17.2: The Past

Deep Time

Time itself seems frozen across the silent, icy expanses of the White Mountains, which form the eastern California border with Nevada. Rosy-fingered dawn blankets the frigid early morning in sparkling-ice vermillion light, witnessed by herds of wild horses as they watch dawn shadows cast into the broad defile of Owen’s Valley, two miles below. Owen’s Valley cleaves the White Mountains from the Sierra Nevada with a rift twice as deep as Grand Canyon. On the western side of Owen’s Valley, the dizzyingly steep eastern front of the juvenile Sierra Nevada bursts abruptly out of ochre alluvial fans, its jagged granitic peaks challenging the indifferent sky.

Figure \(\PageIndex{1}\): A stand of Bristlecone pines growing on pale-white Reed Dolomite in the White Mountains, CA. "Reed Dolomite" by Steven Newton, is licensed under CC BY-NC 4.0. Access a detailed description.

On the other side of Owen’s Valley lie the ancient rocks of the White Mountains, among the oldest in California. One of these antique units is the Reed Dolomite, a white, sugary mixture of calcium and magnesium whose formula is CaMg(CO₃)₂. (Students who have completed a Physical Geology lab course will recall that while limestone reacts readily to hydrochloric acid, the similar-looking dolomite must be powdered before it will fizz.) Dolomite cannot form in substantial quantities in today’s world, but the dolomites of the White Mountains are hundreds of meters thick. From a distance, the Reed Dolomite hills look covered with snow, as the ground often is, but even in summer the bleached dolomite imparts a pallid color.

Figure \(\PageIndex{2}\): Snow-covered carbonate soil hosting a number of Bristlecone pines (Pinus longaeva) in the high-elevaiton White Mountains near the eastern border of California/Nevada. "Bristlecone pine" by Rick Goldwaser is licensed under CC BY-NC 4.0. Access a detailed description.

The Reed Dolomite is the preferred home of the Bristlecone pine (Pinus longaeva), whose weathered trunks twist out of the pale soil like crenulated corkscrews. They are the oldest individual multicellular organisms on planet Earth. This claim is sometimes heatedly disputed; there is a creosote shrub in the Mojave, the Larrea tridentata, that dates to 11.7 ka, however what is being dated are generations of colonial offshoots from a central plant that died long ago, so saying this is the oldest organism is a bit like calculating your age by including your great-great grandparents. There is also evidence for long-survival of bacterial spores in a dormant state, but in terms of continuously active, multicellular, single organisms, nothing is longer in the tooth than Bristlecone pines. The oldest Bristlecone, a particularly gnarled specimen called the Methuselah Tree, was dated in 1957 at 4,789 years, meaning its nascent sprouts saw first light about 2833 b.c. To put that in perspective, Stonehenge was newly begun, construction of the pyramids of Giza was hundreds of years away, and the Trojan War was fifteen hundred years in the future. The Methuselah Tree is an old, old tree. And across the Reed Dolomite slopes of the White Mountains, there are thousands upon thousands of these ancient Bristlecones.

Trees are an excellent tool to assess past climate. Annual tree rings, which can be cored in a non-destructive way, show direct information about conditions. All else being equal, wetter periods produce more tree ring growth and wider rings, while drier or colder conditions stunt tree ring width. Analysis of trends in tree rings can thus establish data about climate.

Dendrochronology

This study and dating of tree rings is called dendrochronology, and it offers powerful windows into past climates (see Figure \(\PageIndex{3}\)). The White Mountain Bristlecone dendrochronology yields data for California climate for over 8,000 years. Other tree species also provide information for shorter time periods. Whether using ancient Bristlecone pines, or fast growing sycamores, there is an ancient story embedded in the trees themselves. What do the trees whisper to us?

Figure \(\PageIndex{3}\): A diagram displaying the dendrochronology principle of differential annual tree ring growth. "Tree rings" by NASA is in the public domain. Access a detailed description.

In the video below, Video \(\PageIndex{1}\), a NOAA paleoclimatologist explains the science behind tree rings and how they provide a record of tree growth under different climate regimes:

Video \(\PageIndex{1}\): Dendrochronology

A video about dendrochronology

Dendrochronology informs us that California’s climate is characterized by extremes. California experiences so much climate variation that positing “normal” conditions is almost meaningless. California has been much warmer and much colder than today. California has experienced much wetter periods than today. California has experienced much drier periods than today; in fact, dendrochronology reveals extended “megadroughts” lasting many decades. While there is not a strict definition of the term megadrought, they can be understood as lasting many decades; in other words, shorter duration climate events, such as the Dust Bowl (1932-1939), are not megadroughts.

Megadroughts

According to dendrochronological data, megadroughts gripped California between 892 A.D. and 1112 A.D., and between 1209 A.D. to 1350 A.D. So as William the Conqueror secured his reign in England after 1066, and as the Black Death claimed its first victims in 1346, California suffered profound, deep droughts that parched the land and challenged its inhabitants.

Figure \(\PageIndex{4}\) shows the severity of California droughts over the previous thousand years using a scale called the Palmer Drought Severity Index:

Figure \(\PageIndex{4}\): California Palmer Drought Index Scale, by Frankson, R., L.E. Stevens, K.E. Kunkel, S.M. Champion, D.R. Easterling, W.. "Drought index" by NOAA is in the public domain. Access a detailed description

This extremism is not limited to the last millennium. Looking further back in time, a 2015 study of pollen collected at Lake Elsinore in Southern California, east of Santa Ana, reveals a period of severe megadroughts between 27.5 - 25.5 ka. These intense, long-lasting droughts changed local vegetation patterns significantly enough to be recorded as pollen changes in lake sediments.

Were such events to repeat today, such prolonged megadroughts would reshape everything about how we live in California. Many argue we are currently entering just such a megadrought.

A cognitive bias for Californians is that the 20th century was actually an unusually wet period. Twentieth century expectations of rainfall, snow melt, and river flows, used in planning for public water projects, are all based on erroneously high assumptions about precipitation. We Californians still talk about drought as if it were a temporary situation, to be addressed by virtuously watering the lawn less; the reality is that this state usually is far drier than can sustain the current inefficient agricultural practices and bloating population growth.

In addition to drought, tremendous precipitation also happens in California. Remember that dendrochronology tells us that California’s climate oscillates between extremes. Sometimes outliers involve more precipitation than usual.

During the winter of 1861-1862, a series of storms flooded California so extensively that the Central Valley became an inland sea for months. Downtown Sacramento became impassable; the newly elected governor, Leland Stanford (who would later found the eponymous Stanford University), had to arrive at his own inauguration via boat. The capitol was temporarily relocated from Sacramento to San Francisco. This event involved atmospheric rivers of precipitation that direct one Pacific storm after another toward the West Coast.

ARkstorms

Could this happen again? The United States Geological Survey (USGS) has researched the possibility of a recurrence of the 1861-1862 event, which they term an ARkstorm event. (This rather awkward acronym means “atmospheric river 1000.”) The upper estimation of damages to California from a repeat of the 1861-1862 scenario could reach $750 billion, on the order of the entire national GDP of the country of Taiwan.

Just as Frost described the end of the world as either by fire or by ice, California’s berserk past climate is a story of apocalypse–severe parching droughts punctuated by catastrophic deluges, oscillating wildly between arid cracked ground swept with wildfires and soaked hillsides collapsing after receiving a year’s worth of rain in a day.

Query \(\PageIndex{1}\)

References

1. Ferguson, C. W. (1979). Dendrochronology of Bristlecone Pine, Pinus Iongaeva. Environment International, 2, 209-214. https://doi.org/10.1016/0160-4120(79)90003-5
2. Gupta, R.S., Steinschneider, S., & Ree, P.M. (2023). Understanding the contributions of paleo-informed natural variability and climate change to hydroclimate extremes in the San Joaquin Valley of California. Earth’s Future, 11, e2023EF003909. https://doi.org/10.1029/2023EF003909
3. Nutt, D. (2023, November 30). California’s Hidden Climate History: A Remarkable 600-Year Tree Ring Analysis. SciTech Daily. Retrieved December 2, 2023, from https://scitechdaily.com/californias...ring-analysis/
4. Cook, B. I., et al., (2016). North American megadroughts in the Common Era: Reconstructions and simulations. Wiley Interdisciplinary Reviews: Climate Change. https://doi.org/ 10.1002/wcc.394
5. Stine, S. (1994) Extreme and persistent drought in California and Patagonia during mediaeval time. Nature 369, 546–549. https://doi.org/10.1038/369546a0
6. Heusser, L. E., Kirby, M. E., & Nichols, J. E. (2015). Pollen-based evidence of extreme drought during the last Glacial (32.6–9.0 ka) in coastal southern California. Quaternary Science Review, 126, 242-253. https://doi.org/doi.org/10.1016/j.quascirev.2015.08.029
7. Columbia Climate School (2022, February 14). Megadrought in Southwest Is Now the Worst in at Least 1,200 Years, Study Confirms. Retrieved November 4, 2023, from https://news.climate.columbia.edu/2022/02/14/megadrought-in-southwest-is-now-the-worst-in-at-least-1200-years-study-confirms/
8. Porter, Keith, et al., (2011). Overview of the ARkStorm scenario: U.S. Geological Survey Open-File Report 2010-1312, 183 p. and appendixes. http://pubs.usgs.gov/of/2010/1312/