We just wrapped up summer without any long term periods of temperatures over 100 degrees. Yes, we had some 3-4 day periods. Now it looks like September is going to be cooler than average. So, the question is how much cooling have we had?
One method is to calculate the Growing Degree Days. What are the growing degree days?
Growing degree days (GDD) are a measure of heat accumulation used by horticulturists, gardeners, and farmers to predict plant and animal development rates such as the date that a flower will bloom, an insect will emerge from dormancy, or a crop will reach maturity.
In the absence of extreme conditions such as unseasonal drought or disease, plants grow in a cumulative stepwise manner which is strongly influenced by the ambient temperature. In other words, GDD values provide a best case outlook as to plants’ pace to maturity. (wikipedia)
The Ice Age Farmer has developed an interactive method for calculating the GDD for every zip code in the US, comparing the difference from last year (2018). The link is HERE.
GDD has decreased in Nevada City, CA to 78.44% of previous value (-21.56% drop) in 95959.
GDD has decreased in Grass Valley, CA to 81.16% of previous value (-18.84% drop) in 95945.
The greatest impact is in the grain, soybean, and corn growing belt in the Northern Hemisphere. Here is a quick look at some Iowa Counties:
Two California grape-growing counties:
A long term reduction in GDD could result in crop failures and the expansion of global hunger. In this video the Ice Age Farmer discusses the impact of early fall frosts.