA 98915, USA