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Vermont Climate Change Indicators

This paper explores how climate change has affected Vermont in recent decades using long-term datasets: specifically the impact on freeze dates, the length of the growing season, the frozen duration of its small lakes, and the onset of spring - as shown by the leaf and bloom of lilacs.

The freeze period in Vermont has got shorter, and the growing season for frost-sensitive plants has got longer by about 3.7 (±1.1) days per decade; as the last spring freeze has come earlier by 2.3 (±0.7) days per decade and the first autumn freeze has come later by 1.5 (±0.8) days per decade. So in the past forty years, the growing season for frost-sensitive plants has increased by 2 weeks. (See Figure 1.)

The frozen duration for small lakes, for which freeze-up and ice-out depend on mean temperatures over longer periods has been changing much faster. Over the past forty years, ice-out has got earlier by 2.9 (±1.0) days per decade and freeze-up has occurred later by 3.9 (±1.1) days per decade, so that the frozen duration in winter has decreased by 6.9 (±1.5) days per decade. As our northern climate has warmed substantially in fall, winter and spring, Stile’s Pond is frozen for 4 weeks less on average than forty years ago. (See Figure 2.)

The first leaf of Vermont lilacs, an indicator of early spring, is closely correlated with the ice-out of our two small reference lakes. Lilac first leaf has also been coming earlier by 2.9 (±0.8) days per decade, while lilac first bloom has advanced more slowly by 1.6 (±0.6) days per decade.

The means that in the past forty years, the growing season for frost-sensitive plants has increased by 2 weeks; and for frost-hardy plants the growing season may have increased by as much as three to four weeks.

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