Study reveals changing climate trends in western US going back 11,000 years

People often say that Phoenix has always been dry; Seattle has always been wet; and San Francisco has always been foggy. But “always” is a strong word.

A study from the University of California, Davis, synthesizes climate trends across the western United States during a relatively recent period of Earth’s history – the Holocene Era, which extends from today to the last 11,000 years. This look at the truly Old West shows that the hallmarks of California’s climate—the misty coastlines that gave rise to towering redwood forests, the rising seas that created productive fisheries, the warm summers and mild winters—began about 4,000 years ago.

It also reveals a time when the Pacific Northwest was hot and dry and the Southwest was hot and wet.

An understudied era: The present

Published in The climate of the past, the study provides a baseline against which modern climate change in the region can be assessed. It also sheds light on a less studied geological epoch – the current one, the Holocene.

“We kept looking for this paper and it wasn’t there,” said lead author Hannah Palmer, who recently received her Ph.D. from the UC Davis Department of Earth and Planetary Sciences. “There are many records of past climate for a single location, but no one had put it all together to understand the big picture. So we decided to write it.”

The authors analyzed more than 40 published studies, examining the interaction between land and sea temperature, hydroclimate and fire activity over three different phases.

The study found:

• Compared to pre-Holocene conditions (the last ice age), the early Holocene (11,700-8,200 years ago) was a time of warm oceans, a warm and dry Pacific Northwest, a warm and wet Southwest, and fairly low fire activity.
• By the mid-Holocene (8200-4200 years ago), that pattern had reversed: the ocean surface cooled, the Pacific Northwest became cool and wet, and the Southwest became drier.
• The late Holocene (4200 years ago-present) is the most climatically variable period. It marks the period when the “modern” climate and temperature patterns were established. The study noted a defined interval of fire activity over the past two centuries that is linked to human activity.

Outstanding interval

The study also assessed the impact of humans on environmental change at that time, noting that the colonization era (1850-present) represents an unprecedented environmental interval in the climate records.

“Humans have lived here throughout the Holocene,” Palmer said. “The climate affected them, and they affected the climate, especially in recent centuries. This paper shows how that push and pull has changed over the last 11,000 years.”

Different answers

“Sometimes people point to recent rain or cold as evidence against climate change,” said co-author Veronica Padilla Vriesman, a recent Ph.D. graduate of the UC Davis Department of Earth and Planetary Sciences. “This study illustrates how different regions respond differently to global climate change. The long-term perspective helps us understand the historical climate of the western United States and how it may respond going forward.”

The study stemmed from a graduate seminar on the Holocene period led by Tessa Hill, a professor in the Department of Earth and Planetary Sciences and assistant vice provost for public scholarship and engagement. Additional co-authors include Caitlin Livsey and Carina Fish. All of the authors were part of Hill’s Ocean Climate Lab at the UC Davis Bodega Marine Laboratory in the Department of Earth and Planetary Sciences.

“Climate records from the Holocene provide a valuable window into the context of anthropogenic climate change,” Hill said. “They give us an opportunity to understand places that may be more or less resistant to change in the future.”