The journals of Henry David Thoreau help scientists in New England investigate global warming's effect on the timing of spring. Thoreau carefully documented the dates the blueberry bushes bloomed.
But even stronger evidence is a photo from May 30, 1868, of a cemetery in Lowell, Mass., that researchers were given as they tracked Thoreau's footsteps, visited area cemeteries and dug into historic records.
In 1868 - and it was not the coldest year on record in those days - the trees were barren. In the photos, nothing is growing in the harsh New England spring.
On the same date in 2005, Boston University biology professor Richard Primack took a picture of the same trees in the same place, using unusual limb shapes for verification. In this photo, everything is in bloom.
"What's amazing about the photo is that it puts it in visual terms," said biologist Abe Miller-Rushing, who worked with Primack. "You can tell someone that spring is coming earlier than it did in Thoreau's day, but to see it puts it in a whole other world."
Primack and Miller-Rushing found that compared to Thoreau's day, Walden Pond's blueberry bushes are blooming more than a week earlier.
Some of the best timing records in the nation are in Massachusetts and they show plants coming about seven to 10 days earlier in general than a century ago, Primack said. Birds, on the other hand, are arriving only a couple days earlier.
Part of the reason plants are budding earlier in the Boston region is the urban heat island effect. But even rural areas of the East are seeing changes.
In Ithaca, N.Y., scientists have seen earlier birds and plant blooms. Wine grapes in Fredonia, N.Y., are hitting mid-bloom about 1 1/2 days earlier each decade. And apple trees in Poughkeepsie and other New York towns like Geneva and Peru are blooming about two days earlier each decade.
Lilacs in Vermont have been leafing three days earlier each decade.
And up the Connecticut, Farmington and Penobscot Rivers, salmon are migrating earlier - by about 6 1/2 days compared to a decade ago. In Maine, the migration is eight days earlier.