Guest essay by David Archibald
This post drew attention to the similarity between the recent warm decades and the period leading up to the extremely cold year of 1740. Now let’s investigate how a 1740-type event might play out. This graph shows the average of the monthly temperatures for the years 1736 to 1739 plotted with the monthly temperatures of the year 1740:
With respect to growing conditions, the 1740 season was a month later than the average of the previous five years and the peak months of the season were 2.5°C cooler. To get a perspective on how a repeat of 1740 might affect growing conditions in the Corn Belt, Bill Fordham, advising the grain industry in the Midwest, has kindly provided an update on the current season:
“So far here in the center of the Midwest, the 2013 growing season is almost identical to 2009 in regards to Growing Degree Days (GDD).
In 2009 48% of the corn was planted by May 12 and 62% was planted by May 19.
In 2013 18% of the corn was planted by May 12 and 71% was planted by May 19.
In 2009, we never received a killing frost until November 5 when the low was at 28F. The Midwest had a huge crop that was wet and light test weight, but never got killed by a frost. In 2009, the total GDD accumulation from May 15 thru September 30 was 2,530 GDD.
The bulk of the corn planted in the Midwest ranges from 2,300 to 2,700 GDD (based on Fahrenheit). With the volcanoes that have been erupting in Alaska and Russia, especially with Mt Sheveluch erupting to 7.4 miles on June 26, I will be surprised if we get through the month of September in 2013 without an early killing frost. If the heat dome and high pressure ridge stays centered in the west and over Alaska until Labor Day, the clockwise rotation will pump the cold air south over the Midwest along with the ash. There are millions of acres at risk in IA and MN, that are 2-3 weeks behind normal.
After silking, it takes 24-28 days to reach the Dough Stage when kernel moisture is about 70% and about 50% of the total dry matter has accumulated in the kernel.
After silking, it takes 35-42 days to reach the Dent Stage when kernel moisture is about 55% and about 70% of the total dry matter has accumulated in the kernel.
It takes about 55-65 days after silking for a corn plant to mature and for the kernel to reach black layer, normally at 30-35% moisture.
A killing frost, <30F, will do damage whenever it occurs before black layer, the earlier the frost, the more severe the damage. A hard killing frost <28F can reduce the yield up to 25%, or more depending on the variety, even a week before black layer.
In 1974 I experienced severe loss on some late planted corn when I got rained out on May 7 and didn’t get back in to finish planting for 3 weeks. The May 7 corn yielded 190 bushels per acre and the May 28 corn yielded 90 bushels per acre, same variety.”
Based on Bill Fordham’s experience of 1974, planting three weeks later reduced the crop yield by 50%. If the peak growth months of June, July and August are 2.5°C (4.5°F) cooler as per the CET record of 1740, that would reduce the GDD by 414.
A repeat of the climate of 1740, with a late planting and reduced heat in the three months prior to harvest can be expected to reduce crop yield by well more than 50%.