Note: the original title Solar Neutrons and the 1970s cooling period was unintentionally misleading as Dr. Svalgaard points out in comments:

What produces Solar Neutrons?
the title of the post is misleading. The cosmic rays are protons, not neutrons, and are not produced by the Sun, but by supernovae in the Galaxy. The ‘neutrons’ are produced in the Earth’s atmosphere when cosmic ray protons collide with air. Neutron Monitors can detect those ‘secondary’ neutrons.

I meant to convey the modulation effect of the sun’s magnetic field on cosmic rays, and hence neutrons. So I’ve truncated the title to: Neutrons and the 1970s cooling period – Anthony

Guest post by David Archibald

The world’s most eminent climatologist was Professor Hubert Lamb, who founded the Climate Research Unit at the University of East Anglia. Professor Lamb was guided by the principle that if a climatologist is to project future climates, he must understand what has happened in the past. In that vein, to understand the cool period coming post solar maximum of Solar Cycle 24, it is apposite to examine the last period of cooling that the Earth experienced. This was the 1970s cooling period. The CIA report on climate written in August, 1974, A Study of Climatological Research as it Pertains to Intelligence Problems, summarised it in these terms:

“Since the late 1960s, a number of foreboding climatic predictions have appeared in various climatic, meteorological and geological periodicals, consistently following one of two themes.

· A global climatic change was underway.

· This climatic change would create worldwide agricultural failures in the 1970s.

Most meteorologists argued that they could not find any justifications for these predictions. The climatologists who argued for the proposition could not provide definitive causal explanations for their hypothesis. Early in the 1970s a series of adverse climatic anomalies occurred:

• The world’s snow and ice cover had increased by at least 10 to 15 percent.
• In the eastern Canadian area of the Arctic Greenland (sic), below normal temperatures were recorded for 19 consecutive months. Nothing like this had happened in the last 100 years.
• The Moscow region suffered its worst drought in three to five hundred years.
• Drought occurred in Central America, the sub-Sahara, South Asia, China and Australia.
• Massive floods took place in the Midwestern United States.

Within a single year, adversity had visited almost every nation on the globe.”

There was a 1970s cooling period – the CIA left a record of it, and by some measures, the 1970s was the coldest decade of the 20^th Century. This is one of those measures:

This is Figure 3 from a paper by Suckling and Mitchell in 2000 which examined variation of the C/D climatic boundary under the Koppen climate classification system for the central United States during the 20^th Century (courtesy of Gail Combs).

The C/D boundary is the boundary between mild winters and cold winters. For the average of the 1970s, the C/D boundary was 200 km south of where it was for the rest of the century. Given that the Great Pacific Climate Shift of 1976 saw a sudden warming, analysis at a finer time resolution is likely to show a much larger move south for the first half of that decade.

What was the signature of the 1970s cooling period in the instrumental record? In terms of the changes in space weather that might have caused that cooling, what was different about the early 1970s was that the neutron count rose back to near-solar minimum levels relatively early in Solar Cycle 20:

If neutron count is a significant determinant of climate, what is happening now? That is shown in the following graph which inverts the neutron count and plots it against F10.7 flux:

F10.7 flux is preferred to sunspot number because it can’t be adjusted by the “sunspot fiddlers” amongst us. What this graph shows is that:

1. there is about a one year lag in neutron count from the F10.7 flux.

2. the divergence between the F10.7 flux and neutron count in the early 1970s.

It looks like F10.7 flux has peaked for Solar Cycle 24 and therefore the neutron count should start climbing again. The current count is not much higher than the pre-Solar Cycle 23 minima in the record.

The Ap index is currently 3.6 which is lower than the minimum monthly levels for pre-Solar Cycle 23 minima. For the last thirty years, the Ap index has been broadly tracking the F10.7 flux apart from the 1970s cooling period:

In the graph above, the Ap Index is shown as 11 month-smoothed. In the big picture, the Ap index did start rising from the mid-19^th Century at about the same time that the glaciers started retreating in 1859. In the early 1970s though, the Ap index had a significant departure from the F10.7 flux and the neutron count. If a higher Ap index is associated with warming, then countervailing effects were much stronger than the high Ap index in the 1970s.

Both the neutron count and Ap Index are now quite close to solar minimum levels in the modern instrumental record, suggesting that they will be particularly weak when the fall of Solar Cycle 24 begins. The question then will be how far south the Koppen C/D boundary will move and what will that do to the Corn Belt growing season? As this figure shows, the Corn Belt is a movable feast:

Meanwhile, the fall of Solar Cycle 24 is upon us. This graph following kindly provided by Mike Williamson show the rise of solar cycles 18 to 24 from the month of minimum. Solar Cycle 24 is the bottom line and appears to be already in a steep decline.

Reference

Suckling, P.W. and Mitchell, M.D.  2000.  Variation of the Koppen C/D climate boundary in the central United States during the 20th century.  Physical Geography 21: 38-45.