cross the Northeast, people opened their doors yesterday to discover something unfamiliar: a chill in the air.

Weeks later than usual, temperatures have finally descended into something like their normal late- December range. The cold snap followed an autumn in which sidewalk cafes stayed open into December, golf courses were packed with plaid and the first World Series game ever played in November felt more like May. The unseasonal curveball that the weather threw at the calendar affected almost everyone, for better and for worse.

Varel Bailey, a cattle and pig farmer in Anita, Iowa, has been spending 5 percent to 10 percent less on grain because cattle eat less when it is not cold.

In New Mexico, Ski Santa Fe, a snow-starved resort, had to postpone its planned Thanksgiving Day opening to Dec. 8, and even now has less than half its skiing areas open.

In the natural world, daffodils bloomed, and the grass at the New York Botanical Garden was a lush green the week before Christmas. In Pennsylvania, a baby killdeer hatched in December instead of April. And in Massachusetts, Bill Davis, a state wildlife biologist, came home from a deer station one recent day with a sunburn.

"It's global warming, dude," said Pito Robles, 28, an auto mechanic who on Thursday was dangling his fishing line into the Hudson River near West 66th Street in Manhattan. "I don't care if the whole planet burns up in a hundred years. If I can get me a fish today, it's cool by me."

In fact, the warm spell that settled over much of the nation this fall may not be caused by global warming, the gradual but potentially calamitous rise in worldwide temperatures that most scientists attribute, at least in part, to fuel-burning human activities like making electricity and driving. When burned, fuels release carbon dioxide and other "greenhouse gases" that trap heat in the atmosphere.

The fall weather could have been a fluke, climatologists agree, because in the short run weather often does unanticipated things.

But the unusually balmy fall seems to fit at least two longer-term patterns. One is that the weather has become significantly more erratic and variable, and that is likely to continue, scientists say.

Another is that the warming trend is destined to continue. This year is expected to be the second warmest on earth since 1860, when temperatures were first measured systematically. Nine of the 10 warmest years have occurred since 1990. And one of the world's leading forecasting agencies, the Hadley Center for Climate Prediction and Research in Britain, says there is a 75 percent chance that next year will be even warmer than this year.

So far, the results of global warming in North America seem to be a mix of good and bad. Since 1960, New England has seen the last hard freeze come a day earlier every three years, extending the growing season. But in the Middle Atlantic and the South, shores are receding as seas, slowly raised by melting glaciers, eat away at coasts.

And Alaska, where warming has been most pronounced, is undergoing a host of changes. Glaciers are receding, permafrost melting, shrubs spreading across previously barren tundra. The start of the Iditarod dog race this year had to be moved north because the trail near the usual starting point was too mushy.

But the growing variability of the weather may be as troublesome as the warming. This weekend, after a mittenless fall, temperatures are plummeting quickly across the country. And the prospect of a sudden, sustained cold snap is making many people uneasy.

Will birds that delayed their ritual migration be trapped up north by frigid weather? Will buds that bloomed in December's warm chicanery burst their pipes and suffer winter damage?

"The party is almost over for people who like warm weather," said Jon B. Davis, a meteorologist for Salomon Smith Barney in Chicago. "Wednesday's 49 degrees in Chicago was probably the warmest we will see until late March. We are headed into a major-league cold pattern."

Labor and Industry
Even if this winter is about to earn its reputation, the fall's warm weather has made its mark — economically, environmentally and in other ways.

It has been a boon to workers at the site of the World Trade Center disaster. Joe Bradley, the foreman of a crew of machine operators struggling to clear rubble at ground zero, said the mild weather allowed them to clean up the site quickly and to recover bodies they might not have otherwise found.

"It gave us an opportunity to get the bulk of the work done," Mr. Bradley said. "It was a gift from God, a real miracle."

Economically, the warm spell has cut both ways, benefiting some industries while hurting others. Warm weather, it turns out, may get people out of their houses, but not necessarily into stores. Foot traffic may have increased, retailers say, but people tend to window-shop. And they are not buying the winter supplies that stores have stocked. As a result, retailers have slashed prices on things like coats, scarves and gloves.

"Warm weather is not good for retailers," said Scott Bernhardt, senior vice president in charge of retail research at Planalytics, a consulting firm in Wayne, Pa., that provides long-range weather predictions and marketing advice to retailers. "A winter like this hurts because retailers don't plan on it."

Snow shovel sales are down about two-thirds for the 7,200 hardware stores in the TruServ cooperative in Chicago, said Ray Winkel, its director of inventory management. But snow blowers are selling well. "Unlike snow shovels, they are not an impulse purchase, and people remembered what a tough winter last year was," Mr. Winkel said.

And other industries are clearly benefiting from the warmth. Scott Arves, president of Schneider National in Green Bay, Wis., the huge trucking company known for its orange fleet, said the mild weather had resulted in hundreds fewer accidents, lower maintenance costs and on-time delivery rates rare in late fall and winter, when a single storm can delay 1,000 Schneider trucks.

Warm weather has depressed demand for natural gas, and, partly as a result, prices have fallen sharply — to $2.60 per million B.T.U.'s of gas to be delivered next month, from $10 last winter. That has helped consumers in the upper Midwest, where gas provides 90 percent of the home heating energy.

In agriculture, the warmth has generally been welcome. Livestock farmers can buy less grain, not only because cows eat less in warm weather but also because breeding stock can stay out in the fields longer to forage. Grain farmers can expect to pay less for fertilizer, because natural gas is a major component.

In states like Florida, rising temperatures may change growing patterns. A 1989 freeze pushed the citrus belt south, said Jim Lushine, a National Weather Service meteorologist, but warmer weather may have the opposite effect. "We could see a return, a northward shift in some of the growing climates in Florida," he added, "and that might actually prove to be a boon for some people."

Flora and Fauna

Some plants and animals have reacted to the warm cycle as if it were a false spring.

Spring peepers are calling, woodpeckers and ruffed grouse have been drumming, said Mr. Davis, the wildlife biologist. More ticks seem to be active late into the year, he said. And in Montana, the Bitterroot Valley has been alive with butterflies that would normally be hibernating.

Most birds that migrate over long distances are not affected by the warmth because they would already have been in the tropics by September. But other species, like ducks and geese, which migrate shorter distances, have stayed north.

Two calliope hummingbirds were first spotted at Fort Tryon Park in Manhattan about a month ago, when they would ordinarily be in Mexico, and were still at the park last week.

The birds that have stayed north could suffer if a sudden cold snap causes ponds and lakes to freeze, making food unavailable for several days, said Dr. Jeff Price, director of Climate Change Impact Studies for the American Bird Conservancy.

Deer may actually benefit from the warmth. The deer population got winter coats on time, but then was less active and consumed fewer calories because of the higher temperatures, Mr. Davis said, adding, "This sends them into winter in better condition to survive."

Many plants may not do as well. At the New York Botanical Garden, Dr. Kim Tripp, vice president for horticulture and living collections, said the prolonged warmth, coupled with a long drought, had left trees and other plants discombobulated and stressed. To survive the winter, Dr. Tripp said, plants have to go through a long process of shutting down: annuals die, perennials kill off their tops, deciduous plants shed their leaves, evergreens pump antifreeze into their needles.

"It's still October for the plants," she said. If it suddenly goes from October to January, water could freeze in the plants' cells and burst them. If the freeze hits anything that has actually flowered, it will not develop buds for next spring, she said.

Several years of these conditions could take a toll on trees, too. Mature canopy could be lost because such stress leaves trees more vulnerable to disease and insects.

"You fight off a cold better when you are healthy and vigorous," Dr. Tripp said. "When you are stressed your metabolism is slower and healing is slower."

The Outlook

Even if this past fall's warm weather was only a blip in the global thermostat, most scientists are beginning to believe that human alteration of the atmosphere is at least partly responsible for a warming trend that has been going on for decades.

"I think it's increasingly clear that humans are influencing the climate," said Dr. Peter H. Gleick, director of the Pacific Institute for Studies in Development, Environment and Security in Oakland, Calif. "I think the debate over whether the climate is going to change is over, and now the debate is what will the impacts be, how bad will they be and what should we do about it."

Dr. Edward S. Sarachik, a professor of atmospheric sciences at the University of Washington in Seattle, said it was especially significant that 2001 had proved so warm. Usually, the conditions in a single year are discounted by climate scientists because weather conditions vary naturally over short time spans, pushed this way or that by unpredictable shifts in sea temperatures and wind patterns.

But in 2001 there was no such push — in particular, no sign of El Niño, the ocean condition in which unusually warm waters spread across the Pacific. El Niño has exerted a powerful influence in many other recent unusually warm years, including 1998, the warmest year on record. But this year, the Pacific has just emerged from the opposite, cooler condition, La Niña.

A milder climate has short-term and long-term consequences for everything from water supplies to beaches.

Higher temperatures, for example, mean reduced snow melt from the Sierras, on which California's reservoirs depend.

"As the earth is warming, less of our precipitation will be snow and more will fall as rain, and that has a whole series of problems for us," Dr. Gleick said. "It means we're going to get more runoff in the winters when we're most worried about floods. And at the other end it means our spring and our summer runoff is going to be reduced — and that's when we need it most."

In Alaska — and much of Siberia — the annual mean temperature has risen 3 to 5 degrees over the last 30 years, said Prof. Gunter Weller, director of the Cooperative Institute for Arctic Research at the University of Alaska at Fairbanks.

"There's no doubt over all that the Arctic has warmed pretty substantially, and that it's causing serious problems," Professor Weller said. Among the effects are increased coastal erosion from melting glaciers, which have raised the sea level in the region, and thawing in the permafrost that has wreaked havoc with roads and could eventually damage structures like the 800-mile- long trans-Alaska pipeline.

Elsewhere, continued warming could produce rising sea levels and more frequent and more severe storms. Seas will rise with temperatures because water expands as it warms, and because melting terrestrial ice will send water flowing into oceans. Hurricanes draw their energy from the heat in ocean waters, so warming could make them more frequent and more powerful.

The consequences would be most serious for the East and Gulf Coasts, which are lined with low-lying barrier islands. A rise of one to three feet in the sea level, which many scientists predict by the end of this century, could send sea water 100 feet or more inland on these stretches of coast, in communities where the federal government is spending tens of millions of dollars a year to pump sand onto shrinking beaches.

The new evidence of human- caused warming comes as the Bush administration is turning its attention back to climate change, which it had approached with skepticism and a determination to choose no remedies that might hinder the use of coal, oil and other fossil fuels.

Most notably, last March Mr. Bush rejected the Kyoto Protocol, the first international agreement that would require industrialized countries to cut emissions of greenhouse gases or face penalties. The last details were approved by almost every other major country at a meeting in November, although the treaty still awaits ratification.

In the next month or two, the Commerce Department, answering a request by Mr. Bush last summer, is planning to issue a new research plan aimed at improving United States climate monitoring and modeling efforts.

Senator Ted Stevens, Republican of Alaska, is one of many from both parties in Congress seeking significant new efforts, especially climate research.

"I have asked that consideration be given in the coming budget to increase activities," Mr. Stevens said, "to zero in on places where they know they can get some firm data and get some firm predictions."

Others in Congress are proposing bills to limit releases of carbon dioxide from power plants, but Mr. Bush rejected such a move last spring.

While officials debate how to deal with the phenomenon, temperatures continue to climb.

Along with whatever warming might be happening because of human activities, federal scientists say, the Pacific is emerging from its most recent cool spell and the warm waters of another El Niño are getting ready to spread.