Is it global warming, or just weird?

It could be just another temperamental New England winter. Or it could be a sign of a more permanent shift in the region's climate.

However you look at it, the weather across Greater Boston has been strange this season, with January ranking as the region's sixth-warmest on record, and temperatures as high as 57 last month sandwiched between cold snaps and snow, according to the National Weather Service.

Locally, the warm weather has meant early plant growth, and some gardeners are thinking twice about whether they are seeing an atmospheric aberration or the possible longer-term effects of global warming.

''It raises concerns with me," said Tom Smarr, horticulture director for the New England Wild Flower Society at its Garden in the Woods preserve in Framingham. ''At the same time, there are quirky cycles that we never really understand fully."

''We're all becoming armchair atmospheric scientists," said Jack Russell, a retail manager at Russell's Garden Center in Wayland who has been gardening for 30 years.

Over the past 111 years of record-keeping, the trend in Greater Boston has been toward warmer average temperatures, according to the National Climatic Data Center. Some see this as a sign that global warming is coming home to roost.

Seth Kaplan, a senior attorney with the Conservation Law Foundation, said the atmosphere's warming is leading to increasingly erratic weather patterns, including more-powerful hurricanes, summer cold snaps, and warmer temperatures in winter.

''Everything we know about expected weather patterns is wrong," Kaplan said. ''This is the kind of unpredictability and instability that should be expected" under global warming, with carbon-dioxide emissions the root cause, he said.

Some local gardening specialists, while noting that New England's weather has always been erratic, say that this winter's mild temperatures could have an effect on the spring growing season. Without an adequate period of winter dormancy, many plants may not grow properly, they say.

''It's part of their life cycle during the year to have a dormant season," Smarr said.

Plants also can be harmed if they're not insulated by snow cover and temperatures suddenly drop, as they did this week.

Michael Arnum, public relations coordinator for the Tower Hill Botanic Garden in Boylston, said its staff members have seen early growth in several plants. While some of the garden's winter programs for the public have been affected, he also noted that the warmer weather has brought more visitors overall.

''It seems like every winter is different," Arnum said. ''You just try to adapt to whatever the conditions warrant."

James Hansen, a climate scientist with the National Aeronautics and Space Administration, made headlines last month by saying the Bush administration was censoring studies citing global warming as a threat to the world's environment. Reports also quoted Hansen as saying that the opportunity to reverse the trend may be shorter than scientists have been projecting.

The National Climatic Data Center reported that this January was the warmest in the past 111 years in the United States, with an average temperature of about 40. Temperatures between August and January also set a record for warmth, with a national average of almost 53.

Last year went into the books as the eighth-warmest on record for the United States, according to the data center.

Newton resident Richard Primack, a professor of biology at Boston University, said the warming is occurring all over the world but he doesn't believe one mild winter is necessarily an indication of climate change.

Primack said he is concerned by the results of his own recent study, which found plants at the Arnold Arboretum in Jamaica Plain are flowering eight days earlier than they were 100 years ago. He also has conducted research on bird migration.

''The evidence of earlier flowering times of plants and arrival times of birds in the spring is a dramatic wake-up call that global warming is happening right in front of us," Primack wrote in an e-mail.

Peter Del Tredici, a senior research scientist at the arboretum who worked with Primack, said the trend over the past 40 years has been toward warmer winters.

One effect he has studied involves the hemlock wooly adelgid, an insect that feeds on the Canadian hemlock. Cold winter temperatures kill the insect and, in turn, save the tree, while milder winters allow the insect to flourish.

Climate change could result in public health concerns as well. Paul Epstein, a doctor and associate director of the Center for Health and the Global Environment at Harvard Medical School, said he has published a study that suggests earlier -- and therefore, longer -- springs are worsening the plight of allergy sufferers.

At Weston Nurseries in Hopkinton, Wayne Mezitt usually sees the witch hazel come into bloom around the second week of February.

This year, he said, the yellow flowers began sprouting before the end of January. Even with the recent cold temperatures, the flowers will be here until spring.

''This will bloom now until April," Mezitt said.

While this season's temperature spikes have been more extreme than usual, he said, he does not expect to see much of an effect on plants.

''I think, in the end, this will come out to be a fairly normal year."

Emily Shartin can be reached at eshartin@globe.com.