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California’s weather extremes found in 600-year-old tree rings

Researchers from Cornell University have investigated the climate history of the San Joaquin Valley. The study reveals a climate marked by intense weather extremes of droughts and floods.

For the investigation, the researchers used tree ring data from over 600 years to reconstruct an extensive record of weather and streamflow scenarios. 

Weather extremes

This innovative research method, which combined paleoclimatic data with synthetic weather generation, ultimately provides a new lens through which to prepare for climate-related risks.

The study sheds light on the weather extremes that have historically plagued the San Joaquin Valley in central California. These climate extremes include prolonged periods of both drought and floods, surpassing anything recorded in recent history in terms of severity and duration.

Broader implications 

The study insights are particularly important given the valley’s status as a major agricultural center and a crucial source of produce for the nation.

Over the last few years, the region has seen a wild swing between severe drought and significant atmospheric rivers. This makes the valley a bellwether for the climate hazards that are facing the rest of California and much of the world, noted study co-senior author Patrick Reed, professor of Engineering at Cornell.

Critical insights

One of the key findings of the study is the role of natural variability in influencing short-term weather extremes. However, human-induced climate change becomes a significant factor over periods exceeding 30 years. 

The last six centuries have witnessed prolonged phases of heavy rainfall and severe drought that can persist for decades. 

Surprisingly, the severity of droughts in the past 30 years rivals some of the worst in the 600-year record, but with a slightly shorter duration. The experts noted that hydroclimatic risks may be underestimated when they are based exclusively on modern records.

Natural variability

One of the most significant implications of this research is its potential impact on policy and planning. 

“Folks typically want to separate out internal variability versus climate change, just to get a sense of the signal change with anthropogenic warming,” said Professor Reed. 

“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.”

Study implications 

The study suggests that the convergence of natural variability and anthropogenic climate change is likely to result in more frequent, severe, and protracted flood and drought extremes than have been seen in the past 600 years. 

The research provides invaluable insights into California’s climate history and serves as a crucial tool for future climate resilience planning.

The study was supported by a National Science Foundation Graduate Research Fellowship. The results are published in Earth’s Future, a journal of the American Geophysical Union.

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