David Archibald

It is now seven years since the global lower atmosphere temperature, as measured by UAH, peaked in 2016.  As it is energy from the Sun that stops the Earth from looking like Pluto, what has solar activity done? There are lags involved so the relationship is not clear cut. And then there is the interaction with a long-cycle terrestrial system that complicates it a bit further. But when that turns the rate of cooling will accelerate.

Figure 1: F10.7 Flux 1948 – 2023

This is the index of solar activity that the solar professionals use, not sunspot number. The solar cycle is driven by the orbital period of Jupiter of 11.86 years, modulated by Saturn and Uranus.

Figure 2: Sunspot Area 1874 – 2023

F10.7 flux correlates closely with sunspot area. This graph of sunspot area by solar hemisphere shows the abrupt jump in solar activity at the start of the Modern Warm Period. What is significant is that the hemispheres have different levels of activity and that trends in activity can last three solar cycles, approximating the orbital period of Saturn.

Figure 3: Ap Index 1032 – 2023

The Ap Index is a geomagnetic index that was adopted in 1932 as an improvement on the aa Index. The 1970s Cooling Period in the middle of the Modern Warm Period is reflected in lower activity of the Ap Index. The first sign that the Modern Warm Period was over was when the Ap Index fell abruptly in 2006, confirmed in June 2008 by it breaking through the floor of activity established in Modern Warm Period. That break has been followed by a change in the character of activity with much lower amplitudes. From that it follows that there has been a big change in the Sun.

Figure 4: aa Index 1868 – 2023

This is the first instrumental record of solar activity. The Sun was more active in the second half of the 20^th century than it had been in the previous 11,000 years. And as it is the level of solar activity that determines the Earth’s surface temperature, the Earth had a mild and much appreciated temperature increase as the system sought a new equilibrium.

Figure 5: Cumulative aa Index 1868 – 2022

What this figure does is take the information underlying Figure 4 and add the difference between the annual figure and the average over the 154 years of the record. This methodology is good for revealing changes in trend. What it shows is that the aa Index had a higher level of activity from 1993 so this is the start of the Modern Warm Period.

Figure 6: Interplanetary Magnetic Field 1966 – 2023

The 1970s Cooling Period stands out as an interval of lower activity that departs from the F10.7 flux of Solar Cycle 20. And as we approach the peak of Solar Cycle 25, the Interplanetary Magnetic Field is near the highs reached in Solar Cycle 23 and higher than in the 1970s Cooling Period.

Figure 7: F10.7 Flux and Oulu Neutron Count

This is where the bulk of the rubber hits the road in terms of the solar effect on climate. Through the Modern Warm Period the inverted Oulu neutron count closely tracked the F10.7 flux. Variation in the neutron count is caused by the magnetic flux of the Sun, carried on the solar wind, pushing galactic cosmic rays away from the inner planets of the Solar System. The galactic cosmic rays hit oxygen and nitrogen atoms in the atmosphere, causing a show of neutrons that reach the Earth’s surface. They act as nuclei for cloud droplet formation in the lower atmosphere. The increased cloud cover increases the Earth’s albedo and more sunlight is reflected back into space, making the planet cooler.

The graph shows that the two series parted company in 2006 at the beginning of the New Cold Period and have maintained that gap. The peak in the Earth’s temperature as measured by the UAH series was ten years later in 2016, consistent with the theory that there is an 11 year lag in the Earth’s climate from changes in solar activity.

Figure 8: Solar wind flow pressure 1967 – 2023

The solar wind flow pressure was lower during the 1970s Cold Period suggesting that it is a variable that affects climate. But otherwise it has a weak to non-existent correlation with other solar parameters and the Earth’s climate.

Figure 9: Solar EUV

The University of Bremen records the Sun’s ultraviolet emissions at 280 nm and produces the data in the graph above. Solar Cycle 25 is tracking later than the cycles of the Modern Warm Period but has reached their amplitude.

Figure 10: Alpha particle/proton density ratio

This is another solar parameter that is different from any other. Once again, it was flat during the 1970s Cooling Period and then responded to the solar cycle, but in a declining trend for the last 45 years.

Figure 11: North America Ex-Greenland monthly snow cover 1971 – 2023

If the Earth was warming, or cooling, you might expect to see that reflected in snow cover at the height of Summer. But that hasn’t changed detectably for nigh on the last 50 years. North America did warm as the 1970s progressed and this is reflected in the significant decrease in snow cover.

Figure 12: Pacific Decadal Oscillation 1890 – 2023

It may be that the Earth’s climate does not slavishly follow solar activity because there is another major driver of climate which has its own cycle. The graph above, from Professor Humlum’s site, is the HadCRUT temperature record plotted as a cumulative change from the average of the period of the record, in the manner of Figure 5. It shows that there are multidecadal trends in the HadCRUT temperature record. As per the annotation, these roughly correspond to the changes in the Pacific Decadal Oscillation (PDO) from positive to negative and back again. HadCRUT has been in uptrend for nearly 50 years and the PDO has been positive for slightly longer.

So we can interpret this all to mean that, even though solar activity fell out of bed in 2006, the reason why snow cover at the height of summer hasn’t increased yet is because the solar-climate nexus has been over-ridden, so far, by the continued positive phase of the PDO. But is the PDO still positive after 50 years?

Figure 13: North Pacific commercial salmon catch 1925 – 2021

As it happens there is a real world parameter which says that the PDO is still in a positive phase. We can thank the North Pacific Anadromous Fish Commission for the graph above which shows the annual salmon catch in thousands of metric tonnes for the six countries around the North Pacific. The salmon catch moves north to Alaska in the positive phase of the PDO and then that reverses in the negative phase. It looks like the fish are heading north still with the Canadian catch in decline from 1990 and the Japanese catch down to 20% of what it was 20 years ago.

It follows that there is a scary conjecture from this interpretation of the data. When the PDO finally turns negative it will hyper-accelerate the solar-driven cooling evident from 2016. The warming that took all of the second half of the 20^th century to achieve will be wiped out in what will feel like just an instant and we will be back to the Little Ice Age conditions of the 19^th century.

David Archibald is the author of The Anticancer Garden in Australia.