Warming Walden

Spring Ahead

Richard Primack, conservation biologist at Boston University, says, "The result of our analysis shows that plants are now flowering one week earlier than in Thoreau's time."

Like Thoreau, Primack makes repeated visits to Walden Pond. He says, "We (are) comparing when plants were first in flower in Thoreau's time and then seeing when they were flowering…today."

While Thoreau's records are the oldest regarding plant flowering times in the area, they are not the only records. Primack notes how in the late 1800s, local shopkeeper and amateur botanist Alfred Hosmer looked at the first flowering time of more than 500 plant species.

Primack, who writes textbooks in the field of conservation biology, noticed that the examples he was using for his books, in his words, "seemed to be from very far away." That led him to start searching for local evidence of climate change.

Primack says the researchers started looking for old records, "So we started going to all different kinds of small scientific gatherings that we have in the Boston area." They soon realized that Thoreau had keep records and, says Primack, "We realized that this was probably the most outstanding set of records that we could find. "

Primack's research is also showing a change in the kind of plants found near Concord from those found in Thoreau's time. He says they realized that "about one quarter of the plants that Thoreau had observed… were no longer present in Concord." Working with a group of Harvard researchers, including Professor Charles Davis, Primack says they found that "species that had very flexible flowering times and presumably very flexible times for leafing out and starting their life cycle earlier, were the species which were tending to persist in Concord."

He adds that shows that "the effects of warming temperatures are already having an impact on the flora of Concord."

Primack says people who keep records of flowering times and bird arrivals where they live can, after a few years, also spot evidence of climate change causing spring to arrive earlier. He notes that a woman now living near Boston has for several decades tracked when the birds arrive at her feeders, and that some species that migrate from the mid-Atlantic states are arriving "sometimes as much as several weeks earlier." He adds there will be temperature changes everywhere in the United States, and that, "This is something that everybody will be able to see…if they start keeping diaries now."

These differences don't happen every year, which is why a diary yields a better long-term record than memory. It also illustrates the confusion that some people have between weather and climate. Primack explains, "Weather is really the day-to-day variation that we experience in terms of temperature and rainfall, snow, dry conditions; and climate is really the long-term averages of these things." He adds, there were warm years and cold years in Thoreau's time, just the same way there are warmer years and colder years today. But, it's just we have much more of these warmer years now than there were during Thoreau's time.

This research appeared the Proceedings of the National Academy of Science, online for the week of October 27, 2008, and was funded by the National Science Foundation, with additional support from a Guggenheim Foundation Fellowship and a fellowship from the Arnold Arboretum of Harvard University.

Elsewhere on the Web:

Walden Pond State Reservation

The Thoreau Reader

USA National Phenology Network