Looking at historical weather patterns | SierraSun.com
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Looking at historical weather patterns

Mark McLaughlin
Photo by Mark McLaughlinAncient tree stumps in the West Walker River are dated to two severe droughts. The most recent ended less than 700 years ago.
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Glacial epochs and historic weather events prove that the Far West is a region of climactic extremes, and climate change, whatever its cause, is likely to have significant impact on the hydrology and ecosystems of the Sierra Nevada in the future. That was the gist of several presentations by climatologists and research scientists at the 2005 California Extreme Symposium held April 22 on the campus of the California State University, Sacramento. Since 1994, the precipitation symposium has been an annual event at CSUS, where scientists can share with the public the latest research focusing on droughts, floods, and the potential effects of climate change in the Golden State with an emphasis on the hydrologically important Sierra Nevada. Since the end of the last ice age, prolonged droughts and epic floods have plagued our region. We live on a planet that is always in a state of flux, with climate phases swinging like a slow and ponderous pendulum between ice ages and global warming. It is probably no accident that human civilization has blossomed during the last few thousand years when Earth is experiencing the relatively mild climate regime of an interglacial period.

Significant weather events can provide clues to the changing climates of the past and the future. Extensive tree ring analysis indicates that severe droughts lasting centuries or more have seared the western landscape in the not-so-distant past. Ancient tree stumps underwater in Lake Tahoe are convincing evidence of climactic change and a long-term dry period that occurred between 4,000 to 6,000 years ago. The submerged trees were first discovered in 1934 by Samuel Harding, a University of California, Berkeley scientist who noticed the drowned stumps after a prolonged drought had lowered Tahoes water level 14 inches below the spillway of the dam. In recent decades the submerged trees have been located off various shores of the lake, the oldest dating to 6,300 years. Scientists theorize that a mega-drought lowered Tahoes water level long enough for a forest to sprout and grow for decades before a changing climate pattern increased annual precipitation in the region and raised the lake.Paleoclimatologist Scott Stine of California State University, Hayward, has published an analysis of tree stumps he located in Mono Lake, which found two prolonged dry spells lasting 100 to 200 years each one ending in the 1100s, the other in the 1300s. In fact, many lakes in the Sierra Nevada dried up completely during these extended drought conditions and rivers that drained the range were reduced to a trickle. Further evidence of historic mega-droughts comes from cored sediments of Pyramid Lake, in which researchers have been able to construct a 7600-year history of droughts throughout the surrounding region. The scientists concluded: Oscillations in the hydrologic balance occurred, on average, about every 150 years, but with significant variability. Over the most recent 2740 years, intervals between droughts ranged from 80 to 230 years, while drought durations ranged from 20 to 100 years. Some of the larger droughts forced mass migrations of indigenous peoples from lands that could no longer support them.

Droughts are a recurring theme and a normal part of the climactic history of the western United States. It is perfectly reasonable to expect more dry periods in the future, even very protracted droughts, with or without the influence of global warming. With that in mind, consider us lucky that the past 100 years in California appears to be the third or fourth wettest century in the past 4,000 years. The historical droughts of the last 150 years have lasted less than a decade, barely a slap on the wrist compared to the epic droughts of the past. It seems a bit nave to assume that the current average runoff from the Sierra Nevada will continue into perpetuity as it has for the last century or so, a wakeup call for smart growth and intelligent water use and planning.There does seem to be a consensus among most scientists that the Earth has warmed a small fraction in the past few decades. The debate gets more complicated when people ask if this warming is part of a natural process, or is it caused by mans activity? Many environmentalists and most governments in the world have said the answer is that man is pumping too much greenhouse gas into the atmosphere. Thats what the Kyoto treaty is all about. Others point out that the Earth has always fluctuated between warm and cool periods, based on climactic change, sunspot activity, oceanic oscillations, and even the axis tilt of the planet. Retired California State Climatologist Jim Goodridge has been collecting precipitation and weather data for California and the Sierra for 50 years. This ongoing weather archiving project has provided Goodridge with data to make accurate analysis of statewide trends in temperature and precipitation. In reviewing his results, it is understandable why Goodridge questions some of the science and rhetoric that greenhouse gas emissions are forcing climate change. Warming temperature trends in California are generally based on regional averages, but when Goodridge breaks down the regional scope into individual records, a pattern of urban bias is revealed. Goodridge has analyzed 228 stations with 50 to 90 years of record to illustrate that while metropolitan areas show a warming trend (influenced by urban waste heat), rural areas in the State actually show a declining temperature trend. Goodridge also likes to point out the relationships between Californias climate and oscillations in the Pacific Ocean as well as solar and sunspot cycles. According to Goodridge, temperature trends in California clearly reflect sea surface temperature trends in the Pacific. The ENSO, or El Nio Southern Oscillation is the best known of these pattern changes, but the Pacific Decadal Oscillation (PDO) has a marked influence on California temperature trends. In parallel with the El Nio phenomenon, the extreme phases of the PDO have been classified as being either warm or cool, as defined by ocean temperature anomalies. The PDO index responds to the location of upwelling of cold water in the eastern Pacific Ocean. When upwelling is suppressed on our coast like the period from 1975 to 1998, California air temperatures rise. Regional temperatures cool when the cold water upwelling returns, a pattern that is now underway and projected to be with us for the next two decades or more. If Goodridge is right, Californias rural air temperatures may continue to buck the warming trend seen in other parts of the globe.Other research based on computer-driven climate models suggest that global warming will raise snow levels, increase the risk of downstream flooding, and reduce the Sierra snowpack. A significantly earlier spring melt will also challenge water managers and reservoir operators, not to mention the possible economic impact on winter sports. No one can state with certainty what the changing climate has in store for us, but even if its more of the same, we can expect quite a ride.Mark McLaughlin’s column, “Weather Window,” appears regularly in the Sierra Sun. His award-winning books, “Western Train Adventures: The Good, the Bad & the Ugly” and “Sierra Stories: Trues Tales of Tahoe, Vol. 1 & 2” are available at local bookstores. Mark, a Carnelian Bay resident, can be reached at mark@thestormking.com.


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