As Earth Warms Up, The Sun Is Remarkably Quiet

If you’re looking toward the sun to help explain this decade’s record global heat on Earth, look again. Solar activity has been below average for more than a decade, and the pattern appears set to continue, according to several top solar researchers. Solar Cycle 24, the one that will wrap up in the late 2010s, was the least active in more than a century. We now have outlooks for Cycle 25, the one that will prevail during the 2020s, and they’re calling for a cycle only about as strong as--and perhaps even less active than--Cycle 24.

Weak solar cycles tend to produce fewer solar storms, those dramatic bursts of magnetized material from the sun that generate spectacular auroral displays and play havoc with satellite-based systems and power grids on Earth. However, solar storms that do emerge during weak cycles can be among the most potent, notes Scott McIntosh (National Center for Atmospheric Research). Just as a catastrophic hurricane can occur in an otherwise quiet season, a quiet solar cycle can still cause devastating space weather, McIntosh told me. “If you look at the record of extreme events from the sun, they most often occur in weak cycles, and they almost always occur in the deep, descending part of the cycle,” he said.

When scientists like McIntosh fret about the potential consequences of a solar storm, they often point to the Big One: the outburst from September 1-2, 1859, that’s been dubbed the Carrington Event. Occurring near the peak of a fairly quiet cycle, the Carrington Event was an extremely intense solar flare aimed directly at Earth. It produced stunning auroral displays around the globe, even in Cuba and Hawaii. The barrage of magnetized particles also knocked out telegraph communications across Europe and North America. A 2013 study from Lloyds of London (see PDF) found that a similar event today could cause up to $2.6 trillion in damage, with up to 40 million Americans losing power for anywhere from two weeks to two years. “While the probability of an extreme storm occurring is relatively low at any given time, it is almost inevitable that one will occur eventually,” noted the report. In fact, we dodged a major solar bullet in July 2012, when a flare roughly as strong as the one in 1859 happened to point away from Earth instead of toward it. See this 2009 post by Jeff Masters for more on how a solar storm can disable electric grids.



Figure 1. Solar material bursts from the sun in this close-up from a video captured on July 9-10, 2016, by NASA’s Solar Dynamics Observatory, or SDO. The sun is composed of plasma, a gas in which the negative electrons move freely around the positive ions, forming a powerful mix of charged particles. SDO captured this image in wavelengths of extreme ultraviolet light, which are typically invisible to our eyes. The imagery is colorized here in red for easy viewing. Image credit: NASA/SDO/GSFC/Joy Ng.


Climate and the solar cycle
The solar cycle, which is about 11.5 years long on average (it varies from 9 to 14 years), can be measured in various sophisticated ways. It’s also trackable simply by counting sunspots, an activity that dates back to the 1600s. The weakest cycles on record occurred during the so-called Maunder Minimum, from about 1645 to 1715. This happens to coincide with the peak of the Little Ice Age, which brought long stretches of conditions far colder than today’s climate to parts of North America and Europe. We can’t pin the Little Ice Age entirely on the Maunder Minimum, though, since volcanic eruptions appear to have kicked things off centuries earlier. It’s now believed that the Maunder Minimum played a minor role at best in sustaining the chill, though it does appear that weak solar periods can lead to colder winters in Europe, based on centuries of data from central England.


Figure 2. International Sunspot Numbers (one of two leading measures of sunspot activity) show the dip in sunspots during the Maunder Minimum as well as the ups and downs of each solar cycle through the mid-2010s. The current solar cycle (Cycle 24, only partially depicted here) reached dual peaks in 2011 and 2014, with a top ISN just over 100. This was the lowest top ISN for a solar cycle since Cycle 14, which peaked in 1906. Image credit: David Hathaway, NASA.


Newly precise measurements confirm that the total solar energy reaching Earth actually doesn’t change all that much from cycle to cycle. As a single cycle ramps up from minimum to maximum, the sun spits out as much as 10 times more energy in extreme ultraviolet wavelengths. However, the sun’s total energy output (irradiance) goes up by a mere 0.1% during a solar cycle, and this boosts global surface temperature by no more than 0.1°C per cycle, according to the Intergovernmental Panel on Climate Change.


Figure 3. Global temperature change (thin light red line), as reported by NASA/GISS, together with the annual total solar irradiance, or TSI (thin light blue). The dark red and dark blue lines show the 11-year moving averages. Data sources: Temperature from NASA GISS, and TSI from 1880 to 1978 from Krivova et al 2007 (data). TSI from 1979 to 2015 from PMOD (see the PMOD index page for data updates). Image credit: Courtesy skepticalscience.com.

What makes a solar cycle?
As with many solar cycles, Cycle 24 had a double peak, in 2011 and 2014, and it’s still on its downward swing, as evident in the Solar Cycle Progression graphics at the website of NOAA's Space Weather Prediction Center. Cycle 24 isn't expected to end until around 2020. (Each cycle is defined as starting when the previous one bottoms out). New approaches to prediction are lending more confidence in scientists’ ability to predict how the upcoming minimum and the following maximum--which should arrive in the mid-2020s--will unfold.

The prevailing notion among solar experts is that plasma flows through the sun in two giant loops that cause the solar cycles to wax and wane. As it flows equatorward, the plasma carries magnetic fields with it, generating sunspots and other features at the surface. Magnetic fields that rise to the surface of the poles during solar minimum are believed to serve as the raw material for the subsequent cycle’s strength. As these magnetic fields drift toward the solar equator, they get stretched and distorted, which helps to trigger sunspots as well as outbursts of charged plasma that can hurtle toward Earth and cause solar storms. The distortion occurs because the gaseous solar sphere actually moves with a faster rotation rate as you go toward its equator, like a sphere made of taffy that’s mounted on a spindle and spun along its midsection. (This NASA animation shows the process in three dimensions, including a rendition of the poorly understood flow beneath the solar surface.)

Unfortunately, the sun’s magnetic fields are very hard to observe, especially near the poles, which complicates the task of predicting the next solar cycle. However, some phenomena related to the polar fields are easier to measure, such as a minimum in geomagnetic activity (the flow of energy reaching Earth via the solar wind). Over the last several cycles, such indices have been extremely well correlated with the strength of the following cycle peak, with correlations as high as 0.99.

Outlook for the 2020s: Another modest cycle
The community of solar researchers has only recently come into consensus on the “polar predictor” method of using polar magnetic fields as the best predictor of solar cycles. A decade ago, various methods produced conflicting results on how strong Cycle 24 would end up. Forecasts based on polar fields at solar minimum did remarkably well; others had more trouble in capturing the cycle’s length and strength. “One of the things we learned is that the difference between the hemispheres is critical,” said McIntosh. The north half of the sun ran about two years ahead of the south during Cycle 24, and that overlap led to the double-peaked maximum (2011 and 2014) while lessening the cycle’s overall peak strength.

Researchers are now trying to push the limits of prediction further. They’re using statistical and dynamical models, plus some data-based intuition, to predict several years in advance how the subsurface magnetic fields will look when they emerge near the poles around 2020, and what, in turn, those fields may tell us about Cycle 25.

• David Hathaway (recently retired from NASA) and Lisa Upton (NCAR and Space Systems Research Corporation) expect a Cycle 25 about as strong as Cycle 24, or perhaps slightly weaker. They published their outlook in November in the Journal of Geophysical Research. Hathaway and Upton used an ensemble model to project the polar fields from now to the end of 2019, with the ensemble showing an uncertainty by that point of about 15%. Natural solar variations in the early 2020s could add to the uncertainty, they note.

• Leif Svalgaard (Stanford University) pioneered the idea of using solar polar fields as prediction tools with colleagues in the 1970s, and he successfully pegged the eventual weakness of Cycle 24 back in 2005. Svalgaard is calling for a weak Cycle 25, but perhaps just a bit stronger than Cycle 24, based on precursors that appear slightly more active this time around.

• NCAR’s McIntosh believes Cycle 25 could extend the recent string of progressively weaker cycles. “We anticipate that the growing degree of overlap between cycles means that Cycle 25 will be weaker than Cycle 24,” he told me.

• Also at NCAR, Mausumi Dikpati will release her outlook for Cycle 25 in a paper to be published later this year. Dikpati and colleagues predicted a stronger-than-average Cycle 24 (as did Hathaway and others). This didn’t materialize, but Dikpati did correctly forecast that Cycle 24 would begin later than usual. Dikpati is now doing a post-mortem on her Cycle 24 forecast, which was based on a pioneering model of the solar dynamo (the flow of plasma that produces magnetism within the sun). As with weather models, she expects that improved data assimilation--bringing the latest observations into the solar dynamo model--will help boost its accuracy.


Figure 4. A composite of 25 separate images from NASA's SDO, spanning one year from April 2012 to April 2013. The image reveals the migration tracks of active regions towards the equator during that period. Image credit: NASA/SDO/Goddard.

Tweaking the time frame of solar forecasts
Even if we have several more decades of a quiet sun ahead of us--a “grand minimum,” which is quite plausible according to recent work published by McIntosh and others--we know that quiet cycles can produce dangerous solar storms, so there’s plenty of motivation to push ahead with solar forecasting. This includes predicting variations that last only a few months to a year or two. Dikpati is leading a team with participants from NCAR, NOAA, Stanford, and the University of Colorado Boulder in order to help advance this type of prediction. Their goal is to use data-infused models to predict solar activity and the likelihood of solar storms a few months in advance. “A seven-day lead-time forecast of weather on Earth covers a period of seven Earth rotations,” Dikpati said in an email. “Similarly, forecasting bursts of solar activity up to seven solar rotations ahead would mean about six months of lead time, since one solar rotation takes about 27 days.”

Every bit of advance notice on the likelihood of dangerous solar storms could be invaluable in a world ever more dependent on reliable power and communications. The potential benefits of cycle forecasts include making satellite projects less risky and more efficient. That’s because the solar cycle can cause air density at low-earth-orbit heights to vary tenfold, vastly altering the atmospheric drag on satellites. Even the most ambitious plans for outer space have a stake in better solar outlooks, according to Svalgaard, who cites this quote from colleague Dean Pesnell (NASA): “A society that travels to other planets needs forecasts of the solar activity visible from any point in the solar system several years in advance.”

We’ll be back with a new post on Friday, including a look at a major late-week/weekend storm that’s still on track to coat large parts of the central U.S. with dangerous freezing rain.

Bob Henson