This is a busy time of year for Richard B. Primack, a biologist at Boston University. He and his colleagues survey the plants growing around Concord, Massachusetts, recording the first day they send up flowers and leaves.
Compared to the last five springs, things are pretty slow around Concord, in large part because of the relatively cold winter and chilly March.
But Primack wouldn’t call this a late spring. “It’s just much later compared to our recent memories of spring,” he said.
Primack knows this because of Henry David Thoreau. During the 1850s, Thoreau carefully recorded the arrival of spring at Walden Pond, one of Concord’s most famous sites. Primack has combined Thoreau’s data with his own and those of other naturalists to create a record of the seasons stretching across 160 years.
As Primack writes in his new book Walden Warming, spring has started earlier and earlier over the decades. It now arrives about three weeks sooner than in Thoreau’s time.
The pattern Primack sees at Walden Pond is part of a grand, planetwide march. Many studies – based both on observations in the field and on satellite images taken from space – indicate that spring is shifting earlier.
The changing spring is one of the most striking impacts attributed to global warming. But in hopeful and troubling ways, new studies are showing that warming’s effects are broader, affecting plants from spring to fall.
Amy M. Iler, a postdoctoral researcher at the University of Maryland, and her colleagues recently exposed some of that complexity in a mountain valley in Colorado, where they and other scientists have been monitoring flowers since 1974.
Recently, they analyzed records for 121 species to look for significant trends in their growth. They published their study last month in The Proceedings of the National Academy of Sciences.
Overall, the flowers have followed the same trend seen around the world. The first flowering in the valley now comes 25 days earlier than in 1974.
In the coming decades, carbon-dioxide levels are expected to rise still more, driving up the planet’s average temperature. Spring probably will continue to advance, and some plants will grow longer into autumn.
Iler worries that the changing flowering times of plants may disrupt their pollination. Some species may end up competing with one another for visits from pollinating bees.
“It’s reshuffling the community,” she said.
