By year’s end, roughly 100 American communities will be left without regular commercial air service, and that number may double next year, according to the Air Transport Association, the industry trade group.

“The guy who is used to taking a nonstop flight on a small airplane now has to drive an hour to an hour and a half to an airport to take a trip,” said David Castelveter, a vice president with the trade group. “It is a crisis of great magnitude and it is having an impact already.”

At least one major carrier could liquidate, the trade group has warned, on top of eight small airlines that have disappeared or filed for bankruptcy protection this year.

The prospect of losing service alarms passengers and politicians alike. “I implore American Airlines, as well as the other carriers considering various cost-saving scenarios, to take into account more than profit when they evaluate routes,” Gov. David A. Paterson of New York said this week after American announced a series of cuts affecting La Guardia and other state airports.

Airlines know the changes are painful. Said John P. Tague, United’s chief operating officer: “There’s going to be a period of adjustment in the last half of the year that will be unpleasant.” But the downsizing of the airlines is unlikely to be reversed anytime soon. Carriers are selling off hundreds of older, less-efficient planes, so the industry would have trouble growing sharply again even if oil prices were to drop and the economy were to rebound quickly.

Well before jet fuel costs jumped, analysts were saying there were too many carriers and that a round of mergers was needed to consolidate the industry.

Image Allon Lefever of Harrisonburg, Va., waiting for a Northwest flight at the Toledo airport. He flies several times a year and says airline cutbacks on routes will make travel more cumbersome. Credit... Fabrizio Costantini for The New York Times

In effect, travelers are seeing the likely results of such mergers  reducing operations to save costs  with only one merger occurring, the combination of Delta and Northwest.