After examining climate data extending back nearly 100 years, a team of Government scientists has concluded that there has been no significant change in average temperatures or rainfall in the United States over that entire period. While the nation's weather in individual years or even for periods of years has been hotter or cooler and drier or wetter than in other periods, the new study shows that over the last century there has been no trend in one direction or another. The study, made by scientists for the National Oceanic and Atmospheric Administration was published in the current issue of Geophysical Research Letters. It is based on temperature and precipitation readings taken at weather stations around the country from 1895 to 1987. Dr. Kirby Hanson, the meteorologist who led the study, said ... that the findings concerning the United States do not necessarily cast doubt" on previous findings of a worldwide trend toward warmer temperatures, nor do they have a bearing one way or another on the theory that a buildup of pollutants is acting like a greenhouse and causing global warming. Several computer models have projected that the greenhouse effect would cause average global temperatures to rise between 3 and 8 degrees Fahrenheit in the next century.