During the exceptionally warm springs of 2010 and 2012, plants bloomed earlier in the eastern U.S. than they have since the American writer and naturalist Henry David Thoreau started keeping records near Walden Pond in 1852.

Many plants now flower several weeks earlier than they did in the 19th century, a response linked to increasingly warmer springs due to global climate change.

For example, in Massachusetts, plants are flowering 20 days earlier now than they were during Thoreau’s time. In Wisconsin, where data on flowering dates was recorded by environmentalist and writer Aldo Leopold in the 1930s, flowering dates are, on average, 24 days earlier.