It’s spring, and everywhere you look flowers are blooming and trees are sprouting leaves. While the season’s arrival is welcome for many of us, researchers say the timing of that arrival is a troubling indicator of our changing climate.

Next to Walden Pond in Concord recently, Boston University professor Richard Primack spotted a small sapling.

“There’s actually a plant I never noticed here before," he said. "Maybe it could be new. This is a bigtooth aspen.”

A newly leafing bigtooth aspen near Walden Pond.
Craig LeMoult/WGBH News

As he got a closer look at the new leaves on the small tree, a crowd of students from Westborough High School walked towards him. They were led by Walden park ranger Jacqueline Kluft, who recognized Primack.

“Everyone, we have a really quick opportunity to talk with an awesome scholar,” she called out to the group.

She’d just been talking with the students about Thoreau, the naturalist and conservationist whose classic book Walden detailed his time living here at the pond. Primack explained to the students that about 17 years ago, he was trying to figure out a good way to measure the effects of climate change in the region.

“And as I started looking I heard about these records that Henry David Thoreau had made in the 1850s — from 1851 to 1858 — about when plants were flowering in Concord when birds where arriving in the spring and when trees and shrubs were leafing out,” he said.

And since 2004, Primack and his students have been documenting the same things.

“And so the information we have from Concord demonstrates that the warming climate associated with global climate change is already affecting the biology of the species here in Concord," he told them.

For these students, a field trip on this nice spring day was both a welcome escape from the classroom, and an ominous warning of what’s to come in their lifetime.

Richard Primack recording his discovery of a newly leafing bigtooth aspen near Walden Pond.
Craig LeMoult / WGBH News

When Primack first set out to measure climate change, he looked everywhere for old records, asking everyone he could, even posting flyers in supermarkets — until an old student of his suggested he check out those Thoreau records.

“And I said, ‘what records are you talking about?’ I said I knew about the book Walden, but I didn’t know about any old records,” he remembered.

Thoreau meticulously documented more than 350 plant species. "It is astonishing how soon and unexpectedly flowers appear, when the fields are scarcely tinged with green," he wrote in 1852. "Yesterday, for instance, you observed only the radical leaves of some plants; today you pluck a flower."

As Primack followed in those footsteps, he held a small notebook.

“You can see that I’ve checked off species that I’ve already seen in flower this year in Concord, and the species that are shown in circles are species that we’re going to be looking for today,” he explained.

Richard Primack records the same species that Thoreau documented in Concord.
Craig LeMoult/WGBH News

As he hiked the trails around the pond, he squinted into the woods. “This is a plant that’s just leafing out today," he said. "This is the black birch.”

When Thoreau saw black birch leaves for the first time in the 1850s, it happened noticeably earlier. “The trees, shrubs and vines like poison ivy and grape are leafing out two weeks earlier on average than in Thoreau’s time,” Primack said.

Leafing time is closely tied to temperature. And the mean spring temperature in the Concord area has jumped from about 42 degrees in Thoreau’s time to nearly 48 degrees today.

Shadbush (also known as serviceberry) blooming at Walden Pond.
Craig LeMoult/WGBH News

Primack’s not just recording plants at Walden Pond. Like Thoreau, he’s searching for blooms and leaves all over Concord. As he drove, he pointed out all kinds of flora along the way.

“So this tree that we’re just driving by is an ash tree,” he said, pointing out the window. He can’t help it. This is how he sees the world.

“So this is something that I’ve just been doing for my whole life, since becoming a botanist at the age of 21 years old," he said. "So I’m driving past every tree and I’m saying 'beech tree, maple tree, apple tree, elm tree, beech tree, apple tree, buttercups, dandelions.'"

Maybe Thoreau did the same kind of thing, but about a quarter of the species he saw in Concord are no longer here — and another third have significantly declined.

“And there are a lot of reasons why the species have been lost from Concord, but at least part of the reason is climate change,” Primack said.

Richard Primack searching for newly leafing trees and blooming flowers near Walden Pond.
Craig LeMoult/WGBH News

Primack rattled off other dangers climate change poses to this area — coastal flooding, more extreme storms, forests killed by insect outbreaks. Not to mention the threats to human public health.

“The fact that it’s slightly warmer, and that spring is happening earlier is actually sort of appealing," he said. "I mean, because New Englanders, we face very harsh winters. But it’s really an indicator of problems that will be coming in future decades.”

That’s the contradiction built in to the research Primack’s doing. Like Thoreau, and like the rest of us, he feels joy in discovering a new bloom or leaves on a tree that was bare just days before. But for Primack, there’s also something troubling in the arrival of spring.