Henry David Thoreau Studies Climate Change (Without Realizing It)

Henry David Thoreau studied climate change unintentionally when he recorded in his journal the exact dates that plants, shrubs and trees blossomed around Concord, Mass.

Henry David Thoreau

Henry David Thoreau

Thoreau started his journal in 1837, taking notes about the flora and fauna he saw during his daily rambles. In 1851, he began an ambitious project to analyze the changing phenomenon of the seasons. For the next 10 years until his death, he took copious notes about his observations. He pressed flowers in a music book, saved specimens in his hat and used his walking stick as a measuring tape. He took the temperature of ponds and streams and watched migratory birds through his spyglass.

Climate Change

After New England’s harsh winters, Thoreau was especially eager for signs of spring. On April 3, 1853, he wrote,

It is a clear day with a cold westerly wind, the snow of yesterday being melted. When the sun shines unobstructedly the landscape is full of light, for it is reflected from the withered fawn-colored grass, as it cannot be from the green grass of summer. (On the back of the hill behind Gourgas’s.)

The bluebird carries the sky on his back.

On May 14, 1852, Thoreau made a list of the dates about 35 species of spring flowers appeared, from the male Acer rubrum on April 28 to a Cerasus Pennsylvanica ? on May 13. He chided himself for not observing closely enough the first common elm and the first red maple. Over a decade of such observation he recorded the exact flowering dates of 500 species of plant, flower and shrubs.

Highbush blueberry

Highbush blueberry

Richard Primack, biology professor at Boston University, pored over the journals and compared Thoreau’s observations a century and a half ago with his own. Primack and his colleagues found one-quarter of the wildflowers Thoreau observed no longer exist around Concord. Scientists consider another third rare. They blame  urban development, pollution and the increased population of people and deer.

Shadbush and Marsh Marigold

But the wildflowers still around Concord bloom earlier than they did in Thoreau’s day.  They generally flower seven days earlier than they did in 1861. Highbush blueberry blooms appear three weeks earlier than they did in the mid-19th century. The shadbush and the marsh marigold do too. Some, like birdfoot violet, rhodora and flowering dogwood, changed by one or two weeks. Some didn’t shift at all.

Marsh marigold

According to the Blue Hill Meteorological Observatory, the average April temperature in Concord is five degrees warmer than it was in Thoreau’s day. Primack blames climate change and the urban heat island that metropolitan Boston has become.

Primack continued to study how climate change affects some plants more than others. The journals of Henry David Thoreau make that possible.

In 2012, Primack wrote in the New York Times,

Despite their dramatic cumulative effects over the last 160 years, these changes would be largely imperceptible without the biological yardstick Thoreau’s records provide.

A Serious Injury

Thoreau, of course, provided more than biological yardsticks. He also offered poetic, even funny, observations about the interrelatedness of all nature.

Fairhaven Bay (Thoreau called it a pond)

On April 13, 1854, he noted in his journal,

The golden-brown tassels of the alder are very rich now. The poplar (tremuloides) by Miles’s Swamp has been out — the earliest catkins — maybe two or three days. On the evening of the 5th the body of a man was found in the river between Fair Haven Pond and Lee’s, much wasted. How these events disturb our associations and tarnish the landscape! It is a serious injury done to a stream. One or two crowfoots on Lee’s Cliff, fully out, surprise me like a flame bursting from the russet ground.

This story was updated in 2020.

Image of Fair Haven Bay By Thoreau's ghost – Own work, CC BY-SA 3.0,


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