The University of California’s Agriculture and Natural Resources (UCANR) has developed a series of webinars titled Insights: Water and Drought which feature timely, relevant expertise on water and drought from experts around the University of California system.
In this webinar, Professor Lynn Ingram, a professor of Earth and Planetary Science at UC Berkeley, discusses the climate history of droughts and floods in California, with a focus on the state’s history of droughts. Professor Ingram’s research looks at climate variation over long time scales in comparison to the modern period, the frequency and severity of past droughts and floods and how they impacted past human societies, and how will future warming impact water resources.
She began by presenting a graph of California’s precipitation from 1870 to 2000, noting that rainfall totals are on the vertical axis and the years are on the horizontal axis. “You can see that precipitation is highly variable,” she said. “Much of this variability is due to El Nino and La Nina, which are conditions over the Pacific Ocean that bring more or fewer or a greater number of storms to California each year.”
She pointed out that 1977 and the dust bowl years of 1928 to 1935 were notably dry, and there’s a longer term average variability that is related to the Pacific Decadal Oscillation which is more of a 20-30 year variability in precipitation.
So how does the current drought compare to the last few hundred years? “For that, we have to look back in the ‘natural archives,’” she said. “We don’t have recorded history beyond the late 1800s, so we have to look into tree rings and sediment cores and things like that to really understand the longer term history of water. If we look at this tree ring record for the past 500 years or so, what these tree ring records show is that multi-year droughts occurred every few decades and the single driest years occurred in 1580, 1923-24, and 1976-77.”
She then presented a slide showing global temperatures for thousands of years before present time which is represented by 0. “Going back in time, the beige color represents the last ice age, and then we transition into our current, warmer Holocene period,” she said. “There were three generally warm time periods within our current warm period. We have the early Holocene period, from 8500-9800 years ago that was also dry. Then we have the mid Holocene Climate Optimum that peaked 4800-5800 years ago, and again that was a very dry period. Then if we move forward to the medieval warm period in the late Holocene which is generally from 1800-600 years ago, it was warmer and again, it was drier.”
“It was particularly dry during the Medieval warm period from about 1100 to 600 years ago, so I’m going to focus on the evidence for why it was drier during that time, in fact with droughts lasting over a century,” Ms. Ingram said, presenting a slide with a graph of the lake level or size over time. “The lake expands during wetter times and during drier times, the lake contracts. This lake is in Eastern California, east of Yosemite Park. What is shown here is age with the present at the top, going back in time thousands of years, plotted against the lake’s level. Higher lake levels are to the left, lower lake levels are to the right, and so it was drier when the lake level was over here to the right, and so you can see that there’s this extended period where the lake was low. It was dry between 1800 and 600 years ago, and you can also see that over 1000 years or so, there’s been some fluctuations wet-dry with a period of about 200 years, so every 200 years it got wet and then it got dry again, wet-dry.”
A researcher, Scott Stein, noticed that throughout the Sierra Nevada, there were places in the lakes and rivers where there were tree stumps, suggesting that at some time in the past, that body of water had dried up. “He actually radio-carbon dated the outer rings of these trees to get the age that the trees died, and what he noticed was that these trees that the radio carbon dates came out into two generations of stumps, meaning two droughts,” she said. “By actually counting the tree rings, he could determine how long the trees had actually grown there, and he found out that there were two droughts, one between 900 and 1100 AD and the one from 1200-1350 AD. These are called the ‘Medieval Megadroughts’ – very extended periods of dry, perhaps 60% of average precipitation occurred during that time.”
Ms. Ingram said at that her laboratory, they work with sediment cores. “One of our ongoing projects has been sediment cores from San Francisco Bay,” she said. “The reason that we’re looking in San Francisco Bay is that its watershed covers 40% of California, so all of the precipitation and runoff over this area drains through the Delta and into San Francisco Bay and then out into the Pacific Ocean.”
The salinity in the San Francisco Bay reflects precipitation and runoff from a very large area of California. She explained that the idea was if they could determine past salinity in the Bay, then they could determine past precipitation over a large area of California. “As freshwater comes into the bay, it mixes with salt water, and the salinity at various locations in the bay is directly controlled by the amount of freshwater coming in to the estuary,” she said. “So we took sediment cores from various locations indicated by these yellow dots, and looked at the chemistry of the sediments and the fossil composition of the sediments. We then dated the cores with radio carbon, and that way we could get a record of past floods and droughts, or wet and dry periods over the Bay’s watershed.”
She then presented a slide with the picture of the sediment core. “It’s maybe 3” in diameter, and it’s generally a fine grained sediment of silt and clay, but it also contains lots of fossils that we can use to look at past salinity,” Ms. Ingram explained. “This is an image of the core split in half, and you can see this general layering of the core with finer grain sediments and coarser grain sands. We first dated this core using radio carbon methods, and then separated out fossil shells from the cores. We looked at both the type of fossils, because different types of organisms live in different conditions of water – some prefer saltier water and some fresher water, so that was one measure of the salinity. We also could look at the chemistry of the shells that reflects salinity using isotopes of oxygen.”
”This is the record that we came up with,” she said, presenting a graph of estimated salinity plotted against depth in the core on the vertical axis. She noted radio carbon ages are listed on the right. “We see shifts in salinity reflecting wet and dry conditions, but then there is this very significant increase in salinity that occurred during that medieval warm period where we saw elsewhere in California that there were drier conditions,” she said. “We see that in the estuary, salinity increased from 15 to 22 parts per thousand over this dry period.”
Thomas Swetnam from the University of Arizona’s tree ring lab studied the fire scars on the giant Sequoias in the Sierra Nevada and he was able to determine that the fire frequency was much greater during that medieval dry period by about 30%, she said.
It was towards the end of that medieval dry period that civilizations in the Four Corners region collapsed, Ms. Ingram said, presenting a slide with pictures from the region. “We have archaeological deposits of shell mounds that appear to be abandoned at the end of that drought,” she said. “The archaeological evidence shows that there was starvation, malnutrition, and even conflict and violence between groups. As resources dried up and dwindled, there was increasing competition for those resources, and finally mass migration in search of more water and resources.”
“So to sum up, the paleoclimate record shows that past periods of warmth are associated with drier conditions in California,” she said. “There were two dry periods that occurred between 19 and 1400 AD, during the medieval warm period; these were two century long droughts that had 60-70% average precipitation. We also see wet-dry cycles over the past 2000 years with periods of 30, 55, 90 and 200 years, so there is other variability in cycles that we see in the records.”
“The past 150 years have been unusually wet when viewed over the past 2000 years, so the 20th century was a wetter century, and this is when all of our water development, population growth and agricultural industry were established, and so it’s possible the climate may now be shifting to a drier regime,” she said.
In terms of future warming, how will that impact the west and California? “We’re already seeing the impacts of the warming that’s been occurring since 1960,” she said. “We’re seeing a reduced snowpack and that will continue into the future. It will be a drier climate with increased evaporation rates so drier soils. More frequent wildfires and increased dust levels as you have drier conditions and as the vegetation changes. It’s also predicted that we’ll have more extreme climate, because as the climate warms, you’re adding more energy and more water vapor to the atmosphere so that will produce larger floods and deeper droughts in the future.”