Volume 2, Issue 11
Weathering Climate Variability
When it comes to weather, most of us are only concerned with the forecast. UC Berkeley professor Lynn Ingram is more interested in old news. Very old. She studies how California's climate has changed over thousands of years. Her research could help prepare us for what tomorrow's weather may bring.
Lynn Ingram is a professor of geology, geophysics, and geography.
"We look at the climate's natural variability and the frequency of events like droughts, wet periods, and floods across different regions in the state," says Ingram, a professor of Earth and Planetary Science and of Geography. "With 35 million people depending on the state's water resources, I think it's important to understand what's happened in the recent past regarding the climate here."
Ingram is a paleoclimatologist, which means that her notion of "the recent past" is somewhat different than most. Her research group analyzes sediment, fossils, and archaeological deposits from the Holocene Epoch, a geological period extending back 10,000 years. Hidden within those samples are clues about the temperature, salinity, precipitation, and other climate variables from days long gone.
Most recently, Ingram and her colleagues conducted a large survey of paleoclimate records from across the state. She's integrating data from other research efforts in the Sierra Nevada region with the results from her own group's fieldwork in the San Francisco Bay. Immediately, she was stricken with the dramatic range of the California climate.
"We've only been measuring climate factors like temperature, precipitation, and salinity here for maybe 80 to 100 years," Ingram says. "The longest drought we've observed is six years. But in the paleoclimate records there are droughts that lasted more than a century. That kind of information is desirable for planning purposes because if a shift occurred in the past, it could probably happen again."
The shells of foraminifera hold clues--in the form of oxygen, strontium, and carbon isotopes--about the San Francisco Estuary's salinity over time. (courtesy the researchers)
About once each year, Ingram, Berkeley adjunct professor Doris Sloan, and their students charter a boat into the Bay where they retrieve sediment cores extracted from the floor of the ocean. The sediment layers are like pages in a book, enabling the scientists to look back in time. Once they return to the laboratory, the researchers analyze the fossils and sediments, measuring isotopes of oxygen, carbon, and strontium. These variables are indicative of water salinity, ocean circulation, temperature, and other factors linked to climate events such as precipitation. For example, Ingram explains, the salinity of San Francisco Bay varies in response to rainfall runoff from the entire watershed covering 40 percent of the state.
"That salinity is then recorded in the chemistry of the fossilized shells," Ingram says.
Based on the paleoclimate records, the researchers have already noticed that California appears to have been much wetter 2,500 to 4,000 years ago. Since then, the dryness has only been interrupted by rapid climate changes. The reason remains a mystery, though as the research progresses Ingram hopes climate modelers will shed light on the patterns. Solar phenomena such as sunspots affecting the climate could be one possible cause of the short-term variations, she says.
While the researchers have so far focused on the Holocene, they're beginning to look back further. The deepest sediment core sample they've retrieved came from 100 feet below the ocean floor, providing 120,000 years worth of data. Right now, Ingram explains, the Earth is in an interglacial period, a warm period between ice ages. But the deep core sample reveals that during the last interglacial period, the climate was warmer and sea levels were higher.
This core sample, split lengthwise, reveals the sediment stratigraphy of the San Francisco Bay. (courtesy the researchers)
"That data might provide a good analog of what this area will look like in the future if global warming continues," Ingram says.
Along with cores extracted from below the Bay, Ingram is exploring California's climate history through samples from marshes and shell mounds, prehistoric refuse piles. The former, a collaboration with Berkeley geography professor Roger Byrne and postdoctoral fellow Frances Malamud-Roam, centers on the study of how marsh vegetation has changed in response to salinity shifts. At the shell mounds, Ingram, Byrne, Berkeley anthropologist Kent Lightfoot, and graduate student Peter Schweikhardt examine entire mollusks for hints of seasonal changes that may have affected their growth. For example, California is known for a Mediterranean climate with a wet winter and dry summer. But has that always been the case? The answer lies in the paleoclimate record.
"I enjoy seeing how different things were in the past," Ingram says. "We take San Francisco Bay for granted, but 10,000 years ago it simply didn't exist. We can't think of the world as unchanging."