Modern scientists trying to understand climate change are engaged in an unlikely collaboration — with two beloved but long-dead nature writers: Henry David Thoreau and Aldo Leopold.

The authors of Walden and A Sand County Almanac and last spring's bizarrely warm weather have helped today's scientists understand that the first flowers of spring can continue to bloom earlier, as temperatures rise to unprecedented levels.

"If you take the old historical records of Thoreau and Aldo Leopold and use those to sort of predict when plants will be flowering in an astonishingly warm year like we had in 2012, the flowering time of plants is exactly what you would predict using this historical data," says Boston University biology professor Richard Primack.

In Concord, Mass., the pattern they found was that for each degree Celsius rise in mean spring temperature, plants bloomed 3.2 days earlier. In Wisconsin, it was 4.1 days earlier for each degree rise.

Primack, together with co-authors in Massachusetts and in Wisconsin, published their combined findings this week in the journal PLOS One.

Their research also shows that record temperatures last spring resulted in the earliest known flowering times for dozens of plants.

In recent years, Primack has been taking a cue from Thoreau to try to understand how global warming is changing the natural world.

Thoreau passionately chronicled the coming of spring around Walden Pond and elsewhere in Concord, Mass., in the mid-19th century.

"So what Thoreau would do is he would go out for walks almost every day for about four hours, and he would record in the spring when he saw the first open flower of a particular species," Primack says.

Thoreau recorded in his journals when hundreds of different flowers opened for the first time, every spring.

"I often visited a particular plant four or five miles distant, half a dozen times within a fortnight, that I might know exactly when it opened," Thoreau wrote about his nature journals.

Over the past several years, Primack's team has documented when these same flowers bloom. They learned that in an average year, flowers will bloom about 11 days earlier than in Thoreau's time.

Then something happened last year that let the scientists glance into the future: Spring temperatures were off the charts.

"In late December 2011 and January 2012, I started seeing irises blooming in the Boston area," says Charles Davis, a Harvard professor of evolutionary biology, who collaborates with Primack. "And you know this is the dead of winter, and you can imagine it sort of rocked my world."

This freakish weather made the scientists wonder: Is it possible to use historical records to predict when flowers will bloom during especially hot years?