The man considered the first environmentalist is now helping scientists track climate change. (Image by Michael Allen Smith.)

Fittingly for a man seen as the first environmentalist, Henry David Thoreau, who described his isolated life in 1840s Massachusetts in the classic of American literature Walden, is now helping scientists pin down the impacts of climate change.

The American author, who died in 1862, is best known for his account of the two years he spent living in a one-room wooden cabin near Walden Pond "because I wished to live deliberately, to front only the essential facts of life." Packed with descriptions of the natural world he loved, Walden is partly autobiographical, partly a manifesto for Thoreau's belief in the rightness of living close to nature. "I never found the companion that was so companionable as solitude," he writes. "Simplify, simplify."

But Thoreau was also a naturalist, and he meticulously observed the first flowering dates for over 500 species of wildflowers in Concord, Mass., between 1851 and 1858, recording them in a set of tables. When Richard Primack, a biology professor at Boston University, and fellow researcher Abraham Miller-Rushing discovered Thoreau's unpublished records, they immediately realized how useful they would be for pinning down the impact of the changing climate over the last century and a half. The timing of seasonal events such as flowering dates is known as phenology, and the phenologies of plants in a temperate climate such as that of Massachusetts are very sensitive to temperature, say the scientists. Studying phenology is therefore a good indicator of ecological responses to climate change.