A group led by David Field of the Scripps Institution of Oceanography analyzed sediment cores drilled off the Santa Barbara coast. The cores contained thin layers of shells from microscopic plankton called forams that rained to the seabed after the animals died.

The cores showed that, as ocean temperatures varied, forams alternated between species that thrive in warmer waters and those dominating cooler waters.

Field found that subtropical and tropical forams started to increase around 1925. The increase became more dramatic after the 1970s.

Part of the ocean warming was due to a cycle of climate variability called the Pacific Decadal Oscillation, which shifts about every 20 years. The last warm cycle lasted from 1977 to the mid '90s.

But Field found that the abundance of tropical and subtropical forams has risen to its highest point in 1,400 years, suggesting that the increase was not just a product of the Pacific Decadal Oscillation.

"There's an additional warming ... that makes the 20th century atypical," he said.

Experts estimate the decadal oscillation accounts for 1 degree Fahrenheit variation in Southern California ocean temperatures. Over the past century, upper ocean temperatures here have warmed nearly 3 degrees.

One likely cause of the extra temperature rise is the production of greenhouse gases, which are linked to global warming, Field said.

Bill Peterson, a scientist with the National Oceanic and Atmospheric Administration, agreed with Field's conclusions of a long-term warming trend.

Peterson also said the Pacific Decadal Oscillation may be speeding up for unknown reasons from its 20-year cycles to three- or four-year cycles.

"It's not behaving like it used to behave," Peterson said.