Geoscience Reference
In-Depth Information
Europe, the changes in climate fed the blossoming of societies. But in the
American West, the changes in climate brought devastation.
We now see that the Medieval period, initially believed to be a worldwide
period of unusual warmth, was not a uniform experience across the globe.
In a seminal 1994 review of the topic, appropriately entitled “Was there a
'Medieval Warm Period' and if so, where and when?,” Malcolm Hughes and
Henry Diaz came to the conclusion that the 500-year period of the Middle
Ages was a time when various regions across the world experienced episodes
of warmer temperatures, but not all at the same time. Furthermore, in some
regions, including the southeastern United States, the Mediterranean, and
parts of South America, there appeared to be very little change in average
temperatures. h e authors proposed that the misnomer “Medieval Warm
Period” be replaced by the term “Medieval Climate Anomaly.” In this topic,
too, we will use the latter term in the remainder of our chapters.
h e article was particularly timely for California and the West,
since the 1980s and 1990s were marked by prolonged droughts alternating
with extremely wet years with damaging l oods. Many people, including
water managers, were becoming increasingly interested in understanding
the region's climate. In this context, information about the lake-level
and tree-stump history of Mono Lake and the increased salinities in San
Francisco Bay provided stark warnings of how climate could af ect our lives
in the future.
back to the marshes
We return to the Suisun Marsh record because it shows that a long period,
including the Medieval Climate Anomaly, was clearly drier in San Francisco
Bay's vast watershed than it was before or at er. However, the marsh record
lacks any evidence of the two “megadroughts” detected in the central to
southern Sierra. To further understand the changes in the bay estuary's
watershed during these droughts, our research group took a second look at
the San Francisco Bay marsh sediments, this time focusing on the clays and
silts that wash into the bay from its watershed. h ese sediments carry mineral
and chemical i ngerprints of their origins, such as the volcanic rocks (mainly
andesites) of the northern Sierra Nevada range and southern Cascades, or
the granites and granodiorites of the central and southern Sierra. As these
sediments are carried into San Francisco Bay, the largest particles settle to
