10
Seasons of the Sun
The Sun, with all those planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the Universe to do.
Attributed to Galileo Galilei
As with the weather on Earth, the Sun and space weather have seasons. More precisely, they have cycles. Close observation of the spots on the Sun or of the intensity of the X rays and ultraviolet light emitted by our star reveals that over the course of about 11 years the Sun’s activity waxes and wanes. In response, the Earth’s magnetic field and atmosphere become more and less disturbed. Scientists have known the pattern for 160 years, but they remain puzzled by what causes the rise and fall of solar activity. And they are only beginning to address a much deeper question: do solar variability and space weather affect the climate on Earth?
Heinrich Schwabe was the first scientist to observe a cycle at work on the Sun. After observing the Sun daily from 1826 to 1843 and recording the sunspots on its face, Schwabe found that the number of spots per month and per year rose and fell, then rose and started to fall again. After he published his results, Edward Sabine and other scientists compared the rise and fall of sunspot numbers with the frequency of magnetic storms around Earth. They found that the number of storms tracked closely with the number of spots, with more storms occurring when the Sun’s face was freckled.
By tracking the Sun methodically for the past 175 years—and extending the observations back another 120 years by compiling and merging disparate records—solar scientists have charted the solar cycle of activity back to the early 1700s. The records confirm that the sunspot cycle lasts about 11 years from minimal activity to a peak or maximum and back to the minimum. The period marked by many sunspots—as many as 200 spots per month, usually about 120 to 130—is known as solar maximum and the quiet period— with fewer than a dozen sunspots or so—is called solar minimum. Some historical solar cycles have lasted as long as 14 years while the shortest ones have risen and fallen in just 9 years (see Figure 15). With the development of better scientific instrumentation, scientists have also detected a 22-year cycle at work, in which the magnetic poles of the Sun flip by 180 degrees and then back again every 22 years. That is, from one solar minimum to solar maximum, the
FIGURE 15. Upon close study of the two-and-a-half centuries of sunspot counts, several patterns emerge. It appears that odd-numbered solar cycles tend to be somewhat more intense than even-numbered cycles. And in the past 100 years, solar activity seems to be intensifying compared to the previous century. Courtesy of NOAA-SEC.
Sun’s magnetic field flips from a north-south orientation to southnorth and then back again in the next sunspot cycle.
Statistical studies of the sunspot cycle also reveal other quirks and oddities that scientists find difficult to explain. For instance, stronger solar cycles (those with more spots) tend to be shorter while weaker cycles tend to last longer. Odd-numbered solar cycles (we are now in the midst of cycle 23) tend to be stronger than even-numbered ones. As the number of sunspots increases, so does the frequency of the Sun’s explosions, but not necessarily the strength. That is, the most intense and effective sunspots, solar flares, coronal mass ejections (CMEs), and magnetic storms do not necessarily occur at solar maximum, but instead are spread throughout the cycles. In fact, contrary to intuition, storms tend to be more extreme after the peak of the solar cycle, when the Sun is allegedly calming down and returning to solar minimum.
Another statistical quirk is that each solar cycle usually has three different peaks: one for the number of sunspots, one for the number of flares, and one for the number of geomagnetic storms. And while flares and coronal mass ejections are much more common at solar maximum, coronal holes and the damaging high-speed solar wind streams they produce are much more common during the approach to solar minimum. The failures of the Anik (1994) and Telstar 401 (1997) satellites as well as the huge solar flares of 1972 all occurred as the Sun was muddling through the middle, minimum periods of its activity cycle.
The progress of sunspots as they move around the face of the Sun may provide a hint of what is happening inside the star. Years after his discovery of solar flares in 1859, British scientist Richard Carrington completed a study of the position of sunspots throughout the solar cycles. He found that sunspots appear closer and closer to the equator as the Sun becomes most active. At solar minimum, the spots tend to appear around 30 to 40 degrees of solar latitude and as the Sun progresses toward the maximum, the spots develop at lower latitudes, nearer to the middle of the Sun.
According to a theory proposed in the 1960s by astrophysicist Horace Babcock, the sunspot cycle and the migration of sunspots
toward the equator is likely a result of the differential rotation of the Sun, whereby the equator spins faster than the poles. The Sun starts with a relatively simple north–south set of magnetic poles. But as the midsection of the Sun spins faster than the higher latitudes—in three years the equator would lap the north and south Poles more than five times—the magnetic field of the Sun gets twisted and pulled in the east–west direction. This twisting and shearing of the magnetic field lines creates two doughnut-shaped “toroidal” fields, one in the north and one in the south. The intensity of these snarled magnetic fields causes them to erupt out of the Sun and create active regions of sunspots. As the solar cycle progresses and the equator continues lapping the poles, the toroidal fields move down toward the equator, carrying their sunspots with them. Eventually the two fields cancel each other, while the process starts all over again at higher latitudes.
Based on several centuries of sunspot watching and the predictions of numerous solar physicists, the process of magnetic twisting and snarling inside the Sun was expected to reach a crescendo in 2000 through 2002. Solar cycle 23—the twenty-third since reliable measurements were first made in the early 1700s—began in October 1996, when the Sun started its ascent out of solar minimum. In September of that year, the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronauticcs and Space Administration (NASA) convened the Solar Cycle 23 Project, an international panel of scientists who were charged with making the best scientific guess at the magnitude of the current solar maximum. Scientists from around the world submitted 28 predictions based on six different prediction methods. Groups studied the relationships between the coming solar cycle and the length of the previous cycle, the level of activity at sunspot minimum, or the size of the previous cycle. Some scientists analyzed measurements of changes in the Earth’s magnetic field at sunspot minimum, which seem to have a statistical relationship to the intensity of the next solar cycle. Still other groups examined the coronal holes and the strength of the solar magnetic field at solar minimum to say something about Babcock’s theory of magnetic turmoil inside the Sun.
But in the end, since scientists are only starting to view the inside of the Sun—and they still don’t know what to look for—the predictions are ultimately educated statistical guesses. To David Hathaway, a solar physicist at NASA’s Marshall Space Flight Center and a contributor to the Solar Cycle 23 Project, the solar cycle prognostication is akin to the weather predictions in the Farmer’s Almanac. “It’s like saying we’re going to have a mild or cold winter,” he notes. “In the end, it’s all statistical inferences. There’s no real physics involved.”
After the Solar Cycle panel compared and analyzed all of the predictions, it was decided that solar maximum 23 would most likely occur in March or April 2000 but could happen as late as January 2001. The panel predicted that the maximum number of sunspots would be about 160, though the range was listed as anywhere from 130 to 190 sunspots. The average solar cycle has a maximum monthly sunspot number of 110. In effect, the new solar cycle should look a lot like the last one, which peaked in July 1989 as the third-largest solar cycle ever observed.1
The Sun has teased scientists as it has meandered through solar maximum 23. The violent sequence of CMEs and flares in May 1998 seemed to announce the approach of a boisterous solar maximum. But then the Sun bobbed and weaved its way up the slope of the predicted sunspot curve (see Figure 16). The monthly sunspot number rose sharply to 137 in June 1999 but dropped to 70 by September. The Sun really picked up its pace in March 2000, reaching a sunspot count of 138 and sustaining active levels through the summer. In July 2000 the sunspot number reached 169, the highest number of the current solar cycle. Living up to the title of “solar maximum,” July 2000 brought the Bastille Day storm, a monstrous space weather event that killed at least one satellite and brought the auroral oval down to the Gulf Coast of the United States.
By the autumn of 2000, the Sun cooled off and less experienced Sun watchers began to speculate that solar max had come and gone. But the solar roller coaster began another steep ascent in March and April 2001, reaching 134 sunspots. More than a
FIGURE 16. A plot of sunspot numbers from January 1994 through October 2001 shows the erratic decline and rise of solar activity during solar cycle 23. The plot includes the projections made by the scientists of the Solar Cycle 23 Project. Courtesy of ISES/NOAA-SEC.
year after the supposed “maximum” the Sun was covered with 150 spots in September 2001. The ride seemed like it wouldn’t end.
Ever since space weather researchers began their media campaign to educate the public about solar maximum, people have been questioning scientists and marking their calendars for the day or month of solar maximum. They continually ask, “Have we reached the maximum?” But in reality the peak of the solar cycle is just the top of a statistical curve. One month may mark the absolute highest point in the cycle, but the Sun roils with maximum activity for years on either side of that historic month. “The sunspot maximum is usually a broad peak,” said Hathaway. “There is a two- or three-year period when activity is quite high.” So while the statistics say that the Sun was at its most boisterous in July 2000, scientists wouldn’t be surprised if the actual surge of solar activity lasted into 2003.
“On some previous cycles, there was some stagnation and hesitation by the Sun on the way to solar maximum—a plateau in the middle of the rise,” JoAnn Joselyn, a researcher at NOAA’s Space Environment Center (SEC) and chair of the Solar Cycle 23 Project. “This cycle is above average in terms of solar flux.”
SEC, which is responsible for monitoring and predicting space weather for the U.S. government, spent much of the late 1990s warning the public and its commercial customers to expect solar proton radiation showers to become more frequent and more intense during the period of solar maximum. The center made its own prediction based principally on the experiences of the past few solar maxima, that the aurora borealis (northern lights) would make several appearances over the continental United States in the next few years, reaching as far south as the Gulf of Mexico at least once. The SEC team wrote that two magnetic storms of the magnitude of the March 1989 storm were possible and the “probability for severe geomagnetic storms will be the greatest during an extended period lasting from 1999 through 2005.”
Even if the Sun does not follow a scientist’s schedule, even if the storms from the Sun during cycle 23 are no worse than any in the past, the current solar maximum and the next one in 2011 are expected to have a much greater impact on society than any we have endured before. Two to three times more satellites are flying now than during the maximum of 1989, and civilization relies on those spacecraft for a lot more information and communications. Electric power grids are serving more customers, but with about the same number of transformers, power plants, and transmission lines as were in operation 11 years ago. “The explosion in technology is intersecting with an extremely disturbed space environment,” said Joselyn. “Electricity is no longer a luxury, and satellites are becoming a critical link in society. There is much higher risk now because we depend more on technology that is vulnerable to space weather.”
Though their lives and careers are separated by four generations, E. Walter Maunder and Jack Eddy are linked intellectually and spiritually by the Sun. At separate times over the past 100 years— Maunder in the 1880s and 1890s, Eddy in the 1970s—the two solar researchers dared to assert what many of their colleagues would not. The clockwork 11-year solar cycle is not constant, and it is probably not the only cycle at work in our star. Over geologic timescales, other cycles and patterns may be unfolding. Studying historical records and bucking conventional wisdom, Maunder and Eddy saw a connection between the Sun and Earth that scientists are cautious to believe but hard pressed to dismiss.
Intrigued by the observations and conjectures from eighteenth-century astronomer William Herschel and nineteenth-century schoolteacher-turned-astronomer Gustav Spörer, Maunder began investigating a phenomenon he would one day call “the prolonged sunspot minimum.” Spörer and Herschel both noted that sunspots seemed to disappear from the astronomical records for nearly 70 years in the late seventeenth and early eighteenth centuries. As the superintendent of the solar division of Britain’s Royal Greenwich Observatory in the 1880s and 1890s, E. W. Maunder had access to hundreds of years of logs, journals, and historical records. So he began to dig deeper into this disappearance of sunspots. His studies confirmed that from about 1645 to 1715, fewer spots were seen on the Sun over those seven decades than can typically be seen in a single active year. Most of the spots that did appear were located near the solar equator and scarcely lasted for more than one rotation of the Sun. From 1672 to 1704, no spots were observed at all in the northern hemisphere of the Sun. And from 1645 through 1705, no more than one sunspot group showed up at a time, and often there was just one lonely spot rather than a group.
Maunder presented his findings and a summary of Spörer’s work to the Royal Astronomical Society in 1890 and again in 1894, titling his papers “A Prolonged Sunspot Minimum.” He argued that the period of low sunspot activity would eventually prove important to understanding the Sun and how it affects Earth. But apparently his work fell on deaf ears, because little mention of it is
made in the science publications of the early twentieth century, and Maunder himself felt compelled to reprise the story with an updated version in 1922. His later work incorporated observations suggesting that very few auroras were observed during the time of the quiet Sun.
Perhaps the initial thought by most scientists was that astronomers had simply missed the sunspots during the so-called minimal period, having lost interest after the initial buzz of Galileo and Christopher Scheiner in the early 1600s. But the historical evidence from the period contradicts that notion. In the latter half of the seventeenth century, scientists such as Johannes Hevelius and Jean Picard actively chronicled solar activity. And there were certainly other great astronomical discoveries made during the period— including the first observations of Saturn and its rings and moons, the observation and study of Halley’s comet, and the calculation of the speed of light from observations of Jupiter’s moons. Moreover, the few sunspots that were observed during the period were usually an occasion for scientific notice and publication. Astronomers Giovanni Cassini and John Flamsteed both paid special attention to sunspots they observed in 1671 and 1684, respectively, noting that such sightings had been rare in recent years.
For whatever reason, Maunder’s sunspot minimum did not capture much attention until American solar physicist Jack Eddy revived the studies in the mid-1970s. Provoked by stories that University of Chicago space physicist Gene Parker told him about Maunder, Eddy reviewed the turn-of-the-century papers. His initial intention was to debunk the theories about solar variability and climate. “I had been taught that while the Sun indeed affects the upper and outer atmosphere of the Earth, purported connections with the troposphere and weather and climate were uniformly wacky and to be distrusted. . . . The claims that were made for associations between weather events and the Sun I thought were pretty preposterous. The trail was, initially, purely historical and driven by my prejudice of trying to find examples from the past that would disprove, once and for all, the notion of strong Sun-weather relations. A devout negativism on this subject was the gospel at the
High Altitude Observatory, where I had been trained.... It needed to be shot at, even after all these years, and dismissed once and for all. So I set out to demonstrate that what Maunder had claimed was really nonsense. . . . I was trying to examine the early origins of Sun-weather claims, like unrolling and deciphering the Dead Sea Scrolls of solar physics. But it was mostly a love of history that took me down the trail.”
To his surprise, Eddy gradually learned that the observations of Spörer and Maunder were anything but nonsense. He expanded on their work by pulling in historical records of auroras, naked-eye sunspots, and eclipses. According to one catalog that Eddy cited, only 77 auroral shows were recorded in the entire world from 1645 to 1715, and 20 of those appeared in 1707 and 1708, when sunspots were present. Of the aurora reports that were made, nearly all of them originated from Scandinavia, despite the fact that London and other locations in Europe typically see 5 to 10 northern light shows per year. Eddy could not find any reports of sunspots viewed with the naked eye in China or other Asian countries, where naked-eye sunspots had been recorded as momentous events since antiquity. The historical records from the Far East typically cite at least one naked-eye sunspot per decade.
Eddy also noted that while astronomy was budding and scientists were chronicling everything they could see in the sky, not one of the accounts of eclipses during the period of Maunder’s sunspot minimum describes the corona of the Sun in any detail. In the usual eclipse, the streamers and detailed structures of the solar wind can be seen extending for some distance as the Moon blocks the dazzling disk of the Sun. Yet at a time when sky watchers would have been more attuned to celestial phenomena, no one was writing about the solar corona or what it looked like through a telescope.
But the clinching evidence for the “Maunder Minimum,” as Eddy came to call it, could be found in the growth rings of fallen trees. As trees lay down their rings each year, they record information about the atmosphere and the environment. Specifically, trees capture carbon dioxide, which consists of a blend of two carbon isotopes—
carbon 12 (C12), which is chemically stable, and carbon 14 (C14), which is radioactive. C14 is constantly being formed in the Earth’s atmosphere when cosmic rays bombard the carbon and nitrogen compounds in the air. That rate of C14 formation varies ever so slightly, but the rate at which the isotope decays is constant (the “half-life,” or time it takes for one-half of the C14 to decay into nitrogen, is about 5,600 years). When the Sun is more active, fewer cosmic rays make it into Earth’s atmosphere, producing less C14; when solar activity is low, cosmic rays flow unimpeded to Earth and create slightly more C14 for trees to absorb.
Combining the records of the solar community with those of environmental scientists, Eddy found that tree rings from the era of the Maunder Minimum contained significantly more C14 than the trees and tree rings formed in the years before or after the sunspot minimum. When held up next to the historical records of auroras and sunspots—and their absence from 1645 to 1715—it became obvious to Eddy that something in the Sun-Earth relationship had changed for those 70 years. The constant solar cycle was in fact not constant at all.
What makes the Maunder Minimum more than just a curiosity of solar science are the stark changes in Earth’s climate that seemed to coincide with the lack of sunspots. In the decades of Maunder’s sunspot disappearing act, Europe and North America endured a period of extremely harsh winters and colder than normal summers; the period has been dubbed the Little Ice Age. Glaciers advanced farther south than they had at any time since the end of the last true Ice Age (about 15,000 years ago). No major floods washed over the lowlands of Switzerland—suggesting that alpine snows and glaciers hardly thawed as they normally do with the rise and fall of the seasons. The average global temperature dropped one degree below the norm, but that was enough of a fluctuation to cause severe drought in the American Southwest and to allow Londoners to skate on the Thames.
Studies of older trees and their rings have since revealed other periods when the Sun appears to have waned and waxed beyond the norms of the 11-year cycle. According to the C-14 record, the
Sun activity dropped off noticeably from 1460 to 1550, another period of global cooling now known as the Spörer minimum. Conversely, from about 1100 to 1250, the Sun waxed into a prolonged maximum of activity, a period that coincides with the “Medieval Climatic Optimum.” Temperatures apparently rose enough to allow the Norse people to first settle Greenland and give it the name that now seems so preposterous. The period also induced a severe, decades-long drought that may have brought on the demise of the Anasazi culture of North America.
While it is tempting to immediately connect the Maunder and Spörer minima and the medieval maximum to changes in Earth’s climate, scientists have remained skeptical for much of the past century. Sure, there have been changes in climate that coincide with changes on the Sun. The Maunder Minimum coincided with a cooling period in many parts of the Earth, and in the 300 years since then, solar activity has steadily increased at the same time as global mean temperatures have increased.
But many of the scientists who run computer models of the atmosphere do not seem to have room for the Sun, or at least for one that varies. And many geophysicists are wary of the solar input because they cannot explain the physics and chemistry of how solar variability could change the climate. The question remains: is the concurrent rise of solar activity and of earthly temperatures a coincidence or a cause-and-effect relationship?
The Sun dominates and drives climate patterns on Earth. Depending on the season of year and the tilt of the Earth’s axis toward or away from the Sun, temperatures climb and fall by hemisphere. The southern hemisphere warms and the northern hemisphere cools as the axis tilts away from the Sun. In summer-time, sunlight beats down directly over one hemisphere and the beams fall more obliquely over the other hemisphere. The tilt of the axis causes one-half of the planet to bathe in longer periods of warming daylight than the other. Effectively, the amount and
intensity of sunlight pouring out from the Sun to Earth do not change, but the angle of Earth’s axis forces more sunlight to shine longer over one region than the other. This sunlight warms not only the surface of the planet but also the atmosphere and the oceans, which both retain heat and drive weather patterns. It is a cyclical—seasonal—pattern, but one that appears quite stable.
Until recently, the radiative output of the Sun was generally thought to be constant—that is, scientists assumed that the Sun emits a steady and unchanging amount of light and heat. But, in fact, the amount of radiation from the Sun does vary. While it would seem that all the spots on the Sun would dim the light a bit, in fact the Sun is brighter during solar maximum because there are more bright active regions. Measurements from the Active Cavity Radiometer Irradiance Monitor (ACRIM) experiment on NASA’s Solar Maximum Mission satellite and the Earth Radiation Budget (ERB) experiment on Nimbus-7 revealed that the Sun’s output changed by 0.1 percent from solar maxima to solar minima in the 1970s through early 1990s.
While the total amount and intensity of sunlight—total solar irradiance—varies by 0.1 percent, other parts of the solar spectrum undergo more significant changes. In particular, high-energy, short-wavelength forms of light are more intense during the Sun’s most active periods, with ultraviolet (UV) radiation increasing by 6 to 8 percent. UV rays don’t contribute much to the total radiative output of the Sun, but they are one of the major contributors to the cyclical changes in irradiance. And since ultraviolet radiation is known to be an important catalyst of chemical reactions in Earth’s atmosphere, small increases in solar UV might have big consequences for the energy balance of the upper atmosphere.
Analysis of the changes in solar irradiance suggest that the variation from solar minimum to maximum could produce a global averaged temperature change of about 0.2 degrees Celsius, or about 0.24 watts per square meter of the Earth’s surface. More recent observations from the Upper Atmospheric Research Satellite (UARS) confirm that this cyclical rise and fall is ongoing. A variation of 0.2 degrees seems trivial and almost silly to worry about.
Predictions of global warming suggest that the global surface temperature could rise by 1 to 3 degrees Celsius (2 to 5 degrees Fahrenheit), numbers that seem negligible when one considers the daily fluctuations most of us feel every day. But in a recent editorial, Jack Eddy put such temperature changes into context: “Why should a change of but a few degrees, 50 or 100 years in the future, be of such concern today? For those of us in middle latitudes, the variation from day to night in the temperature of the air can be 20 degrees C, and in the course of the seasons the daily mean varies through 50 to 60 degrees. What is it about a global average that makes a change of 1 degree profound? A change of 1 degree in the mean implies larger changes in parts of the globe—particularly polar regions—but also smaller ones in others. And a small variation in temperature can signal much greater changes in other conditions, such as precipitation and storms and river flow.” Eddy notes that global mean temperatures have varied by no more than 1 degree over the past thousand years—a period that included the wicked winters and cool summers of the Little Ice Age. A global increase of just 2 degrees C would double the number of unusually hot days on Earth. And the difference between today and the last major Ice Age—when a mile-thick layer of ice covered most of North America—is only about 5 degrees C in the global mean value.
According to Drew Shindell, a climate researcher at NASA’s Goddard Institute for Space Studies in New York, the variations in solar radiation over the course of a solar cycle might affect the ozone layers of the atmosphere. Observations from UARS show that the additional high-energy radiation at solar maximum increases the amount of ozone in the upper atmosphere by at least 1 or 2 percent. The increase in ozone warms the upper atmosphere, and this warm air affects wind patterns from the stratosphere all the way down to the troposphere (the lower layer of the atmosphere near the surface). “When we added the upper atmosphere’s chemistry into our climate model, we found that during a solar maximum major climate changes occur in North America,” noted Shindell. Specifically, westerly winds blow stronger, but changes in wind speed and direction occur all over the
planet. “Solar variability changes the distribution of energy.” Over an 11-year solar cycle, the total amount of energy coming into the atmosphere does not change very much. But where that energy gets deposited varies. This leads to changes in wind speeds and prevailing directions, which create different climate patterns.
Other scientists, such as Henrik Svensmark of the Danish Space Research Institute and Brian Tinsley of the University of Texas at Dallas, suspect that changes in the Sun’s output might affect the cloud cover of Earth. Cosmic rays, which originate outside the solar system when stars go supernova, bombard the atmosphere and change the chemistry a bit. The reaction produces aerosols that help seed and form clouds. The influx of cosmic rays also can cause more electric charge to accumulate on the ice crystals and evaporating droplets at the tops of clouds. This accumulation of charge can in turn affect how clouds reflect light back into space, how much water falls out as precipitation, and how heat is stored and released in the atmosphere.
The link between cosmic rays, solar activity, and climate seems to be the interplanetary magnetic field (IMF). The IMF is blown out from the Sun by the solar wind and the strength of the magnetic field reflects the intensity of solar activity. The IMF causes the cosmic-ray flux (as measured at Earth) to vary by deflecting these high-energy interstellar particles away from the inner solar system. When the Sun’s activity reaches a minimum, the IMF tends to be weaker and more cosmic rays can wash over the Earth’s atmosphere. But at solar maximum, the heightened activity of the Sun and the more potent IMF prevents much of that cosmic radiation from reaching the inner parts of the solar system. During a prolonged sunspot minimum—such as the Maunder Minimum— the IMF would likely be very weak, allowing more cosmic rays to reach Earth and producing more clouds that would reflect sunlight. Could this be a causal link between the Maunder Minimum and the Little Ice Age?
Researchers recently stirred up controversy by unearthing a long-term trend from records of solar and space weather activity. Mike Lockwood and colleagues at Britain’s Rutherford Appleton
Laboratory have found evidence suggesting that the strength of the solar magnetic field in the IMF has doubled over the past 150 years. They deduced this trend by using a combination of solar wind measurements (dating from 1963 to the present) and Earth-based magnetic field measurements dating back to the mid-1800s. Other researchers such as Ernie Hildner, director of NOAA’s Space Environment Center, argue that there is no evidence that the solar magnetic field has been increasing over the last 40 years. The question of whether the IMF has been increasing is not just an esoteric argument. According to the cosmic-ray hypothesis, an increasingly potent IMF would lead to a lower influx of cosmic rays, less cloud cover, and more solar radiation reaching the surface of Earth. An extended period of a weak IMF could lead to greater cloud cover and lower temperatures.
In another connection between space weather and terrestrial weather, there is compelling evidence that dynamic changes in the Earth’s stratosphere may be synchronized to the solar cycle. For nearly three decades, Karin Labitzke of the Free University of Berlin and Harry van Loon of the National Center for Atmospheric Research have worked together to show that a known 10- to 12-year oscillation in the stratosphere of the northern and southern hemispheres matches up fairly well with the four solar cycles since 1958. By analyzing average temperatures and air pressures in the stratosphere, they found that the height of the stratosphere rises by as much as one-third as the Sun progresses from solar minimum to maximum. The stretching of the stratosphere likely affects wind and air pressure patterns in the lower atmosphere. The solar cycle effect is not strictly global; in fact, the stratospheric changes are most pronounced near the poles and between 20 and 40 degrees of latitude. But changing the atmosphere in any one part of the world still affects global weather patterns everywhere, as weather changes in one region and forces changes in the neighboring regions.
“We were all taught that the solar cycle had no influence on climate,” notes van Loon. “Even in recent years, the 0.1 percent difference in solar irradiance has been written off as ‘noise.’ But the solar-stratospheric relationship is more than a statistical
coincidence. Any relationship between changes in solar output and what happens here on Earth is important for understanding long-term climate.”
While the work of van Loon and others correlates the 11-year solar cycle to other 10- to 12-year cycles of activity at work on Earth, a longer view reveals even more Sun-climate connections. Reviewing the past 130 years of sunspot records and temperature measurements, Eigil Friis-Christensen and Knud Lassen of the Danish Meteorological Institute and John Butler of Ireland’s Armagh Observatory found that global temperatures seem to fluctuate in synch with the solar cycles. Shorter solar cycles—that is, those that last fewer than 11 years—tend to produce warming trends on Earth, while longer solar cycles coincide with periods of cooling. As the studies of the sunspot numbers and tree rings show, the overall trend of the past 400 years has been toward greater solar activity and higher temperatures.
Recent studies of other stars and computer models of our own star suggest that solar irradiance—the intensity of the light emitted by the Sun—decreased by 0.15 to 0.4 percent during the Maunder Minimum of the seventeenth and eighteenth centuries, four times the variation observed from maximum to minimum in the 1970s and 1980s. Given such a drastic change over a short period of history, and given that we have methodically observed the Sun and the climate for only a few centuries, there is the reasonable possibility that the Sun-Earth system may undergo much more dramatic variations over longer timescales.
The problem now is time and politics. Scientists can measure and model and mull how Earth’s climate changes over time, but until they have a longer and more substantial set of data about our environment, most will remain skeptical about the Sun-Earth connection to climate. Researchers will either have to develop a way to project solar activity back into ancient times or they will have to wait until their grandchildren’s grandchildren have lived through the next major warming or cooling spell to know for sure. “The Sun’s energy variation does affect weather, and it should not be
discounted,” says van Loon. “But the role of the Sun in climate change is just a big unsolved problem.”
The other problem is that some political groups give some Sun-climate researchers an unfortunate and misappropriated reputation. Groups point to the work of scientists like Svensmark or Butler on the solar role in climate change and claim that their work is compelling evidence that human activity has not contributed to the warming of the Earth. Even the most devout Sun-climate researchers acknowledge that it is irresponsible and at odds with the basic physics of the greenhouse effect to say that industrial emissions of carbon dioxide and other gases have not affected the atmosphere. The increase in greenhouse gases is a measurable fact, as are the global rise in temperatures, the depletion of atmospheric ozone, and the increase in ground-level ozone.
On the other hand, it would be equally irresponsible to reflexively claim that all of the observed global warming is anthropogenic and that the Sun has nothing to do with it. We have to face the fact that the Sun is the primary driver of our climate system and that we are living with a star that changes.
