Washington — Over the next century, the Earth's changing climate may give a whole new meaning to the phrase ''beachfront property.'' The world's oceans have risen four to six inches in the past hundred years, scientists say. And as a blanket of pollution causes Earth's temperature to rise , melting polar icecaps, the ocean is likely to rise even faster - soaking low-lying areas of land and sending storm-tossed waves crashing into coastal cities with increasing frequency.
''It appears that a rise in sea level within the next 100 years of as much as eight feet cannot be ruled out,'' says Joseph Cannon, an Environmental Protection Agency (EPA) associate administrator.
It also appears that mankind has no one to blame but itself for the slow rise in sea levels. Heavy use of fossil fuels is wafting increasing amounts of carbon dioxide skyward, turning the atmosphere into a denser ''blanket'' and causing the Earth to retain more of the sun's heat.
A mysterious haze of carbon-dioxide-rich pollution, for instance, was found over the arctic icecap recently by US National Oceanic and Atmospheric Administration scientists. Industrial complexes in the Soviet Union, a federal report concludes, may be responsible for the haze.
Carbon dioxide concentration in the atmosphere already has increased from 315 parts per million in 1958 to 339 p.p.m. by 1980. Scientists now estimate the concentration will double sometime between 2040 and 2080.
Then Earth's average temperature will rise. The National Academy of Scientists estimates the increase at between 1.5 and 4.5 degrees C. New York City would then have a warmer climate than Charleston, S.C., does today, EPA's Mr. Cannon says.
Polar temperatures would rise even more than the average, and the mammoth ice sheets at the top and bottom of the world would begin to melt around the edges, draining more water into the oceans. The temperature of surface water would increase, causing it to expand and further accelerate the slow rise in the seas.
The world's oceans will thus undoubtedly rise. The only question is how much, an EPA report concludes. By the year 2100, the increase could be as low as 3.8 feet, or as high as 12 feet, the report says.
''Very little work has been done to estimate year-by-year projections of such a rise in sea level,'' says James Titus, an EPA strategic-studies official who helped compile the sea-level report. ''(But) a rise of a few meters would inundate major portions of Louisiana, New Jersey, and Florida, as well as many coastal resorts.''
Louisiana, which because of subsidence is already crumbling into the Gulf of Mexico at a rate of 50 square miles a year, would be particularly hard hit. Such coastal cities as Charleston - with a high-point of 13 feet above sea level - might find high tide uncomfortably high.
Low-lying areas all over the world, such as Bangladesh, would have the same problem.
Perhaps more important, coastal settlements everywhere would be increasingly vulnerable to storms. Mr. Titus estimates that the amount of damage a coastal city can expect to receive from a once-in-a-century storm might by the year 2100 occur every 10 years.
''But I'm convinced we will not abandon our coastal areas, though in some instances'' we may have to retreat, says Sherwood Gagliano, a subsidence expert with Coastal Environments Inc.
City planners, concludes the EPA report, should begin now to plan what measures they must take to protect themselves and their cities against the rising seas.