We know this because we have meticulous records of Walden's plant species and birds, taken by none other than Henry David Thoreau. On nearly every spring morning from 1851 to 1858, the transcendentalist writer explored the woods around the pond, noting the first seasonal blooms of 465 species of flowers.
Since then, other naturalists have revisited the area to maintain and expand on Thoreau's record. Their collective efforts have formed a detailed, long-term study on how the timing of biological events for a given area has changed over the past century and a half.
The changes are striking. Writing in the journal of the Proceedings of the National Academy of Science about their own five-year survey of Walden, Boston University scientists have observed that species are now flowering an average of seven days earlier than in Thoreau's time. Some are flowering three weeks earlier. But many aren't adjusting at all.
When species that depend on each other are unable to shift simultaneously, the whole system gets thrown out of whack. Insects arrive earlier, but starve because the plants they depend on for food have not bloomed yet. Migratory birds arrive to find that the insects that they depend on have starved. And those plants whose flowering time is unaffected by temperature – in Walden they are asters, buttercups, dogwoods, lilies, orchids, roses, saxifrages, and violets, to name a few – do not get pollinated by the insects and birds.
"Climate change is throwing off the synchronicity of nature," one of the authors told the Boston Globe. Since Thoreau's day, Walden's average annual temperature has increased by 4.3 degrees, partly because of the growth of heat-absorbing roads, parking lots, and buildings in the area, but also because of global warming.
The authors found that flowers that can adapt to the temperature by blooming early are flourishing, while those that adhere to a more rigid schedule are dying out.
"For the first time, it shows that climate change is not impacting these plants in a uniform or random way," evolutionary biologist Charles Davis, one of the study's authors, told Discovery Magazine. "It is major branches in the tree of life that are being lost. It happens to be the most charismatic plants – groups that we all know and love: the dogwoods, the orchids, members of the lily family, members of the rose family."
Few areas have been studied more closely over such a long time than Walden, particularly in North America. Wired's Alexis Madrigal notes that, in Europe, weather services monitor things like first blooms and other seasonal changes of living things, but that American weather services do not. But some biologists are hoping to improve Americans' phenological knowledge. Project BudBurst has enlisted thousands of backyard Thoreaus to record when plants in their gardens bloom and add it to an ever growing database. Madrigal writes:
Their data could not only benefit scientists of the present and future, but could aid in providing Americans with direct evidence of climate change, helping to create the political will necessary to address the nation's greenhouse gas emissions.
I'm skeptical that adding flowering times to the already existing mountains of evidence will convince the roughly half of the US population that does not accept the scientific basis of global warming to change their beliefs. As Thoreau himself wrote in "A Week on the Concord and Merrimack Rivers":
You can hardly convince a man of an error in a lifetime, but must content yourself with the reflection that the progress of science is slow. If he is not convinced, his grandchildren may be.
But let's hope that, for once, the gentle Concord poet was wrong. Waiting for two generations hence to solve the climate crisis will doom more than just Walden's flowers.