Fairbanks climate data since 1930 shows the expansion of the growing season at both ends:
The trend is somewhat greater for the first freeze of autumn (1.8 days later per decade) than for the last freeze of spring (1.2 days earlier per decade). Combined, the growing season (based on this 32°F metric) has lengthened by 3.0 days per decade, or nearly a full month since 1930. The last time the growing season was 100 days or less was in 1996, but such a short growing season used to be fairly common.
Localized urban warming does not explain the trend at the official climate site (the airport since 1951, Fort Wainwright in the 1940s, downtown previously), as the data from the university farm paint a similar picture - see below. The farm is at the far western end of campus, in a relatively open and undeveloped location, and has always been far more prone to early and late freezes.
Based on the 32°F threshold, the trends from 1930-present are actually greater at the farm, with the growing season lengthening by 4.2 days per decade. The official climate site has never reported a freeze in July, but it occurred in some of the earlier years at the farm.
For the full history since 1905 at the farm, the trends are 1.5 days per decade later for first freeze, and 1.6 days per decade earlier for last freeze. Note that I used July 15 as the cut-off between "late" versus "early" freeze.
The annual total of growing degree days (accumulated "heat units" above 50°F) has risen tremendously in Fairbanks, with the linear trend amounting to 5% per decade, or about a 60% increase from the 1930 baseline (change from ~750 to ~1200). The chart below shows that a very noticeable jump occurred in the late 1960s, and it's also striking to see that sub-1000 GDD seasons have all but disappeared in recent years.
This increased frost free period has had a substantial effect on the tree diversity; with much more aspen and birch prevalent than in earlier times.. The same can be said for most of the taiga
ReplyDeleteVery interesting, thanks
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