Wednesday, April 26, 2017

Spring Warming Trends

Break-up season is about to get under way across interior Alaska, as seen in the following map from NOAA's Alaska-Pacific River Forecast Center; there is considerable open water on the upper reaches of the Tanana River, and the lower Chena River appears to be mostly open too.  This is supported by a webcam image from Fairbanks showing water pooling on or flowing over the ice.  The ice at Nenana is also deteriorating.





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.

9 comments:

  1. First a question: how is "green-up" defined? Is it when leaves first start to appear, or when they are fully out? There is often a several day period (or longer if they start to appear and then a cold snap sets in) of greening.

    I think what you are doing here is very relevant analysis, because temperature changes of a few degrees here or there are arbitrary to most observers, whereas visible changes in the environment like ice-out and green-up are memorable and create good benchmarks for public understanding.

    Does your theory suggest that there are other forcings at work that cause green-up and break-up, or rather that the temperatures we see at observational sites are slightly inaccurate?

    ReplyDelete
  2. My understanding is that Fairbanks "green-up" occurs when Chena Ridge first has a distinct green tint, as judged by a consensus of observers at NWS/UAF. I've never experienced it, but I believe the date is usually quite unambiguous.

    My approach assumes that temperature is the only forcing with a long-term trend, so this might not be correct, but given that other forcings are relatively minor, the influence of their trends should be small.

    I think I've come up with a better way of backing out the temperature trend from the break-up/green-up dates, using statistical simulation, so I'll post on that soon.

    Thanks for the encouraging words.

    ReplyDelete
  3. If you want to see green-up in Fairbanks keep a daily eye on these trees. Hit the + option to zoom in. The early branch buds that'll soon become leaves are plainly visible:

    http://old.co.fairbanks.ak.us/airquality/CRCurrentPhoto.jpg

    It's south of town obviously and located on a hill overlooking Farmer's Loop Road. Not sure of the elevation but higher than the valley floor and comparable to Chena Ridge. The airport is center right.

    Gary

    ReplyDelete
    Replies
    1. Meant to say north of town looking south.

      Gary

      Delete
    2. There are several searchable recent studies of variable plant phenology during spring. Most I've briefly reviewed infer it's temperature, humidity, and elevation related.

      Do the temperatures and elevation during spring of historical weather observing stations correlate with the temperatures and elevation of the sites used to determine green-up?

      Gary

      Delete
    3. And finally...here's a link to another webcam that views some trees on the valley floor. Compare the coming green-up schedule with the previous link north of town to note any difference in timing.

      http://webcam.morristhompsoncenter.org/axis-cgi/mjpg/video.cgi?resolution=630x430&camera=1&dummy=1488400656815

      Gary

      Delete
    4. At valley floor in Fairbanks the green tips of local aspen and birch tree leaf buds emerged yesterday May 1. More spring on the way.

      Gary

      Delete
  4. Nature abhors straight lines. I think you should break up the timeseries trends at around 1990. Both green-up and break-up look to have earlier times till 1990. Then the trends reverse or remain flat and slowly lead to later times. This could be seen better if you color-coded the scatterplot by year to see drifts.

    This will in turn affect the theoretical correlations. How exactly, I'm not sure. But this might explain the discrepancy between your simulation and actual temperatures. Besides, I would question the model results before the thermometers.

    ReplyDelete
    Replies
    1. Thanks for the comment Eric. You may be right that there are better ways to analyze the trends - surely there are many possible options - but I would argue there's nothing wrong with a single linear trend for a first-order look at how much things have changed.

      I've come up with (I think) a much better approach to the problem, and I'll post on it soon.

      Delete