French and Soviet scientists, using 160,000-year-old ice samples, have obtained the strongest evidence yet to link an increase in atmospheric carbon dioxide to the warming of Earth -- the potentially catastrophic "greenhouse effect."

In three reports published in today's Nature magazine, the researchers said that carbon dioxide levels in the atmosphere dropped by 40 percent and global temperatures fell by about 18 degrees Fahrenheit the last two times that glaciers swept south as far as to St. Louis and New York City.

Carbon dioxide concentrations and surface temperatures then returned to normal as the glaciers retreated.

The reports appear to confirm fears that excess carbon dioxide released into Earth's atmosphere by the burning of fossil fuels would cause a greater global warming than has ever been observed, with harmful effects on climate and agriculture.

The carbon dioxide allows the sun's light to reach Earth's surface, but absorbs heat that would otherwise be radiated back into space, thus trapping heat, as in a greenhouse.

Climatologists estimate that the Earth's surface temperature has already increased by 0.5 to 1.25 degrees Fahrenheit since 1850. This summer, scientists from the National Oceanographic and Atmospheric Administration reported that this temperature increase has been accompanied by a northward shift of rainfall.

If the trend continues, according to a 1983 report by the National Academy of Sciences, agriculture in the southern United States will require massive irrigation just to maintain present levels of production.

"Scientists have theorized about the effects of carbon dioxide on climate since the 19th century," said geologist Eric T. Sundquist of the U.S. Geological Survery in Reston, "but there was little hard evidence to back the theory."

In 1979 and 1980, however, studies of Antarctic ice indicated that carbon dioxide levels had increased about 40 to 50 percent as the last glaciers retreated about 10,000 years ago.