Grand Solar Minimum & Global Cooling
Some scientists believe the Sun could be at the beginning of a grand solar minimum (GSM); a period during which the Sun experiences several back-to-back minimally-active solar cycles. Whether we are indeed at the cusp of a new GSM or not is still debated (Bhowmik and Nandi, 2018). The possibility has captured public attention because there is some indirect correlation between GSM and global cooling.
During solar minimum, total solar irradiance (TSI) can decrease up to about 0.1% as compared to TSI during solar maximum (Lee et al., 1991; Kopp, 2016). Whether a solar-cycle averaged TSI decrease from averaged maximum directly equates to global cooling is not conclusively known. Though some coupled climate models predict a new GSM could offset average global temperatures by up to -0.3°C relative to a scenario wherein no new GSM occurs (Feulner and Rahmstorf, 2010).
Some media personalities, bloggers, and vloggers have taken the indirect correlation between GSM and global cooling to misinform the public that we’re heading into an “ice age”. This is folly, and a gross misrepresentation of the data, and research over the past several decades.
If a 0.3°C decrease can be expected, it would be inconsequential as compared to the projected 3°–5.5°C increase in global temperatures by 2100 relative to the 1986–2005 average (NCA4, Volume I, Ch. 4). The increase in global temperatures driven by increased atmospheric carbon dioxide far exceeds the theorized -0.3°C offset. For those who question the fact that atmospheric carbon dioxide is primary driver of global climate, and whose rate increase is do primarily to anthropogenic activities are welcomed to read why both these facts are true in this blog post.
There is nothing to suggest forcing by a 0.1% decrease in TSI on global climate is going to bring any respite from the increasing rate of global warming, contrary to what we may hear from sources outside academia. All GSM have occurred prior to the second phase of the Industrial Revolution. A key point taken into context in every paper writen and published in peer-review. Carbon dioxide levels in our atmosphere have since nearly doubled.
I explain how carbon dioxide works in this blog post (not like a greenhouse):
The “Greenhouse” Gas - Carbon Dioxide
And how it acts as a primary driver in this blog post:
Climate Change - Carbon dioxide has always been a primary driver
The magnitude of solar irradiance is not constant; not even on a scale of minutes, much less gigayears. It is constantly fluctuating in small amounts. But these fluctuations do exhibit marked periodicities with the most well-known being the ~11-year solar cycle. During a cycle, the Sun goes through a period of relative calm, then a period of increased activity in the form of sunspot-producing intense magnetic field flux. A cycle is measured from the start of one solar minimum, to the start of the next, punctuated by a solar maximum.
The magnitude of average total solar irradiance (TSI) variability between minimum, and maximum is about 0.1% (Lee et al., 1991; Kopp, 2016). But then there are prolonged periods where solar cycles, on average, exhibit unusually calm conditions throughout, and do so back-to-back such that relative calm quells total solar irradiance for decades. Such events are dubbed grand solar minimum, or simply grand minimum.
Scientists have observed, or inferred several grand minimum that have likely occurred over the past 2,000+ years, with the three most recent being the Spörer Minimum (1460-1550 ce), the Maunder Minimum (1645-1715 ce), and the Dalton Minimum (1790-1830 ce). Curiously, there is weak correlation between grand minimum, and slight global cooling, and some admittedly conjectured hypotheses put forth attempting to physically link grand minimum with global cooling. We’ll discuss these hypotheses below, and put them in context of what we might expect should we indeed be at the start of another grand minimum.
Because it has been floating around academic circles for several years that we could be at the cusp of another grand minimum. And while support for this possibility seems to be gaining strength, it is not yet consensus (Bhowmik and Nandi, 2018).
WHY I’M WRITING ON THIS TOPIC
A few days ago friend of mine from the U.K. sent me an e-mail regarding the topic of there being a link between grand solar minimum and global cooling. Presuming we are headed into a new grand solar minimum, he wanted to know what impact it might have on global warming. Would it provide reprieve [by temporarily slowing the rate of temperature increase, halting it, or even reversing it]. He also asked to what extent climate models account for a cooling effect from docile solar activity, if at all.
He honored me by suggesting I make a blog post about it, because I interpret that as him having confidence I can answer these questions well enough and without bias. So to that I humbly thank him, and will do my best to adhere to these high expectations. For others who happen upon this blog post, I hope what’s written here will help make sense of a topic that is often misrepresented in other blogs, YouTube videos, and some mainstream media outlets.
And I suppose this is a secondary reason for me writing on this topic. A cursory search on Youtube, or any search engine results in a cornucopia of misrepresentations of the topic due partial truths, and/or gross misunderstanding of what is known, and isn’t known about grand minimum, and the weakly-correlated periods of global cooling that they’ve roughly overlapped in the past.
THE LITTLE ICE AGE
The widespread general belief among much of the laity is that the poorly-named Little Ice Age was an extensive, unbroken period of global cooling initiated by the Maunder Minimum, and marked by worldwide glacial advancement. The only thing certainly true about that sentence is that the Little Ice Age is poorly named.
Whether or not the Little Ice Age was global is still debated. There are those who suggest it was in fact global (e.g. Chambers et al., 2014), and those who say it only affected the northern hemisphere, and that the cooling was a modest < 1°C relative to late 20th century levels (e.g. Bradley and Jones, 1993; Jones et al., 1998; Mann et al., 1998; 1999; Crowley and Lowery, 2000).
While there is yet no genuine consensus, the fact remains that no scientist can agree on the start and end dates of the Little Ice Age precisely because cooling periods varied from region to region, and area to area. This suggests independent regional climate influences. Furthermore, NASA’s Earth Observatory has noted 3 particularly cold intervals punctuated by periods of slight warming, which further undermines the notion that increased glaciation was a globally-synchronous event.
Worldwide cooling should result in global expansion of glacial ice, but the evidence for such is anecdotal. Climate reconstruction based on glacial length conducted by Oerlemans (2005), showed no great variation from 1600 to 1850 ce. This doesn’t mesh well with the hypothesis of a global cooling event. Moreover, maximum alpine glacial advance in Alaska, New Zealand, and Patagonia each occurred at different times (IPCC TAR - 2001), further suggesting their individual advancements may be due to largely independent regional climate changes. Because of these regional variations, scientists have been unable to come to a consensus as to when the so-called Little Ice Age began.
Be that as it may, Miller et al. (2012) suggest cooling was caused by a series of 4 massive tropical andesitic eruptions which ejected large quantities of sunlight blocking/reflecting ash and aerosols into the atmosphere. This is in parity with said regional variations. Yet Chambers et al. (2014) suggest cooling during the Little Ice Age was caused by a less radiant Sun during the Maunder Minimum. It has been well established that TSI decreases approximately 0.1% as compared to TSI at maximum, and through geologic time, the Sun has been a primary forcing agent on global climate along with carbon dioxide.
However, Steinhilber, Beer, and Frohlich (2009) warn that only considering the rather small forcing by TSI in climate models attempting a physical link to global cooling could prove problematic. They point out that observations have shown high-energy UV irradiance does not exhibit a similarly distinct decreasing trend as TSI during grand minimum as compared to minimum during ~11-year cycles (ibid.).
But apparently not all climate models have found the small decrease in TSI to be a problem when it comes to singularly explaining roughly coinciding cooler global climate. Highly-cited physicist, and climate scientist, Dr. Drew Shindell along with some of his former colleagues at the NASA Goddard Institute for Space Studies plugged the small decrease in TSI during the Maunder Minimum into a general circulation climate model. The result showed cooler modelled temperatures that matched the paleoclimate record during the Little Ice Age.
They posit the idea that cooler temperatures may have been the indirect result of a decrease in UV radiation from the Sun during the Maunder Minimum. A decrease in solar ultraviolet irradiance would theoretically result in a decrease of ozone in the stratosphere, which in turn affects upper-level temperature schemes, which may affect planetary atmospheric waves (jet streams for all intents and purposes).
Stratospheric ozone forms when solar UV breaks apart diatomic oxygen. The resulting free oxygen atoms eagerly combine with other diatomic oxygen to produce ozone. When the Sun emits less UV radiation, fewer stratospheric diatomic oxygen is disassociated, resulting in fewer free oxygen atoms, resulting in the production of fewer ozone molecules, resulting in cooler upper-level temperatures which can affect the jetstream (or more precisely, affects planetary waves).
According to Shindell, altered planetary wave patterns cause the North Atlantic Oscillation (NAO) to switch into a negative phase. The NAO is a weather phenomenon in the North Atlantic of fluctuations in the difference of sea-level atmospheric pressure between the Icelandic Low and the Azores High. These two permanent pressure systems control the direction and strength of westerly winds (Westerlies) into Europe.
Normally (in positive phase of the NAO), Westerlies are strong, dicrecting moderating Atlantic source air deep into Europe thereby moderating their summer and winter temperatures.
However, in negative phase (of the NAO) the Westerlies are supressed such that they are unable to transport moderating Atlantic air into Europe. This translates to extreme summer and winter temperatures. With regard to winter temperatures, matters have been made worse due to loss of summer Arctic sea ice. Models show that air pressure difference with decreased sea ice cover in the Arctic summer is weakened in subsequent winters, thereby allowing cold Arctic air to penetrate more effectively into the mid-latitudes (Dethloff et al., 2012).
Shindell’s circulation model showed the NAO, on average, went into negative phase during the Maunder Minimum, chilling Europe to the bone as indicated in the paleoclimate record for that region.
Unfortunately, few things are in consensus when it comes to hypotheses regarding grand solar minimum and global cooling. Lee and Feldstein (2013) showed that while ozone depletion impacts the jet stream, the effect is primarily in the southern hemisphere only. They believe ozone has limited influence on planetary waves in the northern hemisphere. Gerber and Seok-Woo Son (2014) further warn that ozone and increasing levels of greenhouse gases will largely offset each other in the future.
Scientists at the NASA Goddard Institute for Space Studies found evidence for a mechanism by which greenhouse gases can indirectly influence the jet stream shift by altering tropical convection; the vertical transfer of heat in large-scale cloud systems (Lee and Feldstein, 2013).
To complicate things further, all summers during the Maunder were not signficantly different than summer months of the previous 80 years (Owens et al., 2017). And while the winter of 1683-1684 was one of central England’s coldest on record, the winter of 1686-1687—just 2 years later—was its 5th warmest out of the entire 350-year Central England Temperature (CET) record.
The academic debate as to whether or not the Little Ice Age was driven by a docile Sun seems to be ongoing, and scientific consensus eludes us still. But there is consensus on one thing regarding the Little Ice Age, and that’s when it ended.
It ended about the year 1860, which isn’t coincidentally the close of the first decade of the second phase of the Industrial Revolution, when black carbon (soot) from proliferating factory smokestacks, and coal-powered locomotives across Europe were joined by likewise proliferating factory smokestacks, and coal-powered locomotives across the burgeoning United States.
The end of the Little Ice Age was marked by the sudden retreat of Alpine glaciers after centuries of growth. Ice cores taken from high-elevation sites at three separate locations across the Alps showed large amounts of soot in layers dating from 1860 (Schiermeir, 2013). Mass-balance models showed the negative effect soot may have played on glacial ice by absorbing more heat (Ibid.).
Scientists are also in consensus that solar actiivty has less impact on global climate today than it did prior to the doubling of atmospheric carbon dioxide. This isn’t to say the Sun itself isn’t a main driver of Earth’s climate on geologic timescales, but that its periodic activity no longer has as strong of an influence on shorter-term climate as it may have had in pre-industrial times.
And this is an important point. Because whether or not scientists are able to come to consensus regarding the Little Ice Age’s weak correlation with the Maunder Minimum, or volcanic activity, or some combination thereof, the fact remains that the Little Ice Age, along with the Maunder, and all other grand minimum events, occurred prior to the industrial revolution and subsequent doubling of atmospheric carbon dioxide. Any effects scientists are able to conclude in consensus with regard to pre-industrial climate schemes, may not translate well to today’s climate, wherein carbon dioxide levels are the highest they’ve been in hundreds of thousands of years (according to NASA, and the NOAA).
GRAND SOLAR MINIMUM & GLOBAL COOLING IN A PRE-INDUSTRIAL WORLD
- Solar Activity & Volcanism
As discussed above, correlating global cooling events to grand solar minimum remains difficult. Just as there are anomalies between global temperatures and the Maunder Minimum, there too are anomalies with other grand solar minimum. For example, the warmer-than-average temperatures across China from 1520 to 1620 overlap the last 3 decades of the Spörer Minimum (Jiang and Xu, 1986).
And while cooler-than-normal temperatures of 1816 (colloquially known as the year without a summer), occurred during the Dalton Minimum, they’ve since been shown to have been caused by a volcanic winter initiated by the 1815 eruption of Mount Tambora; one of the largest volcanic eruptions in recorded history. Reflective sulfur aerosols, and sunlight-blocking fine ash acted to reduce TSI. The volcanic winter may have been exacerbated by the 1814 eruption of Mount Mayon.
But anomalies, misaligned timeframes, and other contributing factors affecting global temperatures do not fully negate there being at least some semblance of correlation between grand solar minimum, and cooler-than-average global temperatures. While it has proven difficult to physically link solar activity to climate change, there has been weak evidence suggesting it affects regional weather patterns.
Kelly (1977), Currie (1987), and Labitzke (1987) found weak correlation between regional weather pattern changes and the ~11-year solar cycle. Willet (1974), Hameed et al. (1983), and Reid (1987) linked surface temperature, and precipitation pattern changes to the slower ~80-year solar cycle. Both cycles are roughly associated with cooler, wetter years around solar minimum (Stothers, 1989).
Weak, but “probably statistically significant” periodicities of eruptions seem to correlate to solar activity, such that there appears to be a higher incidence of volcanic eruptions around the time of grand minimum, and (to a lesser extent) a lower incidence around the time of grand maximum (Ibid.).
Challinor (1971), and Currie (1981) detected abrupt changes in day lengths in conjuction with ~11-year cycles, suggesting something is causing the planet to experience sudden jolts in its total moment of inertia (Stothers, 1989). Link (1961), and Kiselev (1981) found similar correlation between day lengths and longer cycles as well, further suggesting something is awry during solar minimum. Further supporting the possibility that sudden changes in Earth’s total moment of inertia are caused by solar activity are papers by Danjon (1962), and Gribbin & Plagemann (1973), who noted relatively abrupt changes in day length right after very large x-class solar flares.
Two hypotheses have been put forth to explain how solar activity could be causing these shifts, and how it can lead to altered surface temperatures on Earth.
One hypothesis posits that torques in Earth’s lithosphere generated from small, relatively abrupt changes in Earth’s rate of rotation might trigger swarms of small earthquakes (Anderson, 1974). Abarca del Rio et al. (2000) found a correlation between day lengths and changes in atmospheric angular momentum, suggesting Earth’s rate of rotation is affected by shifting atmospheric masses. In a later paper, they were able to correlate this phenomenon with solar activity (Abarca del Rio, 2003).
We’ve discussed how shifts in planetary waves can occur due to upper-level temperature changes stemming from decreased ozone production; the latter of which results from decreased solar ultraviolet irradiance during solar minimum. These shifts in planetary waves qualify as changes in atmospheric angular momentum.
It’s possible ‘jolts’ in Earth’s rotation caused by changes in atmospheric angular momentum, in turn causes small earthquakes to occur. These earthquakes can create new, or widen existing small fissures in stressed rock, with particular attention to rock that surrounds volcanic magma chambers.
These fissures could then act as metaphorical relief valves for some of the magma within the chambers to escape into. It’s theorized that this would thus lower some of the pressure within the chambers themselves, thereby reducing the strength of, delaying, or even preventing large eruptions.
Fewer large eruptions equate to less volcanic ash, and sulfur aerosol distribution across Earth’s atmosphere. As such, sunlight would not be blocked, or reflected away (Santer et al., 2014), preventing global cooling.
As it turns out, this hypothesis seems to be supported, albeit weakly. Earthquake records appear to show a greater number of small quakes around times of solar maximum, and a greater number of large eruptions around times of solar minimum (Davison, 1938; Machado, 1960; de Mendoça Dias, 1962; Simpson, 1967; Tamrazyan, 1968). Meanwhile, no correlation has been found between solar activity and very large earthquakes (Gutenberg and Richter, 1963; Meeus, 1976). All this meshes well with the hypothesis that solar activity is linked with volcanic activity.
Singh (1978) even determined that there appear to be a greater number of microearthquake swarms immediately following large X-class solar flares reaching Earth, which falls in line with the Danjon (1962), and Gribbin & Plagemann (1973) papers referenced above.
A second hypothesis proposes the possibility that changes in precipitation accumulations can lead to changes in volcanic eruption frequency, and subsequent atmospheric ash & sulfur aerosol distrubution. Observed increases in rainfall and snowfall around years of solar minimum could temporarily swell reservoirs of groundwater near volcanic vents, whereupon some of that water might seep into magma chambers. If those chambers are near their threshold, then this could trigger an eruption (Stothers, 1989).
I have to interject here to say that the possibility of water seeping into magma chambers is unlikely as the rock surrounding magma chambers would be so hot as to steam away any water long before it would reach a magma chamber. The more likely scenario here would be if a magma chamber itself intruded into an existing reservoir of groundwater, not the other way around. I conferred with an actual geologist, and he concurred this would be the more likely scenario of the two.
Again, these hypotheses attempting to explain physical links between solar activity and volcanism are merely conjectures, and not proven facts, and Stothers admits evidence is weak due the large amount of randomness in the volcanic record. He admits that because of this, solar activity may only act as a minor forcing agent.
Solar Activity & Cosmic Rays
Another avenue by which solar activity could be linked to global surface temperatures involves cosmic rays.
An active Sun emits a relatively strong solar wind which deflects a large fraction of cosmic rays from penetrating Earth’s atmosphere. During solar minimum however, the solar wind is diminished in strength and cosmic rays are better able to penetrate Earth’s atmosphere.
In laboratory conditions, cosmic rays have been shown to enhance cirrus cloud formation. Some scientists believe these lab results should translate to the actual atmosphere (Svensmark et al., 2016), but this is debated (Carslaw, Harrison, and Kirkby, 2002).
If we assume high-level cloud cover is enhanced by cosmic rays, then we can presume it would proportionally increase Earth’s albedo. This in turn would decrease the planet’s average surface temperature by reducing TSI.
As far as I’m aware, the jury is still out on this one.
While physical links between grand solar minimum and global cooling remain in the realm of conjecture, the general agreement is that there is some weak correlation between solar activity and Earth surface temperature. If Feulner and Rahmstorf (2010) are correct, and the planet cools by no more than 0.3° C during grand minimum, then we must consider that decrease against the backdrop of the projected 3°–5.5°C increase in global temperatures by 2100 relative to the 1986–2005 average (NCA4, Volume I, Ch. 4). The near doubling of atmospheric carbon-dioxide from pre-industrial levels is fueling a global temperature increase that far outpaces any cooling effect a grand minimum might otherwise have had on our climate (Ibid.).
No matter what consensus is reached regarding past grand minimum and global temperatures, the fact remains that all grand minimum events occurred prior to the start of the second phase of the Industrial Revolution, when the United States joined Europe in the prolific burning of fossil fuels.
Carbon dioxide levels in our atmosphere are higher than they’ve ever been throughout recorded history, and well beyond. And while many of the papers cited above seem as though they might provide convincing arguments that we’re heading into a period of global cooling, its critical to note that those authors are writing in context of a bigger picture often overlooked by those outside academia (e.g. bloggers, vloggers, and other media personalities).
In fact, Shindell and others at the NASA Goddard Institute for Space Studies who ran models that showed even the small decrease in TSI during the Maunder Minimum could cool the planet, and whose research has been grossly misrepresented by certain media personalities in recent years, writes:
“The period of low solar activity in the middle ages led to atmospheric changes that seem to have brought on the Little Ice Age. However, we need to keep in mind that variation in solar output have had far less impact on the Earth’s recent climate than human actions”
Frank Hill, an astrophysicist at the National Solar Observatory (NSO) who has also seen his research grossly misrepresented by certain ideologues from the laity was forced to add an addendum to his paper (Hill et al., 2011), in which he bluntly states, “We are NOT predicting a mini-ice age”.
Has past grand solar minimum cooled our planet? Probably. Will future grand solar minimum cool our planet?