Of the estimated 7,000 languages spoken in the world today, linguists say, nearly half are in danger of extinction and are likely to disappear in this century. In fact, they are now falling out of use at a rate of about one every two weeks.

Some endangered languages vanish in an instant, at the death of the sole surviving speaker. Others are lost gradually in bilingual cultures, as indigenous tongues are overwhelmed by the dominant language at school, in the marketplace and on television.

New research, reported today, has identified the five regions of the world where languages are disappearing most rapidly. The “hot spots” of imminent language extinctions are: Northern Australia, Central South America, North America’s upper Pacific coastal zone, Eastern Siberia and Oklahoma and Southwest United States. All of the areas are occupied by aboriginal people speaking diverse languages, but in decreasing numbers.

The study was based on field research and data analysis supported by the National Geographic Society and the Living Tongues Institute for Endangered Languages, an organization for the documentation, revitalization and maintenance of languages at risk. The findings are described in the October issue of National Geographic magazine and at www.languagehotspots.org.

At a teleconference with reporters today, K. David Harrison, an assistant professor of linguistics at Swarthmore College, said that more than half of the languages have no written form and are “vulnerable to loss and being forgotten.” When they disappear, they leave behind no dictionary, no text, no record of the accumulated knowledge and history of a vanished culture.

Dr. Harrison; Gregory D. S. Anderson, director of the Living Tongues Institute in Salem, Ore., and Chris Rainier, a filmmaker with the National Geographic Society have traveled in recent years to many parts of the world, the beginning of what they expect to be a long-term series of projects to identify and record endangered languages.

The researchers interview and make recordings of the few remaining speakers of a threatened spoken language, and collected basic word lists.

The projects, some of which extend over three to four years, involve hundreds of hours of audio recordings, development of grammars and preparation of children’s readers in the subject language. The research has especially concentrated on preserving language families that are on their way out.

In Australia, where nearly all of the 231 spoken aboriginal tongues are endangered, the researchers came upon such tiny language communities as the three known speakers of Magati Ke, in the Northern Territory, and the three Yawuru speakers, in Western Australia. In July, Dr. Anderson said, they met the sole living speaker of Amurdag, a language in the Northern Territory that had already been declared extinct.

“This is probably one language that cannot be brought back, but at least we made a record of it,” Dr. Anderson said, noting that the Amurdag speaker strained to recall words he had last heard from his late father.

Many of the 113 languages spoken in the Andes Mountains and Amazon basin are poorly known and are rapidly giving way to Spanish or Portuguese, or in a few cases, to a more dominant indigenous language. In this region, for example, a group known as the Kallawaya use Spanish or Quechua in daily life, but also have their own secret tongue, used mainly for preserving knowledge of medicinal plants, some of which were previously unknown to science.

“How and why this language has survived for more than 400 years, while being spoken by very few, is a mystery,” Dr. Harrison said news release.

The dominance of English threatens the survival of the 54 indigenous languages of the Northwest Pacific plateau of North America, a region including British Columbia, Oregon and Washington. Only one person remains who speaks Siletz Dee-ni, the last of many languages once spoken on a reservation in Oregon.

In Eastern Siberia, the researchers said, government policies have forced speakers of minority languages to use national and regional languages, such as Russian or Sakha.

Forty Native American languages are still spoken in Oklahoma, Texas and New Mexico, many of them originally used by indigenous tribes and others introduced by Eastern tribes that were forced to resettle on reservations there, mainly in Oklahoma. Several of the languages are moribund.

Another measure of the threatened decline of many relatively obscure languages, Dr. Harrison said, is that speakers and writers of the 83 languages with “global” influence now account for 80 percent of the world population. Most of the thousands of other languages now face extinction at a rate, the researchers said, that exceeds that of birds, mammals, fish or plants.