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.