NASA scientists are able to identify for the first time how rapidly sea level is changing through a series of new satellites and observation systems, agency officials announced yesterday.

The question of rising sea levels, which is linked to global warming, could prove critical in coming years as policymakers seek to protect imperiled U.S. coastal areas and communities overseas. Waleed Abdalati, who heads the Cryospheric Sciences Branch at NASA's Goddard Space Flight Center, said yesterday that more than 100 million people could be affected by a three-foot increase in sea level.

"When you consider this information, the importance of learning how and why these changes are occurring becomes clear," Abdalati said at a news conference.

Scientists have been directly measuring sea level since the early 1900s. But until recently they lacked the ability to determine how much these shifts reflected the movement of nearby land.

Four years ago, according to Laury Miller, chief of the National Oceanic and Atmospheric Administration's satellite altimetry laboratory, experts could provide only a range of estimates for how much sea level has increased over the past century. They now know the sea has risen for the past 50 years at a rate of 0.07 of an inch each year, accelerating to an annual rate of 0.12 of an inch the past dozen years.

NASA is using a range of tools to gauge sea level shifts, including satellites that fire lasers at an ice sheet's surface to determine its elevation and two satellites that fly in formation to measure how the land mass is changing below. Some of this technology dates back to 1993; the government launched other programs two years ago.

"I love that mission," Abdalati said of the satellite pair, which the agency began using in 2002.

More than half the recent sea level rise comes from melting ice bodies, according to NASA scientists, while much of the remaining rise stems from warmer ocean temperatures. As ocean temperature rises, its water expands, they said.

Scientists are using new technology to monitor disappearing ice shelves and moving glaciers: One 10,000-year-old ice shelf in the Antarctic peninsula recently melted in just three weeks, according to NASA scientist Eric Rignot. "These glaciers are melting much faster than we thought," said Rignot, principal scientist for the radar science and engineering section at NASA's Jet Propulsion Laboratory.

Scientists who back mandatory cuts in heat-trapping gases linked to climate change, such as Princeton University professor Michael Oppenheimer, cite such statistics in making the case for immediate action. Americans living along the coasts of Florida and Louisiana are vulnerable to higher sea levels, they say, as are residents of Bangladesh and other countries with low-lying coastal areas.

"The message is, global warming is not something for the future," Oppenheimer said in a telephone news conference Wednesday sponsored by the advocacy group Environmental Defense Fund. "Global warming is happening already."

Richard Alley, a professor at Pennsylvania State University's EMS Environment Institute, said once scientists accumulate more information through these advances, they can better inform the public.