Last week we introduced the Frost Days metric as one of the indices that climate scientists use to describe changing climatic conditions over long periods of time. In discussing Frost Days, we described how the number of Frost Days is directly related to the length of the Growing Season, another index of climate condition. This week, we’ll take a closer look at Growing Season Length (GSL) and see what Kari’s analyses indicates is happening with GSL over the long term.
GSL is a Duration Event Index which means that it is a measure of how long a particular type of event lasts in a given year. In contrast, Frost Days is a summation if discrete events throughout the year that meet a certain condition; in this case the number of days where the daily minimum temperature is less than 0 degrees C (32 degrees F). Other duration event indices may not be based on the length of a single event but instead on the number of events meeting or exceeding a given length in a single year. Examples include Cold Spell Duration and Warm Spell Duration where a “Spell” lasts 6 days or more.
GSL is essentially the number of days in a calendar year that plant growth can take place, specifically wheat. You calculate a year’s GSL by finding the first 6 consecutive days where the average temperature is greater than 5 degrees C (41 degrees F) and the first 6 consecutive days when the average temperature is less than 5 degrees C (41 degrees F). You then count up the days between these 2 occurrences. There are other factors that impact the productiveness of a growing season such as rainfall, day length, and frost days, but the GSL gives a good indication on how things are changing (if at all) over time.
Examining Kari’s GSL calculations for the Chesapeake region (Figure 1), it does appear that GSL has increased over the period 1900 to 2014. These calculations were done on data from 18 National Climatic Data Center (NCDC) stations located around Chesapeake Bay. The data were separated into Northern stations and Southern stations and then averaged (Figure 2) as shown by the blue and red lines on the graph. The thicker lines represent the rolling 21 year averages of the same data, which basically gives us a smoother line to look at while maintaining the up or down direction of the trend. Comparing the early 1900s to the early 2000s, the growing season has increased by about a month throughout the Chesapeake region (Figure 1). Interestingly, when you look at the northern and southern stations separately, the results are different with GSL increasing by 17 days in the north and 44 days in the south. As with the Frost Days index, the GSL index for Chesapeake Bay is telling us that the region is getting warmer.
There is the potential that farmers in the Chesapeake region could see some benefit from longer growing seasons by getting a larger yield from their crops. There is also the potential that agricultural pests (weeds, insects, and disease) from southern climates could start finding the Chesapeake region more hospitable. Hotter temperatures could have a negative impact on crop yields and a longer growing season coupled with hotter temperatures may mean that farmers will need to irrigate their crops more. This would put additional strain on groundwater reserves that are already depleted. And if that wasn’t bad enough, a longer growing season may also mean a longer allergy season!
More to come!