Scientists are using Henry David Thoreau’s detailed diaries to analyze the ebb and flow of the natural world. They conclude climate change has significantly altered the budding of wildflowers, and the migration of birds, at Walden Pond and beyond.
GELLERMAN: It's Living On Earth, I'm Bruce Gellerman.
GELLERMAN: On July 4th 1845 Henry David Thoreau came here, to the shores of Walden Pond in Concord Massachusetts. He had built a simple one room cabin on land owned by his friend Ralph Waldo Emerson and for the next two years, two months and two days, Thoreau lived deliberately in the woods, to learn what nature had to teach.
THOREAU (VOICED): I soon found myself observing when plants first blossomed and leafed and I followed it up early and late, far and near, several years in succession.
GELELRMAN: Thoreau kept journals of his comings and goings at Walden and his long rambles beyond; his personal diaries detailing, year by year, day to day, sometimes hour by hour the rhythm and cycles of the natural world.
THOREAU (VOICED): I often visited a particular plant four or five miles distant, half a dozen times within a fortnight, that I might know exactly when it opened.
GELLERMAN: In scientific terms, Thoreau was a phenologist, he studied the timing of seasonal biological events. His unique historical dataset is being mined by scientists today.
PRIMACK: My name is Richard Primack and I’m a professor of Biology at Boston University and I study the effects of climate change on plants and birds in Concord and other places in eastern Massachusetts.
[SOUND OF FOOTSTEPS]
PRIMACK: So, we’re at the edge of Walden Pond and we’re here because 160 years ago Thoreau kept very detailed records about when plants flowered in Concord, when plants were leafing out in Concord. And also when birds arrived in the spring. And so by coming back 160 years later and recording these same events we can actually see how a warming climate has shifted the basic timing of biological processes in Concord.
[SOUNDS OF WALDEN POND]
GELLERMAN: For the past decade, Richard Primack and his colleagues have literally followed in Thoreau’s footsteps, first tracking down his unpublished journals and the records of local naturalists, to create a field guide to over 500 species of wildflowers in Concord and its environs.
PRIMACK: We’re talking about all the common wildflowers. So for example around the pond here there’s high bush blueberry, low bush blueberry, marsh marigold, bulbous buttercup, common violets, bird’s foot violets, black cherry, fire cherry, these are the kind of plants that Thoreau was observing and that we can observe right around Walden Pond today.
GELLERMAN: While Professor Primack compared flowering times, his graduate student Libby Ellwood searched Thoreau’s records of migrating birds.
ELLWOOD: And these records are at Harvard’s museum of comparative zoology so I found them there. And they are these impressive spreadsheets that look like today’s excel spread sheets, where you have rows and columns that have the years and the species. And you can see, for example, maybe he saw an Eastern Phoebe on April 1 of 1853 and from there you could determine over time the changes he saw in bird arrivals. And it was difficult to decipher but just having a few words here and there was much easier than having to decipher extensive journal entries.
PRIMACK: Thoreau had terrible handwriting, very difficult to read,
GELLERMAN: Biologist Richard Primack says reading Thoreau’s journals was just one of the problems he encountered in trying to piece together the historical-phenology data base.
PRIMACK: And also or it’s even more problems working with his data because he started off using common names and then he changed over to scientific names and both his common names and his scientific names are different from the ones we use today. So one of our very major challenges was just to read his handwriting and then to kind of match his names with the names we use today.
GELLERMAN: Walden Pond is still undeveloped but Concord has definitely changed since Thoreau lived here. Perhaps surprisingly, now there are more trees and green space.
During the 19th century this was deforested farmland but today, thanks to conservation efforts it’s heavily wooded.
GELLERMAN: Cars speed by the road right next to Walden Pond. Boston is about 20 miles to the east, and that further complicated Professor Primack’s use of Thoreau’s historic evidence to document climate change today.
PRIMACK: It’s again a little bit deceptive, if you look at Concord it looks like a fairly forested, pastoral landscape and yet it’s really within the urban heat island effect of Boston. So Boston is warmer because of all the roads and parking lots and buildings, and the warm weather of Boston, the warming of the city, actually extends into Concord, and so the temperatures here have risen by about five degrees Fahrenheit because of both global warming and also because of the urbanization of Boston.
GELLERMAN: Richard Primack calculates that global warming is responsible for just about one and a half degrees of the temperature increase. The effect can be seen in the spring.
PRIMACK: What we’ve discovered that plants in Concord are now flowering about ten days earlier than they were in Thoreau’s time. The temperature is just much warmer than in Thoreau’s time and so this really explains the earlier flowering time.
GELLERMAN: Over the years Primack has published his findings; his most recent study appears in the journal BioScience. Climate change has taken its toll on some species; asters, bluets and buttercups, lilies, mints and violets are all on the decline around Concord. Thoreau understood the ebb and flow of nature happens on a timescale of its own.
THOREAU (VOICED): It takes us many years to find out that Nature repeats herself annually. But how perfectly regular and calculable all her phenomena must appear to a mind that has observed her for a thousand years!
GELLERMAN: According to the observations of scientist Libby Ellwood, while plants in Concord are very sensitive to temperature, the effect of climate change on migrating birds is more complicated.
ELLWOOD: Some are arriving earlier, some are arriving later, some don’t seem to be changing their migration dates at all.
GELLERMAN: That’s not to say warming temperatures haven’t affected birds. Almost all of the 22 species of migratory birds Elwood studied have declined in abundance since Thoreau’s time, and then there are birds like the fox sparrow and swamp sparrow…
ELLWOOD: Some of those species don’t actually even migrate anymore. They over winter here.
GELLERMAN: Why don’t they migrate here?
ELLWOOD: Well it’s warm enough where they don’t have to migrate any more. So the winters have become that hospitable to several species that they just don’t even bother going south.
[SOUNDS OF BIRDS, GEESE]
GELLERMAN: Sleepy Hollow Cemetery is near the historic center of Concord, Massachusetts. Overhead, a flock of geese flies in V formation. Here on Author’s Ridge, some of the giants of American 19th century literature are buried. Louisa May Alcott, Nathaniel Hawthorne, Ralph Waldo Emerson and Henry David Thoreau.
THOREAU (VOICED): Time is but the stream I go a-fishing in. I drink at it; but while I drink, I see the sandy bottom and detect how shallow it is. Its thin current slides away, but eternity remains.
GELLERMAN: Henry David Thoreau died May 6th 1862, precisely 150 years ago. He was 44. His tombstone reads simply: Henry.
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