CONCORD, Mass. — It’s hard to channel Thoreau’s spirit at Walden in summertime, with tourists crowding the trails around the writer’s retreat site and splashing in historic Walden Pond. On sunny weekends the parking lot overflows with cars by mid-morning — hardly an ideal place to reflect on human connections with nature.

But this year visitors can see Thoreau in a different light. “Early Spring,” a special exhibit at the Concord Museum, through September 15, 2013, expands on Thoreau’s time at Walden by presenting him as a keen analyst of seasonal changes, whose 19th century writings are informing modern climate science.

Thoreau kept detailed records of spring-time arrival dates of many migratory birds … and much more. (Concord Museum Photograph by David Bohl)

Since 2003 Boston University biologist Richard Primack and his students have used Concord as a living laboratory to measure how rising temperatures are affecting ecological markers, such as the first bloom of wildflowers in spring. As references, Primack and his colleagues use historical records, including naturalists’ journals, museum specimens, and notes from nature clubs. Their sources include data that Thoreau compiled on lengthy walks around the area.

One Set of References: Thoreau’s Notes while Sauntering

“I think that I cannot preserve my health and spirits unless I spend four hours a day at least — and it is commonly more than that — sauntering through the woods and over the hills and fields absolutely free from all worldly engagements,” Thoreau wrote in “Walking,” an essay published after his death in 1862, based on his frequent public lectures.

Thoreau sat for a photograph twice in his life, the last time in 1861. His friend Daniel Ricketson had this replica made in 1862 after Thoreau’s death and sent it to Thoreau’s sister, Sophia. (Concord Museum Photograph by David Bohl)

On those walks Thoreau took detailed notes about events that marked the changing seasons, including snow depth, ice-out for local ponds (the date when ice disappeared), the spring arrival dates for many species of migratory birds, and first flowering dates in spring for numerous types of plants. He compiled the data in his journals, and later organized it into detailed charts. Thoreau also collected thousands of plant specimens, which he carried home between the pages of a book of flute music, then pressed and mounted on heavy paper with their scientific names and collection locations and dates.

“Thoreau was a revolutionary who recognized how much data there was in the natural world,” Primack says. “He saw that things happened very early in warm years and later in cold years, and was analyzing how climatic variation affected the timing of spring events.”

Primack learned about Thoreau’s materials as he was searching for historical data to track climate change impacts locally. He chose Concord because the town had abundant historical records and many citizens involved in activities like botany and birdwatching. (The study of biological events and their relationship to climatic shifts, based on data from these kinds of activities, is called phenology. A U.S.A. National Phenology Network was created in 2006, but is smaller and more decentralized in the U.S. than counterparts abroad.)

When Primack and his colleagues saw some previously unpublished Thoreau materials, they knew they had struck gold. “It was an astonishing find,” Primack says. “There’s a large community of Thoreau scholars who knew about these charts and manuscripts, but they had always studied them from a literary perspective, and the materials weren’t known to people in the biological community.”

Concord Flowerings … Now an Average of 10 Days Earlier

Richard Primack (top right) and scenes from research group doing field work around Concord, Mass. (Concord Museum Photograph by David Bohl)

By comparing Thoreau’s data and other records kept by past and current Concord residents to current information, Boston University researchers have documented that many plant species around Concord now are flowering an average of 10 days earlier than in Thoreau’s time, and that the local mix of species is shifting to favor plants that have highly flexible flowering dates — that is, they are able to bloom early in warm years and late in cold years. The average spring temperature has increased from 42 degrees F in the 1850s to 48 degrees F now , and the average first flowering for 32 selected trees and bushes has advanced from May 15 in Thoreau’s time to May 4 today. Now researchers are studying other phenomena, including the timing of tree leaf-out in spring and the first spring appearance dates for butterflies and other insects.

The Concord Museum exhibit fills two rooms. Thoreau’s notes, plant specimens, and reference books are interspersed with materials from other area nature-lovers. One striking example: two pairs of cupboard doors covered with pencil notes by the late Henry Vose Greenough, a longtime resident nearby, chronicling the arrival of migratory birds in spring from 1930 through 1970.

As visitors move through the rooms, outdoor sounds that they might hear on a solitary nature walk play softly in the background. Spring noises — cracking ice, the chirps of peepers — phase into summer birdcalls, followed by footsteps through dry autumn leaves and gusting winter winds. Displays show members of Primack’s research group in the field, doing many of the same things Thoreau did: cutting plant samples, spotting birds through binoculars (Thoreau’s brass telescope is in the exhibit), and writing down lots of notes.

Encouraging Citizen Science

The exhibit encourages visitors to get involved in citizen science programs that are generating more data on seasonal changes. Touch screens provide links to several. Project Budburst, sponsored by the National Science Foundation, recruits volunteers nationwide to collect data on plant leafing, flowering and fruiting dates. And the New England Wildflower Society has developed an online database to help residents identify 1,200 common Concord-area plants.

Richard Primack with a statue of Henry David Thoreau at Walden Pond, Mass.

“All of the changes that this exhibit shows have happened within a single community. The evidence is very tangible and approachable,” says Gary Clayton, vice president for conservation programs at Mass Audubon. “Media debates have confused this issue for many people, but here you can see climate change happening literally in our back yards.”

Concord is uniquely suited for studying climate change using local historical records. The town is strongly vested in its history, and may have the most extensive environmental records in North America. “There’s probably no place in North America with environmental records as old and complete as Concord’s,” Primack says. “It’s a unique situation, and there are only a few places in the world with as much info on their natural history.”

But Primack sees lessons from Concord that can be applied in other places. “When people go looking for old records, they can find them, especially in big cities and university towns,” Primack says. “Many of those places have elements of what Concord has, and their records can be very interesting to analyze from a climate change perspective.”

The exhibit furthers one of Primack’s central goals: communicating climate change science to the public. “Many people are deeply interested in this issue, and there’s a lot of misinformation about it,” says Primack. “Talking to the public has also helped us sharpen our thinking. When we first started reporting our findings, people would ask why earlier springs mattered, and I realized that we had to be much more explicit that they were an indicator of much more negative things to come. It’s important to show not just what’s happening now, but also what it points to.”

Thoreau put it more succinctly in his journal in 1851: “The question is not what you look at, but what you see.”

Jennifer Weeks is a Massachusetts-based freelance writer specializing in environment and energy stories.