600 years of tree rings reveal climate risks in California

The researchers used the tree rings to reconstruct plausible daily records of weather and streamflow scenarios during the 600-year period.

This new approach, combining paleo information with synthetic weather generation, may help policymakers and scientists better understand -- and anticipate -- California's flood and drought risks and how they will be compounded by climate change.

The group's paper published in Earth's Future, a journal of the American Geophysical Union.

The San Joaquin Valley sits in the southern part of California's Central Valley, a major agricultural hub from which much of the nation gets its produce. Over the last few years, the region has seen a wild swing between severe drought and significant atmospheric rivers, which makes the valley a bellwether for the climate hazards that are facing the rest of California and much of the world, according to Patrick Reed, professor of engineering at Cornell University, and co-senior author of the paper.

The models show how flood and drought extremes have evolved within the San Joaquin Valley and can help clarify how natural variability and climate change can compound each other's effects.

"Folks typically want to separate out internal variability versus climate change, just to get a sense of the signal change with anthropogenic warming," Reed said. "But when we're planning in complex water systems, both are occurring. And we need a sense of what happens when they come together. And what happens is we get extremes we've never seen. This opens the envelope of plausible futures in a much wider sense."

Among the findings:

• A large portion of variability in flood and drought extremes in the San Joaquin Valley can be attributed to natural variability in the short term, but human-driven climate changes are influential at durations longer than 30 years.
• The last 600 years have seen sustained pluvial and drought periods that have lasted decades.
• Estimates of drought occurrence and severity from the last 30 years rival the worst megadrought period in the 600-year reconstruction, but estimates of modern drought duration have been slightly shorter than what is found in the paleo record. Therefore, relying solely on the modern instrumental record can underrepresent hydroclimatic hazards.
• The combination of natural variability and climate change can lead to more frequent, more severe and longer flood and drought extremes than have ever been experienced over the last 600 years.

The research was supported by a National Science Foundation Graduate Research Fellowship.