EXTREME AND PERSISTENT DROUGHT IN CALIFORNIA AND PATAGONIA DURING MEDIEVAL TIME

STUDIES from sites around the world(1-5) have provided evidence for an omalous climate conditions persisting for several hundred years before about AD 1300. Early workers emphasized the temperature increase that marked this period in the British Isles, coining the terms 'Mediaeval Warm Epoch' and 'Little Climatic Optimum', but many sites seem to hav e experienced equally important hydrological changes. Here I present a study of relict tree stumps rooted in present-day lakes, marshes and streams, which suggests that California's Sierra Nevada experienced ex tremely severe drought conditions for more than two centuries before A D similar to 1112 and for more than 140 years before AD similar to 135 0. During these periods, runoff from the Sierra was significantly lowe r than during any of the persistent droughts that have occurred in the region over the past 140 years. I also present similar evidence from Patagonia of drought conditions coinciding with at least the first of these dry periods in California. I suggest that the droughts may have been caused by reorientation of the mid-latitude storm tracks, owing t o a general contraction of the circumpolar vortices and/or a change in the position of the vortex waves. If this reorientation was caused by mediaeval warming, future natural or anthropogenically induced warmin g may cause a recurrence of the extreme drought conditions.