A team of Federal scientists says it has detected an overall warming trend in the earth's atmosphere extending back to the year 1880. They regard this as evidence of the validity of the ''greenhouse'' effect, in which increasing amounts of carbon dioxide cause steady temperature increases.

The seven atmospheric scientists predict a global warming of ''almost unprecedented magnitude'' in the next century. It might even be sufficient to melt and dislodge the ice cover of West Antarctica, they say, eventually leading to a worldwide rise of 15 to 20 feet in the sea level. In that case, they say, it would ''flood 25 percent of Louisiana and Florida, 10 percent of New Jersey and many other lowlands throughout the world'' within a century or less.

Workings of Greenhouse

The forecast, which also envisions widespread disruption of agriculture, is the fruit of analyses and computer simulations conducted by the Institute for Space Studies of the National Aeronautics and Space Administration. The institute, which is in New York City, is part of the space agency's Goddard Spaceflight Center in Greenbelt, Md. The forecast is in an article in the Aug. 28 issue of the journal Science.

Carbon dioxide in the atmosphere, which is primarily a result of mankind's burning of fuels, is thought to act like the glass of a greenhouse. It absorbs heat radiation from the earth and its atmosphere, heat that otherwise would dissipate into space. Other factors being equal, the more carbon dioxide there is in the atmosphere, the warmer the earth should become, according to the theory.