The U.S. West experienced extended droughts and severe floods long before humans contributed in a significant way to atmospheric CO2
The West’s Climate History Suggests An Ominous Future
If thousands of years of western climate history are any guide, the future may hold extremes of drought and flood that we are not well prepared for, according to a University of California expert who has studied climate change since the end of the last ice age.
Lynn Ingram, a professor of geology in the Earth and Planetary Sciences Department at UC Berkeley, spoke last week to the Valley Study Group, meeting in Pleasanton.
She described scientific evidence of droughts lasting for decades and even centuries during the past several thousand years. She also told of floods lasting months that literally covered the floor of California’s Central Valley with water 10 to 15 feet deep.
She said that studies of climate history appear to connect extended droughts with periods of higher temperatures that occurred as a result of natural cycles, suggesting that learning from the past can help us anticipate and deal with a future where temperatures rise because of human activities.
Her talk came two years after she and a former student, Frances Malamud-Roam, co-authored a book that described the extremes of western climate. The book, “The West Without Water: What Past Floods, Droughts and Other Climatic Clues Tell Us About Tomorrow,” received considerable attention nationwide.
Interest in the climate has, if anything, increased since then, spurred not only by daily reports of water rationing and competition for water resources, but also by the tragic consequences of drought, such as the fatal, ongoing Northern California wildfires.
Ingram called California the “epicenter” of an extreme western drought whose signs were evident as early as 2002. It is even possible, she added, that we are at the beginning of a “megadrought,” meaning one that last decades if not centuries, as California has experienced historically.
Independently, as if to add to her fears, the University of Arizona on Monday announced the results of a study showing that snowpack in the Sierra Nevada mountains is at its lowest level in 500 years.
Even during droughts, Ingram noted there can be periods of intense rain. Like everyone else, she said, she hopes that the current shift in Pacific Ocean temperatures called El Niño will bring rain this winter and start to reverse the damage that has been done to forests, agriculture and even human health by lack of rainfall.
Her Pleasanton talk sketched out the manner in which scientists have literally unearthed clues to ancient climate by studying soil and sediment layers around lakes and marshes, fossils, Indian shell mounds and tree stumps. These studies have occurred at sites around California and as far east as the Four Corners region where Utah, Arizona, Colorado and New Mexico meet.
In the San Francisco Bay Area, for example, scientists have read the historical climate evidence in the layered remnants of plants known to thrive either in the presence of salty water or in the presence of fresh water.
During periods of drought, salt-tolerant plants like saltgrass and cordgrass are found farther inland, where brackish water surges as ocean tides overcome weak streams of fresh water from the mountains.
Because one year’s sediment layers on top of the previous year’s, year after year, soil cores extracted with special drills can be chronological windows on thousands of years of natural history.
The cores allow scientists like Ingram to examine bands of mud to determine whether pollen and other materials laid down long ago were from plants that were salt-tolerant, reflecting drought conditions, or that needed fresh water, suggesting more plentiful rainfall.
These readings in turn indicate climate conditions in much of the state, since the Bay drains nearly half of California, from the Oregon border in the north to the Tehachapi mountains in the south.
Ingram’s own studies of marshes in northern Suisun Bay were carried out using Lawrence Livermore National Laboratory’s Center for Accelerator Mass Spectrometry to identify plant remains by measuring the ratios of carbon and oxygen isotopes.
Ingram cited studies, sometimes made using core samples and sometimes with different techniques, that showed prolonged droughts occurring all over the west, from the Sierras to the Great Salt Lake and south to the Four Corners area.
Possibly the most vivid early discovery was itself made during a drought -- the Dust Bowl of the 1920s and ‘30s -- when the surface of Lake Tahoe receded to reveal tree stumps more than 3 feet in diameter preserved under 12 feet of water.
As Ingram told the story in “The West Without Water,” a Berkeley researcher estimated the stumps to be about 150 years old by counting tree rings. From the absence of decomposition, he reasoned that water must have risen rapidly to kill the trees and preserve them. In an era before radiocarbon dating, however, he could not tell when they died.
The invention of carbon dating techniques in the 1950s allowed the trees to be dated to about 4,800 years ago. This time is similar to the dates of trees found submerged in other lakes, like Tenaya in Yosemite, suggesting drought extending across the west.
There were also signs of extended drought in tree rings, both in the narrow banding of growth rings that indicates little rain and in increased scarring from wildfires that occur more often during times of drought.
Ingram sees both lessons and concerns for the west in these studies of past climate. She noted that the western U.S. has warmed by about two degrees since 1890, and that predictions for future snowpack – the source of about one-third of California’s water – are bleak for the last half of this century.
Wildfires are likely to become more common and more dangerous. Human health may be threatened in areas like the Central Valley because of increased dust and respiratory illness.
It may seem counterintuitive, she said, but in the midst of droughts, floods may become more severe. Models show that warmer air will absorb more moisture from the oceans and, in some cases, channel “atmospheric rivers” from the tropics to some part of the state with monster rainstorms carrying the flow of “two to 10 Mississippi Rivers” with hurricane force winds.
Such a storm started in late 1861, she said, lasting 43 days, into February 1862 and flooding California’s Central Valley to depths of 10 to 15 feet. The state went bankrupt. Cattle and farms were swept away and legislative activities had to move from Sacramento to San Francisco. In many places, the water did not recede for months.
Research has shown that such storms occur at intervals of 100 to 200 years and may have been responsible for the sudden rise in the level of Lake Tahoe that killed the 150-year-old trees.
Since the floods of 1861-62, parts of the Central Valley have subsided as much as 30 feet due to over-pumping of the aquifer, she said, so a comparable future storm will be that much more devastating.
While science cannot predict what the future will hold, she said, it can give a range of possibilities. From drought to flood, California faces the possibility of extremes, at “both ends of the spectrum.”
From resource conservation to land use planning and preparing for floods, “we need to begin preparing now.
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