Sea levels were rising by about 1 millimeter (0.04 inches) every year about 200 years ago and as far back as 5,000 years, geologists found from deep sediment samples from the New Jersey coastline. Since then, levels have risen by about 2 millimeters (0.08 inches) a year.

While the planet has been in a warmer period, driving cars and other activities that create carbon dioxide are having a clear impact, the Rutgers University-led team said.

"Half of the current rise ... was going on anyway. But that means half of what's going on is not background. It's human induced," said Kenneth Miller, a geology professor at the New Jersey-based school who led the 15-year effort.

Carbon dioxide emissions come mainly from burning coal and other fossil fuels in power plants, factories and automobiles.

Miller and his colleagues analyzed five 500-meter (1,650-foot) deep samples to look for fossils, sediment types and variations in chemical composition, giving them data on the past 100 million years.

They also analyzed data from satellite, shoreline markers and by gauging ocean tides, among other measures.

"It allows us to understand the mechanisms of sea level change before humans intervened," Miller said in an interview.

His team did not determine whether the rate is accelerating.

The research, funded mostly by the National Science Foundation, also found ocean levels were lower during the dinosaur era than previously thought. They were about 100 meters (330 feet) higher than now, not 250 meters (820 feet) as many geologists had thought, Miller said.

SAMPLES FROM ANTARCTIC DEPTHS

Measurements also showed that, while many scientists had thought polar ice caps did not exist before 15 million years ago, frozen water at the poles did form periodically.

"We believe the ice sheet was not around all the time. It was only around during cool snaps of the climate," Miller said.

In another report published in Science, European researchers using three large samples of polar cap ice found carbon dioxide levels were stable until 200 years ago.

"Today's rise is about 200 times faster than any rise recorded" in the samples, study author Thomas Stocker said in an e-mail interview with Reuters.

The historic data "put the present rise of the last 200 years into a longer-term context," he added.

Trapped gas bubbles in the ice, drilled out from Antarctica depths of about 3,000 meters (9,900 feet), provided scientists with information on the Earth's air up to 650,000 years ago.

Researchers participating in The European Project for Ice Coring in Antarctica measured levels of carbon dioxide as well as methane and nitrous oxide -- two other gases known to affect the atmosphere's protective ozone layer.