The growing season is getting longer in Rexburg Idaho

Spring last frost dates are happening earlier in the year and Fall first hard frost dates are happening later in the year on average than they did about 40 years ago.  This indicates that we are experiencing a climate shift toward a more moderate climate in Rexburg.

I teach course on climate change, so I spent at least part of each day thinking about climate, climate change, and possible effects of that change.  I look at lots of data at the global scale, but I started wondering about what might be happening locally.  So, I started pondering on factors that might indicate whether a shift is occurring in our local climate.  I decided to look at is the dates of last frost in the Spring, and the first hard frost in the Fall.  What did I discover?  Between 1972 and 2011 the last Spring frost has shifted on average from 5 June to 17 May.  And, the date of the first hard frost (temps less than or equal to 28oF) has shifted on average from 20 September to 10 October.

How can I say this?  Let's take a look at the data.

First, though, it's important to understand the difference between climate and weather. Weather is defined as short-term atmospheric conditions or events - anything that lasts between 5-minutes (such as a rain shower) and a year or longer (such as a drought). Climate, on the other hand, is the long-term average of the weather in a particular location.  By long-term average, this typically means that (at least according to the National Snow and Ice Data Center), one needs at least  30 years worth of weather data before they can really say anything about the climate of an area.  Fortunately we have those kinds of data for Rexburg, Idaho, so it is possible to say something about climate trends here.

Lee Warnick, a faculty member at BYU-Idaho and weather spotter for the National Weather Service, kindly provided me with the raw data I analyzed for this posting.  As a disclaimer, the conclusions in this posting are my own and do not necessarily represent his.  

The data set starts in 1972 and extends through 2010-2011.  That's nearly 40 years of weather data.  FYI, the data came from the Ricks College/BYU-Idaho weather station for 1972-1996, Lee Warnick's personal weather station 1996-2009, and from the weather station at the Rexburg Municipal Airport 2009-present.   

The graph below shows the data for the annual Fall first hard frost 1972-2011 in Rexburg, Idaho.  First of all, you will notice that there is a LOT of variability in the data (this is normal for almost any natural system, including weather and climate, and this is why you need at least 30 years of data before you are likely to see meaningful trend).  The line through the data is the trend line of correlation between the date of the Fall first hard frost (temperatures less than or equal to 28oF) and the year.  If there was no change in the average date of the first frost we would expect to see a flat line in this kind of analysis.  The analysis, however, produced a sloped line with the average date of the Fall first hard frost happening later today than it did 40 years ago.  

In the early 1970s, the average date of the Fall first hard frost happened on around 20 September.  By contrast, by 2011 the average Fall first hard frost had shifted to about 10 October.  (For the statisticians out there, the correlation R-value is significant at an alpha level of 0.01.  This means that statistically, there is only 1% of uncertainty in this analysis.) 

When we look at the same data plotted in 10-year sets, we see that during 1972-1981 there were equal numbers of first frost dates in Sept and October, but when we look at 2002-2011 we see that only one first frost date appeared in Sept and the rest happened in October.  The conclusion?  First frost dates are happening later in the year in Rexburg than they used to.

When we look at the other end of the growing season, last frost in the Spring, we see the following:

The graph above reveals a trend showing that the last frost date is happening earlier in the year today than it did about 40 years ago.  In the early 1970s the average last frost date happened around 5 June.  By 2010 the average Spring last frost date had shifted to about 17 May.  (Again, for the statisticians out there, the correlation R-value between Spring last frost date and year is significant at alpha = 0.05.  In other words, there is less than 5% uncertainty in this analysis, so we can be 95% confident that this correlation represents an actual shift in average Spring last frost dates.)

When we look at the Spring last frost data grouped into 10-year periods the data show that the number of Spring last frost dates in June decline and the number in May tend to increase over time.

As did these analyses I started to wonder whether these observed shifts in first and last frost dates are tied to global climate change or if they were driven be something else, such as the sunspot cycle.

The sun goes through an 11-year sunspot cycle during which the intensity of solar radiation fluctuates.  When many sunspots are present the sun releases increased amounts of solar radiation, and when no sun spots are present the sun releases less radiation.

This graph, courtesy of NASA, shows the most recent few sunspot cycles (1945-2010).

I did statistical analyses to see if there are significant correlations between the first frost dates and the number of sunspots present, and between the last frost dates and the number of sunspots present.  There were no significant correlations for either comparison.  

If the sunspot cycle is driving the observed shift shown in the figures above, we would expect to see earlier last Spring frosts and later Fall first hard frosts when the sun has a lot of sunspots, and vice versa.   What the sunspot and frost data correlation analyses show, however, is that there is no significant relationship between these factors and that there is only a negligible relationship.  

The conclusion?  The sunspot cycle does not appear to drive the observed shift in Spring and Fall frost dates.  

Well then what's the take-home message from all of this?  The average dates of Spring last frost dates are happening earlier, and the Fall first hard frost dates are happening later than they did about 40 years ago.

It is important to note that this analysis reflects long-term climate trends and should probably not be used to predict actual first or last frost dates for a particular year - remember that there is still a lot of year to year variability in actual dates of Spring and Fall frost dates.  

My conclusion?  Based on these data, warming of the global climate appears to be driving the observed trends, and we are consequently experiencing a moderation of our local climate.

(Originally posted 10-31-2011)