That's what scientists have been doing.
From a piece in the New York Times...
Primack and his colleagues have used this journal to follow Thoreau’s trails. Many of the species Thoreau saw have disappeared from the Concord area, but by studying 32 spring-flowering native plants from a variety of habitats, the modern researchers have discovered that they are now flowering much earlier. On May 11, 1853, for example, Thoreau noted the blooming of the highbush blueberry (Vaccinium corymbosum), now the most widely grown blueberry for commercial use in North America and, with its distinctive white, dangling, bell-shaped blossoms, an easily identifiable plant. If Thoreau were to search for it today in mid-May, he’d be out of luck, since it now flowers during the last two weeks of April. After the very warm winter of 2011-12, he would have missed it by a good six weeks. Last spring, its appearance in Concord was recorded on the first day of April.
Matching Thoreau’s lists with temperature records kept for over a century by the Blue Hill Meteorological Observatory in Milton, Mass., Primack and his associates have determined that plants in Concord are reacting to warming temperatures by flowering roughly two days earlier for each degree increase in temperature. In Thoreau’s time, the average spring temperature was 42 degrees and the average date of first flowering of the 32 species in the study was May 15. For the years 2004-12, it has changed by 11 days (to May 4) and by 6 degrees (to 48 degrees).