Solar cycle
The solar cycle refers to the approximately eleven-year cycle of solar activity characterized by variations in the number of sunspots on the Sun's surface. Sunspots are temporary dark markings that indicate regions of intense magnetic activity, and their numbers fluctuate between solar maximum, when sunspots are most numerous, and solar minimum, when they are scarce. Although the cycle generally spans around eleven years, it can vary in length and intensity, with some cycles being more active than others. The Sun's magnetic field also reverses during this cycle, resulting in a broader twenty-two-year magnetic cycle.
The solar cycle has implications for Earth's climate, as the Sun's energy output fluctuates slightly with sunspot activity, potentially influencing climate change. While these variations are typically small, prolonged periods of high or low solar activity can cumulatively affect Earth's climate. Historical events, such as the Maunder Minimum in the seventeenth century, which coincided with a cooler climate known as the Little Ice Age, illustrate this relationship. However, the extent to which solar activity alone drives climate changes remains a topic of debate. Some researchers suggest that increased solar activity may have contributed to recent global warming trends, while others argue that human-induced factors, such as greenhouse gas emissions, also play a significant role.
Solar cycle
Definition
Temporary dark markings on the surface of the Sun, known as sunspots, have been observed since ancient times. As seen from Earth, though, most are too small to be observed with the naked eye. It was not until the invention of the telescope that sunspots could be observed with regularity. Sunspots were a regular feature on the face of the Sun in the early seventeenth century; however, sunspots all but disappeared later in that century for a period of time lasting into the eighteenth century. This period with almost no sunspots is called the Maunder Minimum.
By the beginning of the nineteenth century, astronomers had recognized that the number of sunspots increased and decreased according to a pattern. A peak number of sunspots, called the solar maximum, occurs about every eleven years. The cycle of sunspot activity does not have an exact period, however, with some cycles lasting closer to ten years and others lasting close to twelve years. Furthermore, each sunspot cycle does not result in the same average number of sunspots at its maximum. Every sunspot cycle is a little different from the other cycles.
Sunspots are visible signs of magnetic activity on the Sun. Sunspots form in regions of intense activity. These active regions are also the locations of huge explosive releases of magnetic energy called solar flares. The more active the Sun’s magnetic field becomes, the more sunspots will appear on its surface. Astronomers have also found that, during each sunspot cycle, the Sun’s magnetic field is reversed from that of the cycle before. Thus, the magnetic field follows a twenty-two-year cycle, composed of two eleven-year sunspot cycles.
Significance for Climate Change
Since the advent of space-based observations of the Sun, astronomers have found that the Sun’s energy output varies slightly with the sunspot cycle. The more active the Sun is, the more active the sunspot cycle, and the greater the solar irradiance. Since solar energy is the driving force behind Earth’s climate, variations in the Sun’s energy output could potentially result in climate change. The variation in solar energy is very small, typically less than 0.1 percent. Small changes of that magnitude, averaged out over a solar cycle, should not have a long-term effect on the climate. However, not all sunspot cycles are equal in activity. Some are more active than average and some are less active than average. Historical data indicate that sunspot cycles sometimes exhibit trends in activity over long periods of time. An extended period of unusually active or unusually inactive sunspot cycles could have a cumulative effect on the climate.
During the in the seventeenth century, a significant shift toward a cooler climate was observed in many locations. This period of cooling is called the Little Ice Age. (The term “Little Ice Age” is used differently by different writers. Many use it to refer to the climate cooling from about 1300 to 1850, while others use it for the latter half of that interval, when cooling was greatest, beginning around 1550 or 1600.) There is no proof that there was a causal relationship between the two events, but most solar astronomers believe that the Maunder Minimum and the Little are connected. Furthermore, there has been a general trend of increasing activity since the Maunder Minimum. Detailed measurements of solar irradiation go back to the twentieth century. Throughout that time, there has been an observed correlation between solar energy output and the average number of sunspots. Thus, it makes sense that during the Maunder Minimum, the Sun was less active and was providing less solar energy to Earth than normal, resulting in a general cooling of the climate.
If the correlation between sunspot numbers and solar irradiation is consistent, the general increase in average solar activity since the Maunder Minimum would be expected to correspond with a period of increased solar heating of the Earth. Some researchers believe that much of the global warming observed during this time period may be the result of natural solar energy increases. Other researchers, though, dispute the degree of change in solar energy output, suggesting instead that the increase in solar energy reaching Earth may only partially explain the observed temperature increases on Earth during the same period of time. If it is true that global warming trends are the result of increased solar activity rather than human activity, it is unclear whether any human actions could reverse those trends, since they may not be a function of greenhouse gas (GHG) levels. On the other hand, a correlation between solar activity and global warming does not in and of itself establish that atmospheric GHG concentrations are unrelated to warming. Both factors could be contributors to climate change.
The Sun emits more charged particles when it is more active. These charged particles interfere with galactic cosmic rays, changing the amount of carbon 14 produced on Earth. Increased activity results in less carbon 14. These changes can be monitored by measuring the carbon 14 content of artifacts of known age. Such studies seem to support the hypothesis that increases in solar activity correlate with warmer periods in Earth’s climate history and periods of reduced solar activity correlate with cooler periods in history.
Bibliography
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