Basin Watch: Climate history makes global warming dull
According to global warming prognosticators, mean temperatures might rise as much as 10 degrees Fahrenheit during the next century or so. And some experts say that would cause greater rainfall in California and leaner snowpacks. If it happens the way these experts predict, it’ll be tough to cope. Yet, if we find this prospect daunting, think about how humans, animals and vegetation of the Tahoe Basin had to cope with even greater climate changes.
Susan Lindstrom, a Truckee-based archeologist, has provided us with some ideas of what it was like. She has assembled a comprehensive collation of Tahoe’s climate variations since the end of the last ice age. We’ll hit the high spots in this column, but a much better, illustrated, description can be found in Chapter 2, Volume I, of the “Lake Tahoe Watershed Assessment.”
Some 10,000 years ago, glaciers around Lake Tahoe were melting and humans were probably venturing into the basin to hunt and forage. It was the end of the Pleistocene Epoch and the start of the current Holocene Epoch, according to geological reckoning of time.
Reconstructing climate history requires detective work. Clues are found in many places. Swamps, bogs, ponds and lakes contain layers of seeds, pollen, leaves, etc. that tell scientists what kinds of vegetation flourished in the past. Lake and river bottoms contain tree stumps, preserved for centuries by cold Sierra waters. Spaces between tree rings reveal relative wet and dry periods. Fossilized woodrat middens contain layers of leaves, branches, and seeds.
During the Early Holocene, the climate was moist and cool, winter precipitation was still heavy, but temperatures were rising. According to Lindstrom, pollen studies at Osgood Swamp near South Shore show that at the start of the Holocene Epoch, vegetation began to change from cold-dry, semiarid sage to higher elevation types of woodlands.
The Early Holocene was followed by a much warmer and drier Middle Holocene (4,000 to 7,000 years ago). Many lakes in the western Great Basin dried up. Semiarid and mountain woodlands moved to higher elevations. Evidence includes Utah juniper and pinon pines found in woodrat middens at the 10,000 foot elevation in the White Mountains and dated some 5500 years ago. At the same time, semiarid woodland species disappeared from woodrat middens at lower elevations in the Virginia Mountains. The 8,600-year-old record of bristlecone pine tree rings from the Methuselah walk in the White Mountains also showed drier conditions.
Lake Tahoe and Pyramid Lake were among the few lakes that didn’t dry up, but their levels declined. Rooted stumps currently found as deep as 20 feet in Lake Tahoe were dated to a period between 4800 and 6300 years ago, according to Lindstrom. Prehistoric populations increasingly altered the landscape, affecting fauna and flora. Lindstrom explains that archeological evidence shows that the people visiting the basin stayed longer, used the land with greater intensity, and had a broader diet.
During the Neoglacial period, 2,000 to 4,000 years ago, climates became cooler and moister. Many Great Basin lakes reappeared, marshes grew, and there were minor glacial advances in the Sierra. Pollen from Osgood Swamp showed changes from drought-tolerant species to dominance by the kinds of conifers that we see today. Radiocarbon dating of tufa crusts shows that Mono Lake levels rose some 3,450 years ago.
Other evidence, notes Lindstrom, shows that Pyramid Lake levels increased some 3200 years ago. And since the Truckee River is the source of Pyramid Lake’s water, it indicates that Lake Tahoe also had higher lake levels. Pollen records from Taylor Creek Marsh at South Lake Tahoe shows that the marsh was formed by rising ground water levels sometime between 2,900 and 5,000 years ago. Around 2800 years ago, the marsh was inundated, and Lake levels remained high until about 2,100 years ago. Between 1,600 and 2,200 years ago, the area experienced a persistent dry period, followed by strong climate swings between wet and dry periods.
During a period of 175 to 350 years ago, climate cooled in the West, resembling conditions experienced in Europe and New England during their “Little Ice Age.” Lake levels rose and cirque glaciers reformed in the Sierra. It was an important period for Tahoe, because that’s when many of the basin’s current old growth forests began to develop.
At the end of the Little Ice age, Lindstrom explains, drier conditions caused the forests to retreat to higher elevations and fire frequencies to increase. Intervening years have been marked by alternating wet and dry periods, as old Tahoe hands know.
What kinds of things can we learn from historical records? Here’s an example. One of our goals is to restore the basin’s forest to the way it was before European types started cutting them down. Can it be done without recreating the Little Ice Age?
Next time, we’ll look at the basin’s history of land use.
Leo Poppoff is a retired atmospheric physicist with NASA and has been a member of the Tahoe Regional Planning Agency’s advisory planning commission since 1983. He is also a former member of the Lahontan Water Quality Control Board.
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