Saturday, January 31, 2009

California Drought - now and the MWP

California is in a period of severe drought, and this may lead to mandatory water rationing, since even preparations established in better times may well be inadequate this year. But this is not a new discovery. The water that California uses comes from snow melt in the Sierras, and Duffy has found that for longer than the past 100 years there has been a steady trend downwards in the flow of major rivers in CA.

Water flows from the 8 major rivers in the Sierra Nevada – after Duffy

This can be traced back to smaller snowfalls that Stewart has found now melt earlier and with lower volume.

However it is wrong to call this unprecedented. Since back in the Medieval Warming Period, a thousand years ago, things were much worse.
Results indicate an effectively dry climate between A.D. 950 and 1220, corresponding to the Medieval Climatic Anomaly (a warm period in northern Europe), during which Owens Lake approached playa conditions. Wet climates prevailed during A.D. 1220–1480, producing relatively large and deep lakes. Beginning about A.D. 1550, the regional climate turned colder but had frequently oscillating precipitation. Six wet/dry cycles with similar 50-yr duration occurred between A.D. 1480 and 1760, during the later half of which Owens Lake became a playa. Since A.D. 1880, the lake level has steadily dropped from its historic high stand under strong impact of human activity.
A playa is the sort of structure that is often shown on TV where the correspondent is standing on the dried bottom of a lake that has disappeared.

The conditions that caused these droughts, and the evidence for them have been investigated by Scott Stine. If you have the time it is worth watching his video presentation at the California Water Colloquium Series. It is written up by Davis .
Until Los Angeles built the aqueduct and cut off Owens Lake's supply of fresh water, the 112-square-mile lake was the southern terminus of the Owens River, which drains the eastern watershed of the Sierra Nevada. (It dried up in the mid-1920s.) “We now have compelling proof,” says Stine, “that Owens Lake dried up and became a desert playa in the early medieval period. The finding has ominous implications for the future security of Los Angeles' water supply.”

Two years ago, Stine caused a sensation in the science press with his claim that California had endured two epic droughts in the Middle Ages, one of 220 years (from 892 to 1112) and the other of 140 years (from 1209 to 1350). By contrast, the most severe recent drought— which created an unprecedented statewide water emergency— lasted only six years, from 1987 to 1992.

Stine's primary evidence, now broadly accepted, consists of ancient tree stumps that were exposed to view when the 1980s drought and DWP greed reduced water levels by more than 50 feet in Mono Lake (the northernmost catchment of the Los Angeles Aqueduct) and other Sierran lakes and streams.
Boxt has further expounded in this:
A variety of evidence from the Sierra Nevada to the Pacific Ocean suggests that the late Holocene California droughts were particularly acute in the interval between about A.D. 900 to 1300, variously labeled as the Little Climatic Optimum, Medieval Warming Period, and Medieval Climatic Anomaly (or MCA, the term used in this discussion). Graumlich (1993) for example, reports a 1,000-year tree-ring record which indicates more severe and prolonged droughts during the MCA in the Southern Sierra Nevada than during the present century (A.D. 1020 =-1070, 1197-1217, 1249-1365).
Interestingly in her paper Graumich also says
The warmth of the 20th century was exceeded for an extended period of time during the 12th century. This warmth occurred under conditions of preindustrial carbon dioxide concentrations and demonstrates that external climate forcings (e.g. volcanic aerosols, solar variability) may interact with internal oscillations associated with changes in ocean circulation to produce widespread warmth.
Her temperature assessment from tree rings differs considerably from that of Professor Mann et al.
California temperatures over the past Millennium (after Graumlich)

Man came to California at least 13,000 years ago. About 9,000 years ago they migrated to the coast.
Gallegos (1991) suggests that about 9000 years ago people left the desiccating lakes of the Great Basin and moved to the newly formed coastal estuaries (created by the inundation of rivers during post-Pleistocene sea-level rise) along the San Diego coast. His inferences are based largely on similarities in tools and the means by which various species were exploited in the interior desert and coastal estuary areas. Although the evidence is fairly compelling, many questions remain about this early phase in San Diego, particularly because some early coastal Millingstone sites in San Diego and on the central coast now seem to predate the onset of marked Early Holocene aridity.
Even back then changes in climate and the onset of drought were causing problems.

Moving back to the Medieval time
Yatsko clearly documents periods of persistent and severe drought in San Clemente Island prehistory. Using a series of radiocarbon dates from his extensive survey programs, he closely links population trends to these climatic changes, noting especially the very xeric Medieval Climatic Anomaly at AD 1050–1250 and the sharp residential shifts associated with use of the island’s most reliable fresh water.
The drought often led to conflict, and disease.
Jones et al. (1999) broaden the scope of this analysis, providing data on paleoclimatic declines (especially drought) and cultural changes from three areas of California and the Colorado Plateau during AD 800–1350 (the Medieval Climatic Anomaly). They suggest that populations were relatively high and locked into territories, so when severe droughts struck across the west, problems arose that could not be solved by intensification or smaller-scale adjustments. In some areas, major population movements occurred (Southwest, California’s central coast); in others (Santa Barbara) there was a short period of violence and declining health but also increased regional trade and rapid political centralization.

The droughts back in the MWP were longer than we are anticipating and much more severe. If we are, as seems quite likely, returning to the temperatures and climate of the MWP then California may anticipate seeing those conditions again, with droughts that extend for decades. As this one is already carrying us beyond the capacity of the system to cope, it would appear that considerable more planning and preparation is required, since blaming the whole drought scenario on Global Warming is likely neither correct nor helpful in today's circumstance.

1 comment: