THE USE OF LAKES, MARSHES, AND RIVERS IN DEFINING AND DATING THE MEDIEVAL CLIMATIC ANOMALY

Geomorphic, sedimentary, botanical, and archeological evidence from lakes and rivers in California and western Nevada indicates that during upper medieval time the region experienced two century-plus intervals in which effective precipitation remained well below the average values of the modern (instrumental) period. These medieval droughts drew the water bodies to low levels, allowing trees to colonize the newly exposed lake- and stream-beds. Eighty-five radiocarbon dates, together with ring counts from stumps rooted on those relicted beds, demonstrate that the first drought was underway by around CE 900, and that it persisted until approximately 1120; the second drought lasted from roughly CE 1170 to 1350. Evidence from several of the lakes indicates that the half-century that separated the two droughts was substantially wetter than the modern average condition.

In a Nature paper of 1994 I asserted that this drought-deluge-drought sequence of California and Nevada was the local manifestation of a "Medieval Climatic Anomaly" (MCA) that characterized much of the world during the 10th through mid-14th centuries; I argued that, to avoid seeding preconceptions and prejudicing future findings, epithets such as the then-popular "Climatic Optimum" and "Medieval Warm Period" (MWP), which are at most applicable only locally, should be replaced with MCA; and I proffered that a key to understanding the nature of the MCA's aberrant climate lies in reconstructing the prevailing configuration(s) of Earth's circumpolar vortices.

In this presentation I will update the state of our understanding of the MCA in the western U.S.; point out how ongoing conflation of MCA and MWP confuses discussions of medieval climate; issue a warning about one of my own previously reported findings; and discuss the upper medieval configuration of the boreal circumpolar vortex and how it relates to the configuration that has characterized the past four winters of drought in California.