Unusual drought in many areas of the world, also extreme temperatures, tornadoes and blizzards of the last three years, have become the norm, rather than the exception, according to two recently published books about the weather.

Both say that changes in the weather are caused by the sun's activity and predict another little ice age, a return to the frigid winter temperatures and severe drought conditions, such as those prevalent in the early 19th century. These, they say, are likely to persist for several decades.

The consequences of a deteriorating climate are frightening, they say, and man's first concern must surely be for food for the increasing world population. Prolonged drought and temperature extremes will take their heaviest toll on agriculture.

One of the books, "What's Wrong With Our Weather? The Climatic Threat of the 21st Century," by Joh Gribbin (Scribner's, 174 printed pages illustrated, $9.95), asserts that if we are to have food enough in the years ahead, the time to address the problem is now. Unless we do so, the climate threat will become an inescapable disater.

Gribbin received his Ph.D. in astrophysics at Cambridge. He works at the Science Policy Research Unit of the University of Sussex. He has worked for the science journal Nature, is the author of 10 books, and has edited a major academic work, "Climatic Changes," for the Cambridge University Press.

There is little doubt, Gribbin says, that the world's climate has changed and is changing from the benign conditions of the middle part of the century. There is even less doubt that the world food supply and distribution systems are in a crisis state, with no more than one month's supply in reserve, an amount insufficient to compensate for even one bad harvest, one that could result from the natural fluctuations of the weather and climate.

This is the view of not just a few climatologists seeking support for their own work, but of respected government organizations and agencies. A U.S. National Research Council report published in 1976 summed up the situation in the following way: "An understanding of climate change, both natural and man-made, is one of the central issues society must address.. . .

In 1976, food for 4 billion inhabitants is required, in the year 2000, food for at least 6 billion will be needed."

"You may disagree with my interpretation of the evidence," Gribbin says, "but one thing remains clear: The earth's climate is indeed changing, we must learn to understand and live with the threat of climatic change, and I hope this book will contribute to that understanding."

The other book, "Climatic Change and Food Production," edited by Koichiro Takahashi and Masatoshi M. Yoshino, (University of Tokyo Press, Tokyo, Japan, distributed by ISBS, Inc., P.O. Box 555, Forest Grove, Ore. 97116, 433 pages printed in English, illustrated, $52.50) contains the texts of 34 papers, presented at an International Symposium on Recent Climatic Change and Food Production in Tsukuba and Tokyo in 1976. The book also contains comments from conference participants, 100 scholars from all over Asia, Europe and the United States.

The papers describe and characterize climatic fluctuations of recent years and their effect on Asian agriculture, as well as predictions of future climatic patterns for the region.

Takahashi and Yoskino, the editors, are both associated with the Institute of Geoscience, University of Tsukuba, Ibaraki, Japan.Dr. Reid A. Bryson, Institute of Environmental Studies, University of Wisconsin, was a participant in the symposium.

Ultra-long-range weather forecasts indicate a cooling trend may continue for years. Dr. Bryson discussed the posibility of year-in-advance predictions of monthly total precipitation.

The stability of the economy in Japan is based on dependable food supply, with major reliance on the United States, according to Hideo Wada, Hokodate Marine Observatory, Hokkaido, Japan. However, this situation may change in a new climatic epoch. There have been periods in which north Japan suffered from cool summers with a bad harvest of rice and the central part of the United States was plagued by heat and drought with bad harvests of grain. A serious situation in food supply not only for Japan but also for the world is probable, he said.

Following the cool period, in the days of our great grandchildren, it is predicted a warming trend may occur, with possible drastic changes that may lead to complete disapperance of ice, sith an ice-free Arctic Ocean. Such an evolution would soon become irreversible, according to H. Flosh, of the Meterorlogisches Institut, Universiat Bonn, Bonn, Germany.