How Henry David Thoreau Studied Climate Change (Without Realizing It)

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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.

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 lots of notes about what he saw. 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.

Today, biologists refer to Thoreau’s journals to study climate change.

How Thoreau Studied 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 wrote down the exact flowering dates of 500 species of plant, flower and shrubs.

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

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.

Man and Climate Change

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

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

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.

Thoreau studied climate change by noting that man and/or nature change — and by questioning  whether the earth was getting better or worse. On May 4, 1856, he wrote,

…the prudent settler avoids the banks of rivers, choosing high and open land. It suggests that man is not completely at one with Nature, or that she is not yet fitted to be his abode…Either nature may be changed or man. Some animals, as frogs and musquash, are fitted to live in the marsh. Only a portion of the earth is habitable by man. Is the earth improving or deteriorating in this respect? Does it require to be improved by the hands of man, or is man to live more naturally and so more safely?

Thoreau  Studied Earth Day 1855

On what would have been Earth Day during his 37th year (April 22, 1855), Thoreau wrote,

Fair, but windy. The yellow willow catkins pushing out begin to give the trees a misty, downy appearance, dimming them. The bluish band on the breast of the kingfisher leaves the pure white beneath in the form of a heart. The blossoms of the sweetgale are now on fire over the brooks, contorted like caterpillars.

Image of Fair Haven Bay By Thoreau's ghost – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=20413840. Flowering tree By Burim – Own work, CC BY-SA 3.0,, https://commons.wikimedia.org/w/index.php?curid=24781695, This story about how Thoreau studied climate change was updated in 2023.