LESSON 2b: CLIMATE EFFECTS ON CROPS

20 YEAR CYCLE (18.6 YEAR)

A second influence on Midwest climate is the 20-year cycle. This cycle can be seen in drought assesments of the 19^th Century. Tree rings give an indication of periods of significant water stress and periods of ample water in the Midwest. Fig. 2.17 illustrates the tree ring analysis showing moist periods for the western two-thirds of the United States. On this chart where there is no mark, the trees are growing well. Favorable episodes seem to develop periodically. The higher the line on this chart, the greater the number of sites that are showing stress influencing tree growth.

Fig. 2.17 Number of stations reporting drought-stressed trees (1800-1900).

Favorable growth periods come along at rather regular intervals rather than at random. Looking at that pattern, we recognize that there is some periodic aspect to the occurrence of stressful and ideal weather in the western two-thirds of the USA.

The years for this tree ring analysis (Fig. 2.17) are from 1800-1900. The 1800-1900 period has been used because in 1885 someone drew a chart showing crop prices before 1885 and anticipated prices to the year 2000 (Fig. 2.18). Grain was inexpensive from 1812 until about 1820. Then there were two drought events during a 6-year period, which increased the price of grain. During the next 12 years, there was just one drought period, occurring about 1834. Grain prices began to diminish, reaching a low point in 1839. Hence, in 6 years there had been two droughts, and in the following 12 years there was one. The pattern repeated (Fig. 2.18).

Fig. 2.18 Periodic return of drought years and consequent economic hardships.
To view an enlargment, click on the image.

Three droughts in an 18- or 19-year period is the pattern typical for the Midwest. If we look at the years when crop yield has been reduced by greater than 10% (that is, falls to 10% below the trend line or lower), we would consider that to be a drought year (assuming that most of the low yields are due to drought and not to something else).

There have been a couple of times in Iowa, 1915 and 1993, when low yield was due to too much water. We have some slightly low years, lower than would have been expected (1970 and 1974), when the yield was reduced by crop disease and frost conditions, respectively. There can be other factors. But basically three yield-reducing droughts occur in an 18- or 19-year period where the distribution of them is six years with two droughts, and twelve or thirteen years with only one.

From 1800-1885, the pattern was historical and represents actual data (Fig. 2.17). The pattern seemed regular, apparently, to the person(s) who drew this chart back in 1885. They extrapolated the chart, making a forecast into the 21st century, saying, "If it just keeps doing what it has done, we will know when the high and low yield years occur." This chart became very famous because it anticipated two droughts in a 6-year period somewhere around 1934. In 1885, when this chart was made, they had anticipated that two of these severe droughts would be in that 6-year period that included 1934. Then after 1937, there would be 12 years with only one drought. The next droughty time would be in the mid-'50s, the mid-'70s, and around 1988. Thus they have occurred. It indicates: "Once you get past 1993, you will be in the 12-year period of time when there will likely be only one drought during 12 years," which is the time period that we are in now.

That cycle is known as the 18.6-year cycle. It has become quite well known in the Midwest and quite well known around the world. This 18.6-year cycle exists in the center of the large land masses, at least of the temperate continents. It is true in Africa, in South America, on the North American continent, and on the Asian continent. It even appears in the middle of the Australian continent, though that might be marginal to say you could get into the middle of the land mass. Still this 18.6-year cycle shows up.

I found evidence of this cycle in Russia. I first noticed it from the record of the floods. The flood periods on the River Neva at St. Petersburg are marked on the walls of the Peter and Paul Fortress. So, when I found that history, I really had the clue that this cycle does apply everywhere, and it is real. There is some distribution of risk over the years. I think that this cycle is probably the third most important weather factor influencing our long-term crop yield.

Fig. 2.19 Outside wall of Peter and Paul fortress

Fig. 2.20 Inside marks of floods

When you look at this chart, the years of very high yield and low prices are given as exact years. Sometimes yield may be off one year, either way, with the high price off one either way. That is because the years when droughts start seem to be overridden by a more important weather factor. The more important weather factor is an El Niño event.