Researchers are predicting that our Sun is about to enter a period of long-term quiet called a grand solar minimum, a state that might begin as soon as 2020, and could last as long as fifty years. While this period will see a decrease in sunspot formation, magnetic activity and ultraviolet radiation output, the decline in solar radiation isn’t expected to help alleviate the problem of global warming in any meaningful way; however, it may cool the extreme upper atmosphere to the coldest it’s been for the past 70 years.
The Sun follows an 11-year cycle of solar minima and maxima, with sunspot activity and UV output peaking during a solar maximum; conversely, during a solar minimum these activities decrease, sometimes to the point where a number of years might pass with little to no sunspot activity at all. Periodically, the Sun will enter a prolonged period of quiet, called a grand solar minimum. One pronounced example of this was a period called the Maunder Minimum, a 70-year period that lasted from 1645 to 1715. Extensive monitoring of the Sun’s surface by astronomers of the day revealed that sunspot formation had virtually flatlined during the period, with fewer than 50 sunspots being observed in the 28-year period between 1672 and 1699, as opposed to the 45,000 to 56,000 sunspots that would be seen over the same period of time in the 20th century.
By analyzing two decades worth of data on the output of 33 nearby stars that follow cycles similar to that of the Sun, researchers have found that the declining number of sunspots in each subsequent solar cycle of our own star indicates that it is about to enter another such grand minimum, possibly as early as 2020, when the Sun begins its next solar minimum. The Sun’s current solar cycle, having peaked in 2014, has been abnormally quiet, following a trend of steady declines in solar output following a peak in solar maxima in the 1950s.
However, we shouldn’t get our hopes up that the reduced output of solar radiation will grant a reprieve from the advance of global warming, despite predictions that this grand minimum will drop solar UV output by an additional 7 percent below what would be seen during a typical solar minimum, as the researchers predict that it will only drop the temperature of Earth’s lower atmosphere by about a half-degree Fahrenheit (0.3°C). However, in a time when every tenth of a degree counts, we’ll take what we can get.
Indeed, along with the steady decline in solar output since the 1950s, this uneven temperature distribution indicates that the phenomenon of global warming is not due to changes in solar output: solar energy is being trapped near the Earth’s surface by greenhouse gases, warming the lower layers and allowing the upper atmosphere to cool, unlike in a scenario where the Sun’s output increases, in which case the upper atmosphere would be heating up along with the lower layers.
But while the Sun’s prolonged slumber isn’t expected to put much of a dent in climate change, it will have a much more pronounced effect on the extreme upper atmosphere, in particular the layer between 80 kilometers (50 miles) and 600 kilometers (375 miles) above sea level, called the thermosphere. Researchers with NASA who monitor the heat emissions of carbon dioxide (CO2) and nitric oxide (NO) in the thermosphere have charted a steady decline in the temperature of the upper atmosphere over the past few decades. Indeed, the amount of energy in the upper atmosphere is currently only one-tenth of what is usually recorded during a typical solar maximum.
"We see a cooling trend,” says Martin Mlynczak, of NASA’s Langley Research Center. “High above Earth’s surface, near the edge of space, our atmosphere is losing heat energy. If current trends continue, it could soon set a Space Age record for cold." Mlynczak’s team has coined what they call the "Thermosphere Climate Index" (TCI), a new metric that will be added to Spaceweather.com as a regular data feed.
This cooling of the upper atmosphere results in a corresponding contraction of the thermosphere, causing its upper boundary to decrease in altitude as the sphere shrinks. This helps reduce the thermosphere’s meager drag on satellites that orbit through it– for instance, the International Space Station orbits smack-dab in the middle of the thermosphere, at an altitude between 330 and 435 kilometers (205 and 270 miles)–allowing them to maintain their orbits longer. However, this means that space junk also benefits from the lower air resistance, meaning it takes longer for potentially dangerous debris to fall from its orbit and burn up in the lower atmosphere.