Understanding How Earth’s Cycles Mirror Solar Variability
I routinely suggest that changing weather patterns on planet Earth give us insight into changing weather patterns on our main sequence G-type star that we affectionately refer to as the Sun.
As such, I maintain that Earthly climate change is the pulse of the solar system.
Here in this article I will focus on patterns of change on Earth that in the literature is routinely referred to as multi-decadal variability (MDV) and will argue that MDV offers us insight into the pace and trajectory of climatic changes in the Sun over the modern era.
One such body of evidence that I have come upon is shown in Figure 1, which splices together sunspot numbers versus global major hurricane frequency, where the latter is the count of hurricanes with speeds greater than 94 knots (174 kph).
For those who are unaware, sunspots are dark, cooler areas on the Sun’s surface caused by intense focused magnetic activity. They occur when magnetic field lines become tangled, inhibiting convection and reducing surface temperature. These temporary phenomena, lasting days to weeks, are linked to the Sun’s 11-year magnetic cycle (Schwab Cycle).
Figure 1. Royal Observatory Sunspot Number (SSN) versus global major hurricane frequency (Dr. Ryan Maue).
Other solar SSN cycles that have been proposed, based on variable cosmogenic radionuclide content found in the geological record are the Gleissberg cycle (70–100 years), the Suess cycle, or de Vries cycle (210 years) and the Hallstatt cycle (2,400 years). Cosmogenic nuclides, like carbon-14 (¹⁴C), form when cosmic rays strike Earth’s atmosphere, producing isotopes. Their variability in the geological record reflects changes in solar activity, as the Sun’s heliosphere modulates cosmic ray flux.
Stronger solar magnetic fields reduce nuclide production, indicating heliospheric strength fluctuations – a topic for a future article.
Figure 2 shows the magnetic field strength variation from the Sun as recorded at the Wilcox Solar Observatory, as the magnetic dipole flips over the Schwab Cycle. As shown, the period of maximum SSN coincides with the stage wherein the magnetic field strength of the Sun goes through its minimum.
As such, it takes approximately 22 years to complete a full magnetic field reversal cycle (aka pole reversal), which is commonly called the Hale Cycle. Note also that solar activity is not fully reversible over these intervals, as seen in differences in SSN and magnetic field strength from one cycle to the next and while the average duration is 10.7 years, over the observation record of the last few centuries, the cycle length has varied between 9 and 14 years.
Figure 2. Sun’s magnetic field strength variation over two Hale Cycles.
The phrase variable solar activity more times than not is used to indicate that there is a time varying change in the Sun’s total solar irradiance (TSI); however, it is not accurate to equate TSI and solar activity, as the latter is indicative of many physical properties (i.e., SSN, geomagnetic field, coronal mass ejection rates).
The primary instrument used to measure total solar irradiance (TSI) in space is the Active Cavity Radiometer Irradiance Monitor (ACRIM). Other notable instruments include the Total Irradiance Monitor (TIM) on the SORCE satellite and the VIRGO (Variability of Irradiance and Gravity Oscillations) on the SOHO spacecraft. These radiometers precisely measure the Sun’s total energy output.
ACRIM instruments primarily deteriorate through optical degradation of the cavity sensors caused by prolonged exposure to solar UV and extreme ultraviolet (EUV) radiation. Other minor modes of failure include thermal cycling effects and radiation-induced electronics wear, but optical drift is the key challenge for TSI accuracy.
As a result, TSI measuring satellite platform over the past 45 years commonly fail within 10 to 20 years.
Figure 3 shows the TSI records over the modern instrumental record, which shows a significant disagreement in magnitude. This is extremely important to acknowledge the lack of clarity on the question of whether TSI is increasing, decreasing or is stable over the past 45 years. Without this certainty, we can not say if changes in TSI are directly attributed with climatic changes measured over this time frame.
Figure 3. Original daily satellite TSI data as provided by the satellite mission teams at 1 au or “au” 1 astronomical unit – the average distance between the Earth and the Sun.
TSI data from individual satellites (e.g., ACRIM) are combined into a 45 year modern composite time series by cross-calibrating overlapping measurements and normalizing to a common scale (1 au). Data gaps are bridged using statistical models or proxy data (e.g., sunspot numbers). According to Ronan Connolly et al, there have been 21 historical attempts to produce a modern composite TSI record that fall into 6 different categories.
Figure 4 represents 6 different groups in which Ronan Connolly et al argues these 21 differing composite TSI time series falls into, depending on the methods used to splice original satellite data together (Figure 3). Note the appearance of the 11-year Schwab Cycle embedded in each time series.
Figure 4. Six different categories for satellite era TSI composite time series.
Remarkably, each of the six composite groups implies a different history of changes in TSI between each of the four solar minima in the satellite era to date:
- Group A implies almost no variability between 11-year solar minima.
- Groups B and C imply that the TSI increased between the first two 11-year solar minima of the satellite era but has steadily decreased between successive minima since then and that the TSI during the most recent solar minimum was comparable to, or lower than, that of the first solar minimum in 1985.
- Groups D and E imply a similar history but suggest that the first 11-year solar minimum remains the lowest of the satellite era so far.
- Group F implies that the TSI has increased between all four 11-year consecutive solar minima of the satellite era so far. Note that on September 8th, 2025, a new study from NASA’s Jet Propulsion Laboratory titled The Sun Reversed Its Decades-long Weakening Trend in 2008 suggested that overall solar activity has been increasing since 2008.
This uncertainty is extremely important to emphasize.
If we do not know the trajectory of climate change in our main sequence type G star, how can we possibly be certain that modern changes in the Earth’s climate regimes are not originating due to changes in solar activity?
Now to move onto the longer term solar activity record.
Multi-century TSI reconstructions (i.e., not-satellite data) rely on proxies like sunspot records or cosmogenic isotopes (e.g., ¹⁴C), where each have calibration uncertainties and indirect relationships with satellite measured composite TSI time series. Ronan Connolly et al is again referenced in Figure 5, which shows 16 different TSI reconstructions from the broader literature that extend back to the 17th and 18th centuries, which categorizes these into high and low variability groups.
Note that the y-axis of each TSI time series is an anomaly (i.e., deviation from average) which is why there are positive and negative numbers displayed on either side of 0 Watts per square meter. There is over a 5-fold difference in the minimum to maximum variabilities shown between the left-hand high variable TSI reconstructions and the right-hand low variable TSI reconstructions.
See the rest at josephfournier.substack.com
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