It is well known that break-up at Nenana has tended to occur earlier in recent decades, and of course this is directly related to the long-term warming trend. Warmer spring weather has also caused trees to leaf out somewhat earlier in Fairbanks; we know this because the annual dates of "green-up" have been recorded since 1974 by visual observation of the east-facing slopes of Chena Ridge. The chart below shows the green-up data along with the April-May mean temperature from Fairbanks airport. Note that the high degree of variance prevents the trends from being statistically significant despite last year having the earliest green-up and warmest April-May period on record.
The chart below shows a corresponding analysis for break-up at Nenana, except that here I've plotted the April-May mean temperature for the Southeast Interior climate division (which shows a slightly higher correlation with break-up than temperatures in Fairbanks). In this case the trends are marginally significant over this period of 43 years.
Here's a scatter plot of green-up and break-up dates versus April-May temperature, showing the rather high degree of correlation for both physical phenomena. Spring temperatures aren't the only influence on timing of green-up and break-up, but they do explain most of the year-to-year variation.
In view of the high correlations, it's interesting to consider what the green-up and break-up dates can tell us about long-term temperature trends. For example, if there was no long-term trend in green-up and break-up dates, then it would be difficult to believe that temperatures have risen, even if the thermometers have reported a warming trend; we might infer that urbanization trends or other factors were creating "artificial" warming in the temperature data. However, the fact that green-up and break-up dates have advanced supports the warming trend observed by the thermometers.
We can take this idea a step farther by examining what temperature trend would be needed to optimize the correlation between temperature and green-up or break-up dates. I performed this analysis by systematically applying small adjustments to the temperature trend and re-calculating the correlations each time; the results are shown below. Note that the correlation coefficients are around -0.85 or so, with small variations depending on how the temperature trend is adjusted.
The result of the analysis is rather fascinating: for both Fairbanks green-up dates and Nenana break-up dates, the correlation with temperature is optimized when the reported warming trend is increased by 0.2°F/decade. This reflects a change from +0.4 to +0.6 °F/decade in Fairbanks April-May temperatures and from +0.6 to +0.8 °F/decade in the Southeast Interior climate division. This appears to suggest that the actual rate of warming, which is reflected in the dates of green-up and break-up, has been higher than reported by the thermometers at this time of year.
I'll look at break-up dates from some other rivers around Alaska in a subsequent post. For now, I'd welcome comment from readers on the idea I'm proposing here: that physical date markers with long-term histories can be used to infer true temperature trends and thereby test whether historical weather data is adequately measuring the changes that have actually occurred.