Tree ring-reconstructed late summer moisture conditions, 1546 to present, northern Lake Michigan, USA

Abstract

Drought can affect even humid regions like northeastern North America, which experienced significant, well-documented dry spells in the 1930s, 50s, 60s, and 80s, and proxies tell us that in the years before instrumentally recorded climate, droughts could be even more severe. To get a more complete picture of pre-recorded climate, the spatial coverage of proxy-based climate reconstructions must be extended. This can better put in context past, current, and future climate, and it can lend anthropological and historical insights. With regard to tree rings as climate proxies, however, there is increasing evidence that relationships between tree growth and climate can be inconsistent over time, in some cases decreasing the utility of tree rings in the representation of climate. We developed a chronology from white cedar Thuja occidentalis tree ring widths for the period 1469-2015 C.E. with which we modeled the relationship between growth and July-September moisture conditions (Palmer Z index). The relationship was consistent across the period of instrumentally recorded climate, 1895-present, and the model explained 27% of variability. Therefore, we used the model to reconstruct July-September moisture conditions from 1546-2014. We found the most variable century to be the 20th, the least the 18th. The severest decade-scale droughts (≤0.75 SD from mean) occurred in the 1560s, 1600s/10s, 1630s, 1770s/80s, 1840s, and 1910s/20s, the severest pluvials (≥0.75 SD) in the 1610s/20s, 1660s/70s, and the 1970s/80s. The occasional occurrence of severe droughts throughout the reconstruction, increasing variability in the 20th century, and expected climate change-enhanced late summer drought, portend a future punctuated with severe droughts.