by Judith Curry

We can conclude that the evidence provided is sufficient to justify a complete updating and reviewing of present climate models to better consider these detected natural recurrences and lags in solar processes. – Jorge Sánchez-Sesma

In pondering how the climate of the 21st century will play out, solar variability has generally been dismissed as an important factor by the proponents of AGW. However, I think that it is important that scenarios of future solar variability and their potential impacts on climate should by considered in scenarios of future climate change.

I have been cursorily following the literature on this topic. I have recently been in communication with Jorge Sanchez-Sesma. He has new paper that was just accepted for publication in Earth System Dynamics, an interactive open-access journal published by the EGU. I am featuring this paper in a post since it provides important new analysis and insights on this topic, and also provides a useful assessment of the literature and current state of knowledge on this topic.

The significance of this paper is reflected in the EGU metrics link that indicates that this paper has been downloaded 1531 times so far (before it has been formally published).

Evidence of cosmic recurrent and lagged millennia-scale patterns and consequent forecasts: multi-scale responses of solar activity to planetary gravitational forcing [link]

Jorge Sánchez-Sesma

Abstract. Solar activity (SA) oscillations over the past millennia are analyzed and extrapolated based on reconstructed solar-related records. Here, simple recurrent models of SA signal are applied and tested. The consequent results strongly suggest the following: (a) the existence of multi-millennial (9500-year) scale solar patterns linked with planetary gravitational forcing (PGF), and (b) their persistence, over at least the last glacial– interglacial cycle, but possibly since the Miocene (10.5 Ma). This empirical modeling of solar recurrent patterns has also provided a consequent multi-millennial-scale experimental forecast, suggesting a solar decreasing trend toward Grand (Super) Minimum conditions for the upcoming period, AD2050–2250 (AD 3750–4450). Taking into account the importance of these estimated SA scenarios, a comparison is made with other SA forecasts. In Appendixes A and B, we provide further verification, testing and analysis of solar recurrent patterns since geological eras, and their potential gravitational forcing.

The Introduction provides some background information on this topic:

Solar activity (SA) has non-linear characteristics that influence multiple scales in solar processes. For instance, millennia-scale solar oscillations have been recently detected, like those of about 6000 and 2400 years, with important and interesting influences in the near, past and future climate. These millennial scale patterns of reconstructed SA variability could justify epochs of low activity, such as the Maunder Minimum, as well as epochs of enhanced activity, such as the current Modern Maximum, and the Medieval Maximum in the 12th century.

Although the reason for these SA oscillations is unclear, it has been proposed that they are due to chaotic behavior of non-linear dynamo equations, or stochastic instabilities forcing the solar dynamo, leading to on-off intermittency, or planetary gravitational forcing with recurrent multi-decadal, multi-centennial and longer patterns. It should be noted that all proponents of planetary forcing have forecasted a solar Grand Minimum for the upcoming decades, but one of them has also forecasted a Super Minimum for the next centuries. In addition, during recent decades, statistical forecasts (with physically based spectral information of reconstructed records) of solar magnetic activity predict a clear decrease in SA, reaching a minimum around AD2100.

These different cosmogenic radionuclide-based reconstructions of SA present variations for the past millennia, and large uncertainties appear in reconstructions of the solar modulation of galactic cosmic rays from different proxies, 10 Be and 14 C, and of changes in the geomagnetic shielding influence. However, these reconstructed records provide, especially when considered all together, the most objective information as elements for detecting and eventually modeling and extrapolating multi-millennial-scale solar oscillations, trends and absolute levels.

The Discussion summarizes the findings and puts them into context:

Our analysis establishes the following:

Solar System dynamics generate lateral forces with multi-millennial scale (~9500 years) oscillations similar to those shown by solar activity. There is a suggested lagged response of around 67 centuries,of solar activity to the gravitational forcing (lateral force). The maximum forces F precede the maximum solar activity TSI, meaning that increases (decreases) of force F produce lagged increases (decreases) of TSI. Taking into account that the Sun’s rotation axis is tilted by about 7.25_from the axis of the Earth’s orbit, the PGF are able to generate meridional forces and consequently meridional circulations in the Sun. The lagged response appears to increase with forcing periods with a non-linear logarithmic function that implies temporal-scale influences and possible connections with meridional circulations in different deep layers of the Sun; The similarity of the ~9500-year TSI with the average SSN 10.5-year cycle, with scales differing at almost 3 orders of magnitude, suggests a self-similar process with a mechanism possibly linked to recurrent PGF in different scales.

Our experimental multi-millennial-scale analogue forecast of TSI, supported mainly by recurrent oscillations over the last glacial–interglacial cycle, shows a lowering trend toward a minimum for the coming decades. Our forecast also confirms previous efforts by several authors who have forecasted a solar Grand Minimum for the upcoming decades. Although the complete physical basis of this recurrent process is missing, there are several examples of physical and theoretical evidence that also support our findings.

In this work, we have forecasted a continuation of the solar decline for the next decades, which is supported through precursory signals during recent decades:

A steady and systematic decline in solar polar magnetic fields, starting from around 1995, which is well correlated with changes in meridional-flow speeds.. A decline in solar wind micro-turbulence levels. Based on extensive interplanetary scintillation (IPS) observations at 327 MHz, obtained between 1983 and 2009, a steady and significant drop in the turbulence levels in the entire inner heliosphere, starting from1995, was detected. A significant reduced ionospheric cut-off frequency to radio waves, normally about 30 MHz, to well below 10MHz.

Also, in this work, we have forecasted a Grand solar minimum, with sustained low solar activity for the next 2 centuries, which has been supported through a number of recent studies and their findings:

The continuation of this decline in solar activity is estimated to continue until at least 2020, and there is a good possibility of the onset of a Grand solar minimum from solar-cycle 26 onwards (2031). Based on the S04 SA record, it has been shown that gradual (abrupt) changes in solar surface meridional flow velocity lead to a gradual (abrupt) onset of grand minima, and that one or two solar cycles before the onset of grand minima, the cycle period tends to become longer. It is noteworthy that surface meridional flows over Cycle 23 have shown gradual variations, and Cycle 24 started 1.3 years later than expected.

From the Conclusions:

The tested existence of the ~9.5 kyr period recurrent pattern suggests that SA is characterized by solar dynamics with long-term patterns. Considering that it has been suggested that the modulating oscillations of SA, around 84, 178 and 2400 years, are possibly related to the Sun’s rotation rate and impulses of the torque in the Sun’s irregular motion, our results also suggest that similar mechanisms on the solar dynamo must be proposed for solar oscillations of around 9.5 kyrs.

With all of these recurrent and persistent phenomena, we have presented, tested and verified an experimental multi-millennial forecast technique for SA. We have provided elements and recent supporting studies on precursor signals of an entering into a Grand minimum SA mode. The extreme duration of the last solar minimum is important evidence of longer cycles, similar to those presented before the start of the Maunder and Spörer minima.

We can conclude that the evidence provided is sufficient to justify a complete updating and reviewing of present climate models to better consider these detected natural recurrences and lags in solar processes.

Excerpts from Reviewer #2

One of the reviewers [link] made some good comments about the general significance of the paper:

It addresses an ongoing discussion in the scientific community regarding the current declining solar activity and whether solar activity will die down in the near future to the level and duration, to justify it as being called a “Grand-Minimum.” This paper on the other hand takes and entirely different line of thought in that it looks for long term signatures in solar activity, and identifies, based on empirical modelling of solar activity, a 9500 year solar activity cycle which suggests “Grand Minima” like conditions for the period 2050-2250.

In conclusion, I agree entirely with the author that looking for short term signatures or time scales of practical consequences i.e. in years is in essence “missing the forest for the trees”. One needs to look at long term signatures and this study by the authors has yielded the first multi-millennial scale solar activity oscillation which is modulated, as the author suggests, by solar dynamics, essentially the solar dynamo and helio-seismology, with long term patterns. This is a significant step forward in gaining an overall perspective of the sun climate system and I hope the solar community can now take this forward.

JC reflections

This is a remarkable paper in many ways. This paper has a single author — Jorge Sanchez-Sesma, who is a climatologist (not a solar physicist). I have been in contact with Jorge and will be posting an interview with him in several weeks. He has a remarkable story to tell.

This paper indicates that the case is increasingly compelling for millennial-scale variations in solar activity. The arguments for a forthcoming Grand Solar Minimum are also increasingly compelling.

To what extent a Grand Solar Minimum will influence the Earth’s climate remains uncertain. As discussed on a previous blog post IPCC: solar variations don’t matter, the IPCC AR5 Ch 8 stated:

Nevertheless, even if there is such decrease in the solar activity, there is a high confidence that the TSI RF variations will be much smaller in magnitude than the projected increased forcing due to GHG.

The previous post also describes different perspectives on this from Svensmark and a 2013 NRC report (see also Effects of solar variability on climate; 21st century solar cooling.)

Solar indirect effects on climate remain at the knowledge frontier, and are associated with substantial uncertainty and ignorance. This uncertainty and ignorance is not a rationale for ignoring solar effects on the 21st century climate (and 22nd, 23rd centuries). And anyways, is the solar uncertainty (we understand the sign) really so much more greater than that associated with the effects of clouds on climate (see my recent post The cloud climate conundrum), where even the sign of the feedback is uncertain and the magnitude of cloud forcing swamps greenhouse gas radiative forcings.

But we are starting to see some ideas emerge as to how these solar effects and processes could be included in climate models. Independently of climate models, the statistical forecast technique used by Sanchez-Sesma provides the basis for creating alternative scenarios of the 21st century climate. I find his arguments about lags to be particularly important as we sort out the solar-climate effects.

Tackling the variability of solar activity and solar indirect effects seems more tractable than the cloud-climate problem and untangling the myriad of scales of ocean oscillations, so I would hope to see much more emphasis put on unraveling the solar-climate connections.

The policy significance of this issue is clear: if we are headed to a mid-20th century solar minimum, or a Grand Solar Minimum for the next two centuries, this will offset greenhouse warming to some extent. The extent of the offset depends on whether climate sensitivity to CO2 is on the larger or smaller end of the range of estimates, and the magnitude of the solar impact. But the sign of the solar offset is becoming increasingly clear: towards cooling.