Stable isotope values of shallow groundwater emerging from springs in a high-elevation basin in the central Sierra Nevada of California decrease with increasing geochemical groundwater ages. From water recharged in 1960 to water recharged in 1990, a 1.2‰ and 11‰ increase occurs in δ18O and δD, respectively. Historic temperature records from surrounding areas show a 1.3 °C increase in mean annual air temperature over the same time period. This temperature change alone is not great enough to explain the observed increase in δ18O. We suggest that changes in atmospheric circulation patterns, which have been documented in this region, account for the remaining increase in δ18O. The time scale of these climatic changes, recorded in both the historical and the hydrological records, is extremely short (decadal scale) and suggests that the δ18O of groundwater can be a useful tool for studying climatic and atmospheric circulation changes over very short time intervals. It also suggests that circulation changes must be considered along with temperature when interpreting paleoclimate data.