MAY 10, 1853, was a warm day outside Concord, Massachusetts—an early spring day when a New Englander outdoors would “begin to think of thin coats,” noted Henry David Thoreau. Walking from Concord towards Saw Mill Brook, Thoreau jotted down what he saw. “The deciduous woods were in their hoary youth,” he wrote, “every expanding bud swaddled with downy webs.” Nodding trillium had flower buds, and hornbeam was about to bloom. Pear trees had blossomed, and the butternut buds were the most pronounced of all the woods’ hickories. He heard the spring’s first veery. “It is remarkable,” wrote Thoreau, “that I saw this morning for the first time the bobolink, gold robin [most likely a northern oriole], and kingbird.”

Remarkable, too, that he kept such meticulous records. In fact, on almost every spring morning between 1851 and 1858, long after his private tenure at Walden Pond, Thoreau explored the ponds and shady woods around Concord, observing nature. For day after day, year after year, he searched for the first blooms of more than 300 plant species and watched for the first arrivals of migrating birds.

Today, nearly 160 years later, Thoreau’s detailed observations form the basis of a long-term study of how climate change is altering the timing of seasonal biological events—or phenology—and how such shifts may in turn impact the wildlife and wild places of an entire region. Researchers from Boston University have assembled a vast array of biological data—arboretum specimens, old photographs and the observations of local citizens, in addition to Thoreau’s journals—to produce a baseline of springtime events for the Concord area. Comparing these data to the results of their own exhaustive, five-year effort to walk, literally, in Thoreau’s footsteps, the scientists can now tell a story that New England’s favorite naturalist-philosopher might never have imagined: As Massachusetts warms, flowers are blooming, trees are leafing out, and birds are arriving as many as three weeks earlier than they did in the mid-nineteenth century. “If Thoreau were alive today, he would be very concerned about this,” says Richard Primack, a biology professor at Boston University and lead researcher on the project.

NOTING NATURE Thoreau, famous for his prodigious note-keeping, recorded his seasonal observations in tables sketched on large sheets of surveyor’s paper. “I take infinite pains to know all of the phenomena of the spring,” he explained in one journal entry. Thoreau intended to publish a book about the unfolding of spring in the woods around Concord, but his death in 1862 derailed the project, and his notes were scattered among library collections across the country.

Four years ago, however, Primack learned that an independent New Hampshire scholar named Brad Dean had spent 10 years tracking down these original sheets, making copies and reassembling the data. By then, Primack, author of A Primer on Conservation Biology, was looking for studies demonstrating physical evidence of global warming. He and graduate student Abraham Miller-Rushing couldn’t believe their good fortune. Still, it took Primack’s team nearly nine months to decipher Thoreau’s famously poor handwriting and archaic species names and plug the information into a usable spreadsheet.

At the same time, the scientists’ sleuthing uncovered a trove of other regional records to augment Thoreau’s notes. At Harvard University’s Arnold Arboretum, one of the oldest public botanical gardens in the United States, they were able to compare the flowering times of 229 plants in 2003 with records of flowering times of the same individual plants going back as far as 1885. In Concord, they found a collection of images from a photographer, Herbert Wendell Gleason, who between 1900 and 1921 took and dated photographs of many of the plants and places mentioned in Thoreau’s journals. From these, the scientists gleaned flowering data on 17 species of wild plants, including pink lady’s slipper, which flowered six weeks earlier in 2005 than in 1917.

Some of the richest sources of data turned out to be citizen-scientists in the mold of Thoreau himself. From 1888 to 1902, a Concord shopkeeper named Alfred Hosmer, inspired by Thoreau’s writings, recorded the first flowering dates of more than 700 plant species in the area. A passionate nature aficionado named Pennie Logemann provided flowering records between 1963 and 1993. And for more than half a century, Middleborough, Massachusetts, resident Kathleen Anderson has kept meticulous track of the timing of bird arrivals, plant flowerings and spring choruses of frogs and toads on her 100-acre farm. “I keep a stack of those desk calendars with one full page for each day of the week,” she explains, “and I was pretty intense about it. I noted weather conditions, temperature, rainfall, and whatever I happened to notice. Were the Canada mayflowers blooming? Were the juncos around? It was for my own enjoyment. It never occurred to me that these records would be of any use or interest to anyone whatsoever”—until she was contacted by Primack and Miller-Rushing, who crunched her observations into their expanding database.

The researchers, meanwhile, were making their own detailed observations. For the past five years, Primack and Miller-Rushing have traveled to Concord three times a week in spring and summer, walking the woods to ask the same questions that Thoreau asked: When do the flowers bloom? When do the birds return? So far, they have amassed another 100,000 data entries about the phenology of springtime plants and birds.

WHAT THE FLOWERS SAY Pooling their data, the researchers have discovered that many plants in the Concord region are flowering more than a week earlier today than when Thoreau made his observations. Highbush blueberry—one of Thoreau’s favorite wild edibles—is blooming some two weeks earlier than it did 150 years ago. Yellow wood sorrel can be found in bloom about a month earlier. During this same period, Primack says, long-term weather data show that the average temperature of a Concord spring has increased by approximately 4.5 degrees F.

Much of the temperature rise in the intensely developed Northeast is due to what’s known as the urban heat island effect—parking lots, streets and buildings absorb heat while vegetation loss lessens the release of cooling water from trees and other plants. But at least some of it can be attributed to global warming, says Primack. And on Anderson’s farm, many of the wild creatures that appear regularly each spring seem to be responding. Wood ducks are arriving about a month earlier than they did 30 years ago, for example, while ruby-throated hummingbirds show up more than 18 days sooner.

Scientists say such changes have the potential to wreak ecological havoc if interdependent species do not shift in concert. Many birds, for example, have evolved to time their spring migrations to take advantage of a flush of food sources. In New England, warbler species such as the black-throated blue warbler and American redstart feed heavily on leaf-eating caterpillars, which peak in abundance after leaf out and before leaves mature and grow tough.

In northern Europe, biologists already have found troubling evidence that one migratory bird, the pied flycatcher, has suffered from getting out of sync with its springtime food source. In the past, flycatchers arrived from their West African wintering grounds just as winter moth caterpillars were hatching. But warmer springs have pushed the caterpillar’s emergence date two weeks earlier—unbeknownst to flycatchers that are still 2,800 miles away. In regions where the timing of caterpillar abundance has shifted the most, researchers have documented a 90 percent decline in flycatcher numbers. In the United States, a similar “potential for mistimed relationships is very real,” says Primack, “but it is understudied.”

A COLD HARD LOOK To increase much needed data on global warming’s impact on U.S. species, some scientists propose identifying and training a network of modern-day Thoreaus. According to Primack, Miller-Rushing and other researchers, there is the potential for a rich interaction between scientists and members of the general public interested in gathering observations on natural phenomena such as plant flowering and the arrival of migratory birds. Countries such as England, Belgium and Canada have long embraced monitoring programs that rely, in part, on observations of nonscientists. Recently, a consortium of U.S. government agencies and academic institutions, with funding from the National Science Foundation, launched just such an effort, the National Phenological Network, to help researchers collect and disseminate information about seasonal changes.

“We desperately need a wall-to-wall, coast-to-coast network of phenological observation points—literally thousands of points on par with what is being done with meteorological observations,” explains Julio Betancourt, a biologist with the U.S. Geological Survey’s Desert Laboratory in Tucson, Arizona, and one of the network’s founders. Volunteer observers are an important part of the process. The group’s Project BudBurst, begun as a pilot program in spring 2007, will launch nationally in January 2008. Volunteers from across the country are asked to choose from a long list of plants to watch for signs of a particular phenophase, such as budburst, first leaf or first flower, and to report observations online.

“So much of the discussion about global climate change has centered on numbers—fractions and degrees of fractions,” says biologist Mark D. Schwartz of the University of Wisconsin–Madison, who is helping to coordinate the network’s startup. “But when you talk about how lilacs are blooming six days earlier than they were 30 years ago, people start relating to the issue. And tell them that they can involve themselves in the process of documenting these changes, and that makes it very real.

”That’s something Kathleen Anderson understands well. “This kind of work should inspire more people to be more observant,” she says. At the age of 84, she still keeps notebooks handy at home, in the car and in the kitchen. “And it really doesn’t matter where you live. If you look closely, you’ll find enough things to interest you in the little bit of land that is around you.”

After all, as Thoreau told his friend and sometime walking companion, Ellery Channing, in 1859, “There is nothing but the seasons.” By which he might have meant that the seasons will tell all, to those who wish to hear.

Writer T. Edward Nickens is based in North Carolina. To find out how to participate in the National Phenological Network, go to www.usanpn.org.

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