Record warm temperatures in 2010 and 2012 resulted in the earliest spring flowering in the eastern United States in more than 150 years, researchers at Harvard University, Boston University and the University of Wisconsin have found.

"We're seeing plants that are now flowering on average over three weeks earlier than when they were first observed - and some species are flowering as much as six weeks earlier," said Charles Davis, a Harvard Professor of Organismic and Evolutionary Biology and the study's senior author. "Spring is arriving much earlier today than it has in the past."

To explain spring's early arrival, Davis and his co-authors, Boston University biology Professor Richard Primack, BU postdoctoral researcher Elizabeth Ellwood and Professor Stanley Temple at the University of Wisconsin, point to temperature increases resulting from global climate change.

Using data collected in Massachusetts and Wisconsin from the mid-1800s to the present day, they show that the two warmest years on record - 2010 and 2012 - also featured record breaking early spring flowering.

Significantly, researchers found that the early arrival of spring was predicted by historical records, and that plants haven't shown any sign of reaching a threshold for adjusting to warming temperatures.

"It appears that many spring plants keep pushing things earlier and earlier", Davis said.

To conduct the study, Davis and colleagues relied on two "incredibly unique" data sets.

"The data were initiated by Henry David Thoreau in the mid-1800s," Davis said. "He was making observations on flowering times across Concord, Massachusetts for nearly a decade. In central Wisconsin, the data were collected by environmental pioneer Aldo Leopold beginning in the mid-1930s.

"The striking finding is that we see the same pattern in Wisconsin as we see in Massachusetts," Davis said. "It's amazing that these areas are so far apart and yet we're seeing the same things-it speaks to a larger phenomenon taking place in the eastern United States."

"Thoreau and Leopold are icons of the American environmental movement and it is astonishing that the records both kept decades ago can be used today to demonstrate the impacts of climate change on plant flowering times," Primack said.

While it's clear that continued monitoring of flowering times is needed, Davis also expressed hope that the study provides a tangible example of the potential consequences of climate change.

"The problem of climate change is so massive, the temptation is for people to tune out," Davis said. "But I think being aware that this is indeed happening is one step in the right direction of good stewardship of our planet." Davis continued.

"When we talk about future climate change, it can be difficult to grasp. Humans may weather these changes reasonably well in the short-term, but many organisms in the tree of life will not fare nearly as well."