June 6, 1912 brought the volcanic eruption of Novarupta (Katmai) in Alaska, the largest volcanic eruption of the 20th century.

The gases and particulates released from this eruption circulated across the Northern Hemisphere and suppressed solar radiation, reducing surface air temperatures across portions of Europe and North American for a period of 16 months. The 1913 growing season was cooler than normal in many areas, and farmers did not produce a good crop.

June 12, 1991 brought the second-largest terrestrial volcanic eruption of the 20th century, when Mount Pinatubo in the Philippines blew up and ejected tens of millions of tons of material into the atmosphere. It was speculated at the time that the gases and particulates ejected by this eruption would have atmospheric effects similar to those of Novarupta earlier in the century. However, few anticipated the widespread and long-term climate fingerprint. Satellite measurements picked up aerosols and particulates from this eruption and were able to track the plumes as they encircled the Northern Hemisphere.

The impact on the atmosphere in terms of diminishing solar radiation was evident in many areas of the Northern Hemisphere for up to two years. Later studies by climate scientists showed that significant reductions in surface temperature values were measured during both 1992 and 1993 summers, especially in the mid- to high-latitude positions of North America.

One of the climate consequences for Minnesota was a very cold summer in 1992. The growing season produced the fewest growing degree days of any season since 1915, and some of the state’s corn crop was damaged by frost on the summer solstice, June 21. This had never happened before in history — a damaging frost on the longest day of the year!

However, the most amplified climate effect in Minnesota occurred in July 1992, with colder temperatures than anyone had ever experienced during that month. It was nearly 2 degrees F colder than the next-coldest July — 1915 — and what climatologist call a “statistical singularity.”

Temperatures averaged from 6 degrees F to 9 degrees F below normal for July. The temperature did not reach 90 degrees F anywhere in the state, a rare occurrence. In fact, many climate observers reported less than 10 days that reached 80 degrees F. Portions of the Red River Valley reported morning lows in the upper 30s on July 29, setting new daily records for the date. The growing season produced late-maturing crops and yields below trend. The statewide corn yield was 114 bushels per acre.

The Mount Pinatubo climate effects carried over into 1993, producing another cooler-than-normal growing season, but not as extreme in July. This time, the cooler-than-normal temperatures combined with an extraordinarily wet spring and summer as well. Statewide corn yields fell off to just 70 bushels per acre in 1993, the lowest of the modern era.

The bottom line to remember is that large-scale volcanic eruptions elsewhere in the world can certainly have significant effects on our growing-season climatic patterns. They always have and they always will.

Seeley is an Extension climatologist with the University of Minnesota.