Coastal upwelling in the California Current was weaker 6,000 years ago than today, according to new studies

The California Current is a major influence on the climate of western North America and on the productivity of both terrestrial and marine ecosystems along the West Coast. But 6,000 years ago, the California Current wasn't quite what it is today, according to a team of researchers at the University of California, Santa Cruz.

New findings suggest that coastal upwelling in the California Current was significantly weaker 6,000 years ago than it is today as a result of differences in Earth's orbit around the Sun. This hypothesis is based on a combination of results from computer modeling of the California regional climate and analysis of geologic records. Noah Diffenbaugh, a graduate student in Earth sciences at UCSC, is presenting the new results this week at a meeting of the International Union for Quaternary Research (INQUA) in Reno, Nevada, July 24 through 30.

The past 11,000 years is known as the Holocene, or "Age of Humans," when people became the dominant shapers of regional and global environments. Archaeological records indicate that the mid-Holocene (8,000 to 3,000 years ago) was a particularly important period of global climatic and cultural transition, with important innovations in agricultural, architectural, and ceremonial complexity occurring throughout the world.

Geological records suggest that these increases in cultural complexity coincided with regional climatic change, and evidence for such changes has been seen in archaeological and geological records from the Pacific Coast in the California Current region. It has not been clear, however, exactly how climate and culture interacted in the California Current region during that time.

The high biological productivity of California's coastal waters is a direct result of the spring and summer upwelling season, which brings deep, cold, nutrient-rich waters to the ocean surface, supporting a diverse and complex ecosystem. While ancient peoples would likely have been affected by changes in the California Current through time, the geologic record of mid-Holocene California Current upwelling is ambiguous, with some studies indicating increased mid-Holocene upwelling and others reduced.

In order to better understand the nature of coastal upwelling in the California Current at that time, the UCSC researchers employed a high-resolution regional climate model. The regional climate model was developed by a team led by Diffenbaugh's adviser, professor of Earth sciences Lisa Sloan. The model showed that 6,000 years ago, the California Current was affected by differences in the amount of solar energy reaching the Earth due to predictable changes in Earth's orbit.

At the INQUA Congress, Diffenbaugh will present results from the climate model in conjunction with geologic data from the California Current analyzed by a group of researchers headed by John Barron of the U.S. Geological Survey (USGS) in Menlo Park, California.

The combined results indicate that, relative to the present, the mid-Holocene California Current upwelling season was longer and less vigorous than today, with reduced seasonal contrast. Such a change in California Current activity would have impacted marine ecosystems and perhaps also affected ancient human communities relying on those marine resources.

The comparison of geologic data and climate model results presented by the UCSC-USGS team helps to explain the evolution of California Current activity during the Holocene. It may also provide insight into the environmental factors that shaped anthropological change in the region during the culturally important mid-Holocene.

The primary results to be presented at the INQUA Congress were published recently in Paleoceanography, a peer-reviewed journal published by the American Geophysical Union:

--Diffenbaugh, N.S., L.C. Sloan and M.A. Snyder, Orbital suppression of wind-driven upwelling in the California Current at 6 ka, Paleoceanography 18(2): 1051 (2003).

--Barron, J.A., L. Heusser, T. Herbert and M. Lyle, High-resolution climatic evolution of coastal northern California during the past 16,000 years, Paleoceanography 18(1): 1020 (2003).


Note to reporters: You may contact Noah Diffenbaugh after the meeting at 831-459-3504 or